A transcription terminator signal necessary for plasmid Collb-P9 replication

Replication of the Inclα plasmid CoIIb-P9 requires the repZ gene, which encodes an essential, unstable initiator protein termed RepZ. Although many functional features of the CoIIb-P9 replicon resemble those of structurally unrelated IncFII plasmids R1 and NR1, the role of transcription of repZ towards the replication origin is poorly understood. Using a series of deletion and substitution mutants of the CoIIb-P9 replicon, we found that RepZ prefers to act in cis and that a spacer sequence between repZ and the origin is required for replication. This spacer element, referred to as CIS, retained strong transcription terminator activity. Efficient transcription terminators, whether Rho-dependent or -independent, were capable of replacing CIS function for in vivo replication; CoIIb-P9 replicated better as transcription terminated more efficiently within CIS. When the CIS element was substituted for by a strong Rho-dependent terminator, such as λ tR1 or E. coli trp tt', in vivo replication of these recombinant replicons became dependent on the Rho factor, in contrast to the authentic CoIIb-P9 replicon.