Genetics of the replication and maintenance functions of the hemolytic plasmid pSU316. Cloning of an IncFIII determinant.

Two miniplasmids have been constructed from pSU306, a Tn802 insertion derivative of the IncFIII-IncFIV hemolytic plasmid pSU316. One of these, pSU3027, is a low copy number plasmid expressing both IncFIII and IncFIV incompatibilities, but is rather unstable, and probably lacks a putative par gene. The other, pSU3025, is maintained in about 340 copies per genome equivalent and expresses only IncFIII incompatibility. Most of the PstI-generated fragments from pSU3027 have been cloned in pBR322. One of the resulting plasmids, pSU3135, contains an insertion of 0.5 kb in the vector molecule, and expresses IncFIII, but not the IncFIV incompatibility. These results allowed us to identify and locate several genes involved in the control of pSU316 replication and stable plasmid maintenance.