The Tra1 region of broad host range IncPα plasmid RK2 encodes proteins essential for its promiscuous conjugative transfer and includes , the site at which nicking occurs to initiate transfer replication. Unregulated expression of the Tra1 region genes would be likely to place a major burden on the host. To investigate the control of these genes the three transcriptional promoters from this region were cloned by PCR and inserted into promoter probe vectors. The strength of and was estimated to be six to eightfold less than the strong promoter which is required for expression of genes for vegetative replication of RK2. The promoter was about one-tenth the strength of the other two. These promoters are not repressed by products of the central control operon of RK2. However, and , which are arranged as back to back divergent promoters in the region, are repressed by TraK which constitutes part of the relaxosome necessary for nicking at A second relaxosome protein, TraJ, represses is not repressed by any relaxosome proteins. All three promoters are repressed by TrbA, which is encoded at the start of the operon containing the rest of the transfer genes (the Tra2 region). These circuits provide: (i) an autoregulatory way of ensuring production of enough relaxosome proteins without overburdening the host; and (ii) a means of coordinating expression of both blocks of transfer genes.


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