Plasmids belonging to incompatibility group P are of particular interest because they can transfer between, and be stably maintained in, almost all Gram-negative bacterial species. The segment of the IncPα plasmid genome between the key regulatory gene and the vegetative replication origin, , encodes a series of operons co-regulated with replication and transfer functions by the KorA protein. To determine which of these genes are likely to have an important role in IncP plasmid survival the equivalent region of the distantly related IncPβ plasmid R751 was sequenced. Sequence comparisons show that the operon (formerly the locus, which is also responsible for a cryptic tellurite-resistance determinant) is completely absent from R751. Similarly in the region, which encodes genes associated with the KilE phenotype of unknown function, and , which we proposed arose by a duplication of and , are also completely absent. The genes that are conserved are (formerly , responsible for the KilC, and recently proposed to be involved in overcoming restriction barriers during transfer), (an ORF interrupted by Tn 1 insertion in RK2), (a transcriptional repressor which controls the and operons), and and . A striking feature of the organization in R751 is the lack of the strong transcriptional termination signals which are present in IncPα plasmids. The degree of divergence between the plasmids facilitates the identification of motifs of probable functional importance in the primary protein sequences.


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