Functional organization of the plasmid pT181 replication origin.

Replication of the staphylococcal plasmid pT181 is initiated at the origin (ori) with the introduction of a site-specific nick by the plasmid-encoded initiator protein RepC. Deletion analysis showed that a sequence of about 70 base-pairs is required for full ori function, including the ability to compete with a co-resident wild-type origin for the trans-acting RepC protein. A shorter sequence of 43 base-pairs is sufficient for origin function in the absence of competition. Single and double point mutations within these 43 base-pairs were used to determine the sequence requirement for replication within the minimal origin. Deletion mutants and point mutants were tested in replication and competition assays in vivo and in vitro, and in a RepC-mediated nicking assay.