Plasmid copy-number control and better-than-random segregation genes of pSM19035 share a common regulator

Transcription initiation of the copy-number control and better-than-random segregation genes of the broad-host-range and low-copy-number plasmid pSM19035 are subjected to repression by the autoregulated pSM19035-encoded ω product in Bacillus subtilis cells. The promoters of the copS (Pcop1 and Pcop2), δ (Pδ), and ω (Pω) genes have been mapped. These promoters are embedded in a set of either seven copies of a 7-bp direct repeat or in a block consisting of two 7-bp direct repeats and one 7-bp inverted repeat; the blocks are present either two or three times. The cooperative binding of ω protein to the repeats on the Pcop1, Pcop2, Pδ, and Pω promoters represses transcription initiation by a mechanism that does not exclude σARNAP from the promoters. These results indicate that ω protein regulates plasmid maintenance by controlling the copy number on the one hand and by regulating the amount of proteins required for better-than-random segregation on the other hand.