Further studies on the inviability of Escherichia coli K12 rho mutant strains carrying IncFII plasmids.

Previous results (Baumberg and Lovett 1977) showed that many conjugative plasmids, most notably those of the IncFII group, were only transferable to Escherichia coli K12 rho mutants at very low frequencies, and that this appeared to be due to a lethal interaction between plasmid and rho- allele. Experiments reported here were designed to examine this phenomenon further, and in particular to test the possibility that uncontrolled plasmid replication in the presence of the rho mutant allele occurs. The rifampicin resistance allele rpo203 antagonizes the effect of rho201 on conjugal plasmid transfer; since the former is known to counteract the latter's effect on transcription termination, this result indicates that the plasmid-rho interaction stems directly from the lack of transcription termination caused by a rho mutant allele. Direct estimation of Rldrd19 replication after conjugal transfer to rho+ and rho201 recipients showed no difference according to rho status. Also, cotransduction of rho101 and 201 with ilv::Tn5 into recipients carrying miniplasmid derivatives of R1, which retain a restriction fragment containing all the DNA known to be involved in plasmid replication, was normal and the rho- transductants retained the miniplasmids. It therefore appears that the lethal interaction between the Inc FII plasmids and rho mutant alleles does not involve plasmid replication functions.