INVOLVEMENT OF KIL AND KOR GENES IN THE PHENOTYPE OF A HOST-RANGE MUTANT OF RP4

Plasmid pRP761 is a derivative of the promiscuous plasmid RP4, which has a transposon Tn76 insert 1.8 kilobases from its EcoRI site within the trfB region. This mutation was pleiotropic, having 3 effects: the plasmid is unstably maintained in Escherichia coli, it reduces the growth rate of its host and it has suffered a reduction in host-range. pRP761 has reduced expression from its korA and korB genes and Tn76 has inserted between them. Fragment exchange experiments showed that this is the only mutant region in pRP761 and is solely responsible for the pleiotropic effects. A spontaneous deletion derivative pRP761-6 has lost Tn76 and its adjacent kilA and korA genes; it has reacquired stability, does not inhibit host growth but is still reduced in its host-range. The provision of cloned korA+ in trans complements the first 2 phenotypic effects in pRP761 to a large extent, but neither korA+ alone nor korA+ with korB+ complements the host-range reduction in pRP761 or pRP761-6. A possible explanation for these results is that there is a site between korA and korB, affected by the Tn76 insert, that is essential to stable replication of these plasmids in some of their bacterial hosts.