Functional analysis of replication and stability regions of broad-host-range conjugative plasmid CTX-M3 from the IncL/M incompatibility group.

Plasmid CTX-M3 (89 kb) isolated from Citrobacter freundii from a Warsaw hospital is a mosaic plasmid with replication functions 100% identical with those of pMU407.1 of the IncL/M group, conjugative operons with up to 60% homology to ColIb-P9 (IncI) and stability functions originating either from NR1(R100) (IncFII) or ColIb-P9 /R1/NR1 plasmids. We established the broad-host-range for pCTX-M3 and defined its minireplicon in Escherichia coli. We analyzed the role of stability cassettes and showed that the par operon consists of three orfs parA (stbA), parB (stbB) and nuc with a centromere-like region located upstream of the operon. Deletion of the par operon strongly destabilized pCTX-M3 despite the presence of the pemIK toxin-antidote system identical to that on NR1(R100) plasmids. Deletion of the pemIK operon had no effect on plasmid stability.