Control of pT181 replication I. The pT181 copy control function acts by inhibiting the synthesis of a replication protein.

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Abstract
PT181 is a fully sequenced 4.4‐kb 20 copy Tcr plasmid from Staphylococcus aureus. Its replication system involves a unique unidirectional origin embedded in the coding sequence for a plasmid‐determined protein, RepC, that is required for initiation. When joined to a 55 copy carrier plasmid, pE194, pT181 excludes autonomous isologous replicons by inhibiting their replication. Two types of spontaneous pT181 copy mutants have been isolated, one that eliminates sensitivity to this inhibition and another that does not. A spontaneous 180‐bp deletion, delta 144, eliminates both the inhibitory activity and sensitivity to it. This deletion increases copy number by 50‐fold and RepC production by at least 10‐fold. It is located directly upstream from the repC coding sequence and the deletion‐bearing plasmid supports the replication of inhibitor‐sensitive plasmids in cells containing active inhibitor. This effect is probably due to the overproduction of RepC by the delta 144 plasmid. On the basis of these results, it is suggested that RepC synthesis is negatively controlled by an inhibitor that is encoded directly upstream from the repC coding sequence and acts as a tareget set in the same region. It is likely, therefore, that pT181 replication rate is determined by the level of RepC.