Field of the Invention: This invention relates to a method for screening compositions which interfere with any pathology specific to a transcriptional transactivation event. More specifically, this invention relates to methods for identifying antiviral agents by virtue of their ability to interfere with viral transactivation elements.

Background of the Invention:

It is now appreciated that, during some viral infections, certain stages of infection are characterized by the coordinate induction of different classes of viral genes. This developmental regulation can be mediated at the level of transcription initiation by specific viral proteins through a process known as transactivation. (See, e.g., Wagner. E..Adv. Viral Oncol. 3:239-270 (1983); Weinheimer, S.P. and Si. McKnight, J. Mol. Biol. 195:819-833 (1987)). Transactivation is accomplished by the combinatorial interaction of viral proteins, host cell proteins and DNA.

In order to more fully appreciate the complexity of transactivation and to more fully understand this invention, a brief review of eukaryotic transcription is required. In general, the field of eukaiyotic transcription has been recently reviewed by Ptashne, M. Nature 335:683-689 (1988), Lewin, B. £sll 61:1161-64 (1990), the entire contents of which are incorporated herein by reference for purposes of background only. Briefly, eukaryotic genes contain more information than appears in the final messenger RNA (mRNA) transcript This information occurs outside of the transcriptional unit (e.g. the TATA box), or within the unit (e.g., as introns, polyadenylation sites, etc.) The final mRNA species is the result of extensive post- transcriptional modification events (e.g. splicing, addition of the 5'-cap, truncation and addition of a polyA tail.). For purposes of this invention, the elements outside of the transcriptional unit which influence transcription initiation are of greater importance for the reasons outlined below.

In eukaryotes and animal viruses, the mechanism of transcription initiation is complex. Upstream from the site of initiation is often a TATA box which is important in RNA polymerase recognition. In yeast the TATA box is typically 60- 100 base pairs(bp) upstream while in higher eukaryotes such as Drosophila or human, the TATA sequence is usually 30 bp upstream. Upstream activation sequences (UASs, also known as enhancers, upstream promoter elements, distal promoter elements or cis-acting elements) are usually located 100 to 400 bp upstream; however, some UASs may occupy variable locations upstream,

downstream, and within introns of the genes they regulate These sequences bind specific eukaryotic transactivator proteins. For UASs close to the site of transcription initiation, the orientation of the sequence is critical, while for those far from the initiation site the sequences may occur in either orientation (See, Figure 1). Three eukaryotic RNA polymerases have been defined based on the type of

RNA they transcribe and by their sensitivity to the mushroom toxin, α-amanitin. RNA polymerase I makes large ribosomal RNAs and is insensitive to α-amanitin, RNA polymerase II makes messenger RNA (mRNA) and is sensitive to α-amanitin. RNA polymerase DI makes a variety of very small stable RNAs including 5S ribosomal RNA and transfer RNAs and is partially sensitive to α-amanitin. Most of the small RNAs that form the small nuclear ribonucleoproteins (snRNP), however, are made by RNA polymerase π. These polymerases are composed of 8-12 polypeptide chains, including two large subunits with sequence similarity to the β and β' subunits of E. coli RNA polymerase. In the discussions which follow, the emphasis will be on RNA polymerase II (RNA pol II) (See, Figure 2).

The eukaryotic transcription reaction can be divided into two parts, formation of the basal complex and activation by transactivators bound to the UASs. Assembly of the basal transcription complex determines which genes will be transcribed, whereas, assembly of transactivator complexes determines the rate and timing of transcription.

As presently understood, the first step in transcription and prior to the binding of RNA pol II, a number of transcriptional factors assemble at the TATA box. The first of these is TFIID (Transcription Eactor H __). It is thought that the binding of TFIID to the TATA box is the rate determining step in the initiation of transcription. TFIID may function by blocking nucleosome formation at the TATA box and thereby facilitate polymerase association. Transcription Factor IIA(TFIIA) and Transcription Factor IIB(TFIIB) then bind to TFIID give rise to the DAB complex. The genes for many of these general transcription factors have been cloned and sequenced. Once the DAB complex has formed, RNA pol π is able to bind. However, prior to binding, TFIIF (composed of two subunits, RAP30 and RAP74) must first associate with polymerase. After the complex, called RNA pol HA, has associated with the DAB complex, the C-terminal domain of the large RNA pol II subunit (RBP1) is polyphosphorylated by a proposed C-terminal domain kinase. The modification is postulated to increase the affinity of the polymerase (now called RNA pol HO) for the DNA ~50-fold and, hence, to commit the polymerase to transcription of a particular gene. Following phosphorylation, factors TFIIE, G, and

H bind to yield the basal complex. In this state RNA pol πθ is poised to initiate transcription. As the polymerase begins transcription and moves down the DNA molecule, TFIID is thought to be left behind to help initiate another round of transcription. The rate of transcription from the basal complex is usually low. Interaction of the complex with upstream transactivators is required to promote high levels of transcription.

The structures of transactivators have now been examined in some detail (See, Figure 3) and a number of general themes have emerged. The most striking is that the transactivators are modular, each having a specific DNA recognition domain and a unique activation domain. The DNA recognition domain guides the element to a particular UAS where the activation domain can then specifically interact with the basal transcriptional apparatus.

The DNA recognition domains characterized so far fall into four general classes or families (Fig. 3). These are (i) the homeodomain or helix-turn-helix family, represented by Oct-1, Oct-2, Pit-1 and unc-86, now denoted POU proteins (Herr et al., 1988); (ii) the zinc finger family, represented by TFIIA and the steroid hormone receptors (Kaptein, R.. Curr. Opin. in Struct. Biol. 1:63-70. (1991)); (iii) the dimeric, leucine zipper family, represented by jun and fos (Kerpolla, T.K. and T. Curran. Curr. Opin. in Struct. Biol. 1:71-79. (1991)); (iv) and the dimeric, helix- loop-helix family, represented by E12 and E47 (Murre et al., £sll 56:777-83

(1989)). In addition, there are many transactivators which fall into none of these families and may contain other DNA recognition motifs.

The activation domains characterized thus far also fall into four families (Fig. 3). There are the activators with unstructured, acidic domains represented by HSV VP16 (Dahymple, M.A. et al., Nucl. Acid Res. 13:7865-79 (1985)); Cress, A. and S.J. Triezenberg, Science 251:87-90 (1991)); activators with serine/threonine rich activation domains represented by Oct-1 and Oct-2 (Clerc, R.G. et al., Genes Dev. 2:1570-81 (1988)); Sturm, R.A. et al, Genes Dev.2:1582-99 (1988)), activators with proline-rich domains represented by CTF (Santoro, C. et al, Nature 304:218-224 (1988)); and activators with glutamine rich domains represented by Oct-1, Oct-2 and Spl (Courtney, AJ. and R. Tjian, £ell 55:887-98 (1988)). These activation domains are often small and compact usually ranging from 30 to 100 amino acids. The independent nature of the DNA binding and activation domains is evidenced by the fact that activation domains can be linked to heterologous DNA binding domains via recombinant methods and remain competent to activate transcription of reporter genes linked to UASs-specific for the new binding domain. Deletion analysis of Spl has shown that its activation domain has as many as four

separate regions (Courtney and Tjian, supra. 1988). This transactivator has two glutamine-rich domains, a third positively charged domain and a fourth carboxyl- terminal 30 amino acid domain. The latter two domains display no obvious similarity to each other nor to the glutamine-rich activation domains. Less is known about the structure and function of the basal transcription factors. Based on the structures of TFIID from six different species, the molecule has two distinct domains: a variable N-terminal domain and a relatively conserved C-terminal domain. The C-terminal domain (or conserved core) is roughly 180 amino acids long. Among the six species there is between 80 and 90% homology in this region. This core contains two direct repeats at either end and a conserved, basic repeat between them. In addition, there are homologies to myc and the eubacterial protein, σ^O, involved in promoter recognition.

The N-terminal domain is more variable and it's function is a matter of debate at this time. It has been suggested that this domain may interact with transactivation factors in a species-specific manner. It has been shown that a

Drosophila TFIID with a truncated N-terminal domain, and yeast TFHD, in which the N-terminal domain is absent, function equally well when compared to endogenous or cloned TFIID in a basal transcription assay. However, the truncated Drosophila or the yeast TFIID will not allow Spl -promoted transactivation, even in the presence of Drosophila crude nuclear extracts. These data are consistent with the notion that the C-terminal domain may be important for interactions with basal factors and that the N-terminal domain may serve a regulatory role, i.e., the species- specific interaction with upstream activation elements or with adapters or . Much less is known about adapter molecules. The existence of these molecules was suggested from the results of several groups (Berger, S.L. et al., Cell 61:1199-1208 (1990)); Kelleher m, R.S. et al., Cell 61:1209-15 (1990)); Pugh, B.F. and R. Tjian, Cell 61:1187-97 (1990)). Berger, et al. supra (1990), using a GAL4- VP16 chimeric transactivator, have demonstrated indirectiy that there must be a class of molecules that bridge the transactivators and the basal complex. Pugh and Tjian (1990) supra have shown that cloned TFIID from Drosophila cannot alone substitute for TFHD-depleted extracts, implying that there must be TFIID associated factors (1-6). TAFs have now been purified, but not as yet cloned. These data suggest that in certain cases, adapter molecules may form bridges between proteins in the basal transcriptional complex and proteins associated with upstream activation sequences and thus modulate their activity. It should be noted that little information is currently available concerning these molecules, and the generality of these observations is yet to be demonstrated.

Transactivators are known to bind to specific upstream activation sequences. UASs are usually found in clusters containing binding sites for many different transactivators. Activation of basal transcription by transactivators is, therefore, proposed to be a combinatorial event, i.e., the cell cycle, developmental, or hormonally regulated expression of a given gene results from the unique combination of different transactivators binding to their specific response elements. The experiments of Berger, et al. supra (1990) illustrate the amplification of basal transcription produced by a single transactivator. They measured the transactivator (GAL4)-dependent transcription in cerevisiae nuclear extracts under different conditions. Using a DNA template without a UAS or with the GAL4 UAS but no transactivator, they observed only a low level of activity. Using the same GAL4 UAS template in the presence of a transactivator containing the GAL4-DNA binding domain fused genetically to the VP16 acidic activation domain, transcription increased 100-fold. Hence the binding of the transactivator to its upstream activation sequence significantly increased transcription.

It is an object of this invention to provide various regulatable pathology- specific transactivating elements, to transform host cells with such elements and to employ the transformed cells in screens for the identification of compounds which will interfere with transactivation and therefore represent potential antipathogenic agents. Pathology-specific elements can be found in, but are not limited to, cancer and in bacterial, viral, and fungal infections.

Brief Description of the Figures

Figure 1 illustrates a diagram of Prokaryotic and Eukaryotic Gene Structure. Figure 2 illustrates the formation of Basal Eukaryotic Transcription

Initiation Complex.

Figure 3 illustrates a diagram of the functioning of transactivators, adapters and basal transcription factors.

Figure 4 illustrates two preferred targets of viral transactivation.

Brief Description of the Invention

This invention relates to a method for identifying antiviral compounds comprising: a. culturing a recombinant cell comprising a differentially expressible gene encoding a growth-inhibiting viral-transactivating element under a first set of culture

conditions which permit cell growth but substantially no expression of said element to provide a population of screenable cells; b. shifting the cells to a second set of culture conditions under which said element is expressed resulting in cell growth inhibition; c. contacting the inhibited cells with compositions comprising suspected antiviral compounds; and d. identifying those compounds which relieve growth inhibition. This invention relates to a method for identifying antiviral compounds comprising: a. culturing a recombinant yeast cell comprising a plasmid encoding a chimeric gene GAL4-VP16 operatively linked to a yeast CYC1 promoter in a nutrient medium comprising glucose as the carbon source; b. plating the glucose grown yeast cells on a solid nutrient medium comprising lactate as the carbon source to induce the expression of the chimeric gene resulting in inhibition of yeast cell growth; c. contacting the inhibited yeast cells with compositions comprising potential viral inhibitors and d. identifying said inhibitors by their ability to promote cell growth.

Detailed Description of the Invention

For ease of description, this invention will be described with reference to one well-known transactivating system; namely, the VP16 protein of Herpes Simplex Virus (HSV). The invention is, however, broadly applicable to any viral transactivation system, in which transactivating elements can be differentially expressed and, when they are expressed, result in an identifiable phenotype in the expressing cells. Accordingly, in addition to the Herpes virus family, this invention contemplates employing the transactivation elements from papilloma virus (E2), HIV1 (TAT).

In further describing the present invention, the following additional terms will be employed, and are intended to be defined as indicated below.

"Fusion protein" is a protein resulting from the expression of at least two operatively-linked heterologous coding sequences. The protein comprising VP16 and GAL4 peptide sequences of this invention is an example of a fusion protein.

A coding sequence is "operably linked to" another coding sequence when an RNA polymerase will transcribe the two coding sequences into a single mRNA,

which is translatable in a single reading frame into a single polypeptide having amino acids derived from both coding sequences. The coding sequences need not be contiguous with one another so long as the expressed sequence is ultimately processed to produce the desired protein. A coding sequence is "operatively linked to a promoter when the transcription of the coding sequence is regulated by said promoter.

"Recombinant" polypeptides refer to polypeptides produced by recombinant DNA techniques; i.e., produced from cells transformed by an exogenous DNA construct encoding the desired polypeptide. "Synthetic" polypeptides are those prepared by chemical synthesis.

A "replicon" is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo: i.e., capable of replication under its own control.

A "vector" is a replicon, such as a plasmid, phage, or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment.

A "double-stranded DNA molecule" refers to the polymeric form of deoxyribonucleotides (bases adenine, guanine, thymine, or cytosine) in a double- stranded helix, both relaxed and supercoiled. This term refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus this term includes double-stranded DNA found, inter alia, in linear DNA molecules (e.g., restriction fragments, viruses, plasmids, and chromosomes. In discussion of the structure of particular double-stranded DNA molecules, sequences may be described herein according to the normal convention of giving only the sequence in the 5' to 3' direction along the nontranscribed strand of DNA (i.e., the strand having the sequence homologous to the mRNA).

A DNA "coding sequence of or a "nucleotide sequence encoding" a particular protein, is a DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropriate regulatory sequences.

A "promoter sequence" is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3' direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bound at the 3' terminus by the translation start codon (ATG) of a coding sequence and extends upstream (5 ^* direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence will be found a transcription

initiation site (conveniently defined by mapping with nuclease S ), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Eukaryotic promoters will often, but not always, contain "TATA" boxes and "CAT" boxes. Prokaryotic promoters contain Shine-Dalgarno sequences in addition to the -10 and -35 consensus sequences.

DNA "control sequences" refers collectively to promoter sequences, ribosome binding sites, polyadenylation signals, transcription termination sequences, upstream regulatory domains, enhancers, and the like, which collectively provide for the expression (i.e., the transcription and translation) of a coding sequence in a host cell.

A control sequence "directs the expression" of a coding sequence in a cell when RNA polymerase will bind the promoter sequence and transcribe the coding sequence into mRNA, which is then translated into the polypeptide encoded by the coding sequence. A "host cell" is a cell which has been transformed, or is capable of transformation, by an exogenous DNA sequence.

A cell has been "transformed" by exogenous DNA when such exogenous DNA has been introduced inside the cell membrane. Exogenous DNA may or may not be integrated (covalently linked) into chromosomal DNA making up the genome of the cell. In prokaryotes and yeasts, for example, the exogenous DNA may be maintained on an episomal element, such as a plasmid. With respect to eukaryotic cells, a stably transformed cell is one in which the exogenous DNA has become integrated into the chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cell containing the exogenous DNA.

A "clone" is a population of cells derived from a single cell or common ancestor by mitosis. A "cell line" is a clone of a primary cell that is capable of stable growth in vitro for many generations. Two DNA or polypeptide sequences are "substantially homologous" when at least about 80% (preferably at least about 90%, and most preferably at least about 95%) of the nucleotides or amino acids match over a defined length of the molecule. As used herein, substantially homologous also refers to sequences showing identity to the specified DNA or polypeptide sequence. DNA sequences that are substantially homologous can be identified in a Southern hybridization experiment under, for example, stringent conditions, as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art. Ss£, e.g..

"Current Protocols in Mol. Biol." Vol. I &ϋ, Wiley Interscience, Ausbel £i a (ed.) (1992).

A "heterologous" region of a DNA construct is an identifiable segment of DNA within or attached to another DNA molecule that is not found in association with the other molecule in nature. Thus, when the heterologous region encodes a yeast gene, the gene will usually be flanked by DNA that does not flank the yeast gene in the genome of the source yeast Another example of a heterologous coding sequence is a construct where the coding sequence itself is not found in nature (e.g., synthetic sequences having codons different from the native gene). Allelic variation or naturally occurring mutational events do not give rise to a heterologous region of DNA, as used herein.

As mentioned above, the present invention will be illustrated with reference to the Herpes Simplex Virus (HSV) transactivation system, but the invention is not limited to such a system. The general area of HSV transactivation has been recently reviewed by (OΗare, P. eiaL, Antiviral Chgm, and Chemotherapy, 2(i):l-7 (1991); Roizman, B and A.E. Sears, Virology (2nd Ed.), Fields, B.N. and D.M. Knipe (Eds), pp 1785-1841 (1990)). , the entire contents of which are incorporated herein by reference for purposes of background only.

The HSV genome is a linear DNA molecule, -150 kb in size. It consists of two components, the long unique region (L) and the short unique region (S) separated by and bounded by inverted repeats. During replication and packaging these regions are able to invert relative to each other. As a consequence, viral genomes extracted from virions or infected cells consist of four equimolar isomers that differ solely in the origination of the L and S component. The HSV genome contains three origins of DNA replication, two in the S component (orig) and one in the middle of the L component (orijj. Viral DNA circularizes immediately after infection without requiring de novo protein synthesis and replication is thought to occur via a rolling circle mechanism although this has not been conclusively demonstrated. To initiate infection, herpes viruses attach to a variety of cell surface molecules. Heparan sulfate proteoglycans have been shown to play a role in virus attachment Evidence has been presented that suggests there is an HSV glycoprotein D receptor but the precise molecule has not been identified. The viral proteins gB and gC have been shown to be essential in the process of HSV attachment Attachment activates a process mediated by the viral proteins gB and gD which causes the fusion of the envelope and the plasma membrane. The de-enveloped

capsid is then transported to the nuclear pore where the DNA is released into the nucleus.

The HSV genome encodes at least 72 open reading frames. Approximately 67 genes products have been identified (Roizman, B. and A E. Sears, Ah"- Re - Microbiol. 41:543-571 (1987)). Genetic and biochemical studies have implicated a number of these proteins in viral DNA metabolism and transcription. Many of the functions served by viral gene products may also be provided by the host cells.

Although HSV genes are not clustered in accordance with a temporal order of expression, sets of genes are expressed in an ordered fashion. The first set of genes to be expressed, the immediate early (IE) or α-genes, are induced by the transactivating protein, VP16, which is brought into the cell in the tegument of the virus and is required for their initiation. The upstream activator sequences (UAS) of several α-genes contain the cis-acting elements required for VP16 mediated induction. The immediate early genes encode six proteins, i.e., ICPO, ICP4, ICP6, ICP22, ICP27, and ICP47. The ICP6 gene encodes the large subunit of the viral ribonucleotide reductase. While the functions of ICP22 and ICP47 are unknown, although ICP22 may play some role in regulation of late gene expression in some cell types. ICPs 0, 4, and 27 act as specific transactivators for the initiation of transcription of α, β, and γ genes. Among these, ICP4 plays a major role in regulating the expression of all three kinetic classes of HSV genes. The delayed early (DE) or β-genes are expressed at peak levels between 5 and 7 hours post infection and primarily encode enzymes involved in nucleic acid metabolism and replication. These include DNA polymerase, major DNA binding proteins, a trimeric helicase/primase, ribonucleotide reductase, etc. The final phase of protein synthesis involves expression of the γ-or late genes. The γ-proteins identified to date, are all structural components of the virion. Expression of γ-genes is heterogeneous with respect to timing and dependency on DNA replication. Whereas the expression of γj genes (e.g., gB, gD, VP16, and the major capsid protein, VP5) is only slightly reduced by inhibition of viral DNA replication, the expression of 72- genes (e.g., gC and gE) stringently requires DNA synthesis.

These and several other studies suggest that viral gene transcription and DNA synthesis are tightly coupled, either indirectly via regulation of essential enzymes or directly as with the γ2-genes; hence, interference with viral transcription should be an effective method for shutting off viral DNA replication. VP16, ICP4, ICPO, and ICP27 are the major transcriptional regulators in HSV. VP 16 is important as the earliest acting regulatory protein and promotes the expression of ICPs 0, 4, and 27. ICP4 is the best characterized transcriptional regulator of the α-

proteins, being involved in up-regulating expression of α, β, and γ-genes. Late in infection, it down-regulates its own expression. ICP27 is required for late gene expression, while little is known about ICPO. It appears that ICPO and ICP27 cooperate with ICP4 to regulate the expression of β- and γ-genes. As discussed above, replication of herpes simplex virus during lytic infection involves three phases of viral protein synthesis, α, β, and γ. The coordinate induction of the different classes of viral genes is regulated by specific viral proteins at the level of transcription initiation, i.e., through a process of transactivation. These processes are accomplished by the combinatorial interaction of viral proteins, host proteins and DNA. The proteins involved in this sequence of events may be grouped into three classes: Transactivators, which bind to DNA upstream from the structural gene; basal initiation factors, which associate with RNA polymerase; and adaptors, which may act as bridges between transactivators and basal factors. For any given gene, combinations of these proteins result in formation of assemblies whose composition can vary with the cell cycle, with developmental stage or in response to hormones or other signals. In HSV, the virally encoded transactivators give rise to unique ensembles containing specific viral proteins. In the absence of the viral proteins, these ensembles will not form and hence transcription of viral message will not occur, at least under biologically relevant conditions. In Figure 4 VP16 is shown specifically associated with the ubiquitous eukaryotic transcriptional activator, Oct-1; a factor called host cell factor (HCF) which has not been characterized; and two proteins of the basal transcription initiation complex. TFIID and TFIIB. Mutational structure/function studies on VP16 and Oct-1 are extensive, making this transactivation assembly one of the best characterized in the literature.

Because VP16 is an important transactivating protein of HSV but not humans, drugs which interfere with the function of VP 16 are strong candidates for antiviral therapy. This invention provides a useful high throughput screening . protocol for identifying such drug candidates. The invention is predicated on the observations of ≤Lal (Cs 61:l 199-1208 (1990)) and Gill and Ptashne (Nature 334:721-724 (1988)) that the GAL4-VP16 fusion product could be used as a transcriptional inhibitor. However, to incorporate these observations into a functioning screen, a variety of reagents needed to be developed.

This invention provides a screenable population of cells useful for the detection of a class of antiviral agent whose mechanism of action, inter alia. includes interference with viral transactivation. In its broadest embodiment the invention provides a recombinant cell capable of differentially expressing a

pathology inducing specific transactivating element. The term viral transactivating element as employed herein refers to a protein having a minimum of two domains functional in transcriptional transactivation, at least one of which is of viral origin. The first domain, as described hereinabove, is the so-called activation domain and the second domain, also described hereinabove, is the so-called DNA recognition domain.

Both domains can be found on a single,viral protein such ICP4 of Herpes, or E2 of papilloma virus. Alternatively, the transactivation domain and the DNA recognition can be found on peptides from two distinct sources. For example HSV VP16 represents a transactivating domain that cannot bind DNA. During natural infection, VP16 as described above, associates with a host DNA recognition domain Oct-1. This invention contemplates the use of both types of transactivating elements. Furthermore, this invention provides a chimeric transactivating element wherein the genetic information encoding an activation domain from one source is fused with the genetic information encoding a DNA binding domain from a heterologous source. The GAL4-VP16 construct is an example of such a chimeric element

Regardless of the particular form the transactivating element may take, it is important for effective screening that the element is differentially expressible in the recombinant cell, which is to say that the element should be regulatable, i.e., be capable of being turned on or off under defined conditions. In other words, for purposes of the screening system disclosed herein, constitutive expression of the element is not appropriate.

A variety of regulatable promoters are known which may be used to control the expression of the transactivating element. The promoters fall into two general classes, inducible or derepressible, depending on the nature of the interaction of the promoter and various associated regulatory components. Either class of promoter or a promoter from a mixed class may be used. Useful promoters include: GUP1, GAL1-10, CYC1 for yeast especially .J cerevisiae. or metallothionen for Drosophila especially 52.

The promoter chosen must of course, function in the host cell used in the screen. Useful host cells include: lower eukaryotes algae, fungi and the like, and higher eukaryotes such as Drosophila and mammalian cells in culture. Of course the skilled artisian would select the appropriate host/promoter combination. Drosophila and yeast are particularly preferred.

Regardless of the host cell selected, it is important that the cell display an identifiable phenotype corresponding to the expression of the transactivating

element a convenient phenotype is cell growth inhibition. Although not wishing to be bound by any particular theory or mechanistic explanation for the inhibition, it is believed that when expressed in the recombinant cell, the transactivating element competes with endogenous transactivators for the basal transcription apparatus thereby reducing the efficiency of host cell transcription and host cell growth. Such a competition is referred to in the vernacular as "transcriptional squelching."

As employed herein, cells are cultured under permissive conditions, i.e., those under which the viral transactivating element is not expressed, to provide a population of screenable cells. The culture conditions are then shifted, to cause expression of the transacting element Under these conditions, cell growth is inhibited. Under these latter conditions, the cells are contacted with a composition suspected of having molecules capable of interfering with the function of the viral transactivating element. If such a property is exhibited by the composition tested, then the transactivating element will not be able to enter into unproductive assemblages with the host basal transcription apparatus, thereby relieving the growth inhibition. Although it is convenient to contact the cells with the compositions to be tested after shifting to the non-permissive growth conditions, it is possible to have the compositions present during the early growth stages of the protocol.

Compositions which may be screened include a variety of organic molecules, separately or in combination, or complex mixtures of organic molecules such as from natural product extracts, soil extracts, and fermentation broths. Once identified as yielding a positive, growth-promoting response in the primary screen, the potential antiviral agents can then be tested in secondary screens to establish their efficacy on virally-infected cells directly. Of course, if complex mixtures are tested, such as natural product extracts, it may be desirable to subject the mixture to various chemical and/or physical separation techniques well-known in the art in order to isolate the active compound from the mixture. It also may be desirable to use the protocol herein as a bioassay to monitor the presence of the molecule of interest during any purification process. In addition to the screening protocol disclosed and claimed herein, the invention also includes regulatable viral transactivating elements and recombinant cells transformed therewith. Also included within the scope of this invention are antiviral agents identified by the use of the screen disclosed.

A particularly useful screening system employs a GAL4-VP16 chimeric transactivating element under the regulatable control of the yeast CYC1 promoter.

CONSTRUCTION OF GAL4-VP16 EXPRESSING YEAST STRAIN UNDER THE CONTROL OF THE CYC1 PROMOTER, UAS2UP1

GAL4-VP16 is a fusion protein (Sadowski, I. et al., Nature 335:563-64

(1988)) in which the acidic activation domain of the herpes simplex virus I protein, VP16 (Triezenberg, S.J. et al., Genes & Devel. 2:730-742 (1988); Greaves and

OΗare, J. Virol.63:1641-50 (1989)) is fused to the DNA-binding domain of GAL4

(Johnson and Dover, Proc. Nafl. Acad. Sci. USA 84:2401-2405 (1987)). In this case, GAL4-VP16 was excised from a plasmid, pJR3, on a Bam HI fragment and inserted into ρlG265UPl-ATG (made by cleaving pLG265UPl (Guarente, L. et al., £__\ 36:503-511 (1984) with Xho and Sacl (removing the CYC-1 ATG) and inserting the Xho-Sac-1 fragment from pLGSD5-ATG). The final construct contains the GAL4-VP16 gene driven by UAS2UP1 of CYC-1 on a 2-micron plasmid with the URA3 marker.

Yeast cells transformed with the vector can then be used to screen for antiviral compounds of interest according to the following protocol. It is preferred to employ yeast strains auxotrophic for a selectable marker carried by the plasmid (e.g., ura3), as well as being nystatin resistant to aid in permeability of the compositions.

PROTOCOL FOR SQUELCHING ASSAY

Yeast cells fS. cerevisiae RSI 88N> of the genotype (mata, ade2, His3, Lev 2, trpl, ura3, ca l) which were also nystatin resistant carrying the CYC1 driven GAL4-VP16 construct were grown up in syntheticura-liquid medium containing 2% glucose). The cells were stored at 4°C as stock for later use. Before plating the cells were transferred to a medium comprising 2% raffinose as the carbon source and culture for at least one round of cell division. In each assay, an 8 ml suspension of the above cells (at A^y=0Λ) was applied to a 20 x 20 cm^ plate containing synthetic-uracil solid medium containing 2% lactate (instead of 2% glucose or raffinose). Wells were made and 100 ul of testing samples, dissolved in 50% DMSO/50% MeOH, were added. After incubation at 30°C for 5 days, most cells fail to grow up except those surrounding wells containing the inhibitor of GAL4- VP16 production. Wells containing 0.5% glucose may be used as a positive control. A positive for the assay would be a ring of yeast colonies around a well containing an active compound thereby identifying the compound as a potential antiviral agent.

The recipe for synthetic-ura medium

0.67% yeast nitrogen base without amino acids (Difco) 2% glucose

2% Bacto agar (for solid medium) 20 ug ml of the appropriate amino acids needed by the strain (Leu; His; Trp)