Summary: The central control region (Ctl) of IncP plasmids is associated with two phenotypes: the coordinate expression of replication and transfer genes; and the ability to increase the segregational stability of a low-copy-number test plasmid. This region of the IncP plasmid R751 shows significant sequence divergence from the IncPα plasmid RK2 sequence, and two genes, and present in the IncPα region are missing in the IncP Ctl. In other respects the organization of the Ctl is basically the same. Although the two key global regulatory genes and are highly conserved, studies on their ability to repress transcription from a variety of IncPα and IncP plasmid promoters suggest differences in operator recognition by KorA and synergy with other repressors. The products of and genes are conserved; KfrA shows least conservation and, while retaining the ability to act as a transcriptional repressor, appears to have completely different DNA-binding specificity. The genes required for the plasmid segregational stabilization (partitioning) phenotype - and the KorB operator O3 - are conserved and contribute to a more efficient plasmid stabilization than the IncPα equivalents. This may indicate that the Ctl plays an especially important role in partitioning of IncP plasmids, since they lack the second stability region () found in IncP plasmids.


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