Studies on the regulation of colicin Ib synthesis: replication of the Col Ib-P9 plasmid during colicin induction.

The Col Ib-P9 plasmid content of colicinogenic cells has been measured by both sedimentation analysis of cleared lysates and by deoxyribonucleic acid hybridization to purified Col Ib-P9 factor deoxyribonucleic acid. The results show an independence between mitomycin C-induced colicin synthesis and Col Ib-P9 replication. In addition, the ratio of the various molecular configurations (i.e., covalently closed circles or open circles) of the Col Ib-P9 plasmid does not change upon induction of colicin synthesis. Heat treatment of a colicinogenic strain carrying a thermosensitive dnaB mutation on the bacterial chromosome leads to complete inhibition of Col Ib-P9 plasmid replication and the simultaneous induction of colicin Ib synthesis in all cells of the colicinogenic population. These studies rule out mechanisms or regulation of colicin Ib synthesis based on the induced replication of the Col Ib-P9 plasmid. Our studies show that in a Col Ib-P9 colicinogenic population there is, on the average, one plasmid per cell.