The gene of I1 plasmid Collb-P9 was previously shown to alleviate DNA restriction by type I enzymes and to promote conjugative transmission of the unmodified plasmid to a restricting host. To clarify the ecological role of , its distribution was determined on plasmids from 23 incompatibility groups using hybridization to the coding sequence as an assay. Hybridizing sequences, shown by nucleotide sequencing to have at least 60% identity with , were detected on plasmids belonging to the I complex (IncB, I1 and K), the F complex (IncFV) and the IncN group. The homologues were found to specify an antirestriction phenotype which was enhanced by genetic derepression of the plasmid transfer system. loci map in plasmid leading regions but show no consistent association with a particular type of origin-of-transfer or a leading region gene of the (single-stranded DNA-binding protein), (plasmid SOS inhibition) and (host killing) families. It may be significant that plasmids are authentic enterobacterial plasmids and that type I restriction systems are associated historically with members of the Enterobacteriaceae.


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