PURPOSE: To prepare replicon having high manifestation ability of gene, by linking a phosphate bond protein gene to replicon.

CONSTITUTION: (i) Plasmid (e.g., pBR 322) obtained by extraction from Escherichia coli (e.g., K-12 strain C 600), etc. followed by purification is treated with a restriction enzyme (e.g., EcoRI), so that it is scissored, to prepare cleavage DNA (replicon). While, (ii) a phosphate bond protein gene of Escherichia coli is scissored with the same restriction enzyme that is used in the above-mentioned treatment, and (iii) the prepared cleavage DNA is linked to the cleavage DNA obtained by the process (i) with T4 ligase.