Structural Organization and Control of Expression of the sop Operon of Linear Plasmid Prophage N15

Stable inheritance of bacterial chromosomes and low-copy-number plasmids depends on the active partition of replicated molecules between daughter cells. The partition mechanism is well known for circular plasmids F and P1. The mechanism of partition of linear replicons was studied with the example of bacterio-phage N15, which persists as a linear plasmid with covalently closed ends in the lysogenic state, rather than integrating into the Escherichia coli chromosome. Since stable inheritance of N15 is due to the sop operon homologous to sop of the F plasmid, the control of expression of the N15 sop genes was analyzed. The sop promoter (Psop) contains a binding site for bacterial IHF and five CTTTGC copies, which overlap the –35 and –10 elements. The Sop proteins were shown to interact with a Psop-containing DNA fragment in vitro. Transcription of the sop operon is regulated by the Sop proteins: SopA represses Psop, and SopB enhances the repression, having no effect on the promoter activity in the absence of SopA. In N15 lysogenic cells, Psop proved to be repressed. This regulatory mechanism was assumed to ensure production of SopA and SopB in amounts required for the segregation stability of N15 and to neutralize occasional fluctuations of their concentration in the cell.