EVOLUTION OF THE KORA-ORIV SEGMENT OF PROMISCUOUS INCP PLASMIDS

Plasmids belonging to Escherichia coil incompatibility group P are of particular interest because they can transfer between, and be stably m aintained in, almost all Cram-negative bacterial species. The segment of the IncP alpha plasmid genome between the key regulatory gene korA and the vegetative replication origin, oriV, encodes a series of opero ns co-regulated with replication and transfer functions by the KorA pr otein. To determine which of these genes are likely to have an importa nt role in IncP plasmid survival the equivalent region of the distantl y related IncP beta plasmid R751 was sequenced. Sequence comparisons s how that the kla operon (formerly the kilA locus, which is also respon sible for a cryptic tellurite-resistance determinant) is completely ab sent from R751. Similarly in the kle region, which encodes genes assoc iated with the KilE(+) phenotype of unknown function, kleC and kleD, w hich we proposed arose by a duplication of kleA and kleB, are also com pletely absent, The genes that are conserved are klcA (formerly kilC, responsible for the KilC(+), and recently proposed to be involved in o vercoming restriction barriers during transfer), klcB (an ORF interrup ted by Tn1 insertion in RK2), korC (a transcriptional repressor which controls the klcK and kle operons), and kleA, kleB, kleE and kleF. A s triking feature of the organization in R751 is the lack of the strong transcriptional termination signals which are present in IncP alpha pl asmids. The degree of divergence between the plasmids facilitates the identification of motifs of probable functional importance in the prim ary protein sequences.