title: Unique type of plasmid maintenance function: postsegregational killing of plasmid-free cells.
abstract: The stability locus parB+ of plasmid R1 has been found to specify a unique type of plasmid maintenance function. Two genes, hok (host killing) and sok (suppressor of killing), are required for the stabilizing activity. The hok gene encodes a highly toxic gene product, whose overexpression causes a rapid killing and a concomitant dramatic change in morphology of the host cell. The other gene, sok, was found to encode a product that counteracts the hok gene-mediated killing. The parB+ region was inserted in a plasmid with a temperature-sensitive replication system. At nonpermissive temperature, the parB+ plasmid was maintained in the population for a significantly longer period than the corresponding parB- plasmid. Coupled to this extended maintenance, a large fraction of the population was shown to be nonviable plasmid-free cells with the characteristic hok-induced change in morphology. Based on these findings, we propose that the parB+ locus mediates plasmid stability by killing cells that have lost the parB+ plasmid during the preceding cell division, thereby ensuring that a growing bacterial culture predominantly consists of plasmid-containing cells.
mesh_terms: D002455:Cell Division; D002874:Chromosome Mapping; D004261:DNA Replication; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000254:growth & development; D005786:Gene Expression Regulation; D005798:Genes, Bacterial; D010957:Plasmids; D013489:Suppression, Genetic; D014158:Transcription, Genetic
chemical_list: D004269:DNA, Bacterial