Transfer of the IncJ plasmid R391 to recombination deficient Escherichia coli K12: Evidence that R391 behaves as a conjugal transposon

A study of the IncJ plasmid R391 confirmed a low frequency of transfer between recombination proficient (recA⁺) Escherichia coli (10^-5 donor^-1). Reanalysis of its transfer to recombination deficient (recA) E. coli revealed an equivalent transfer frequency to and from all mutants tested. Extrachromosomal DNA could not be detected in either recA⁺ or recA transconjugants, while R391 proved refractory to curing in both backgrounds implying a high degree of stability. The integration of R391 into a specific region of the chromosome was demonstrated by its transfer as part of the ekogenote mobilised from the transfer origins of Hfr strains BW6165 and JC158. Transfer of R391 coupled to recA independent chromosomal integration has significant implications as to the nature and classification of the element. We propose that R391 behaves like a conjugal transposon.