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Evolution of colicin BM plasmids: the loss of the colicin B activity gene

Australian National University

ABSTRACT

In E. coli, colicins B and M are usually encoded adjacently on the same plasmid. However, in a collection of E. coli isolated from Australian vertebrates, a significant fraction of colicin BM strains were found to lack an intact colicin B activity gene. The colicin B and M gene region was sequenced in 62 strains with varying colicin BM genotypes and it was found (with one exception) that all plasmids encoding colicin M and the B immunity gene have an identical colicin gene structure, possessing a complete colicin B immunity gene and a 130 bp remnant of the B activity gene. A phylogenetic analysis and investigation of virulence factors encoded on the plasmids suggested that they have evolved twice from two separate but similar events. Colicin B immunity was found to be non-functional in strains that have lost colicin B activity, but colicin M still appears to be produced despite the absence of the SOS box believed to regulate its production in colicin BM strains. The presence of a remnant of the microcin V operon next to the truncated colicin B activity gene indicated that these plasmids evolved as a consequence of gene transfer between colicin BM and microcin V plasmids. We suggest that these transfer events most likely involve the transfer of some microcin V genes and associated virulence factors onto ColBM plasmids

¹ E-mail: david.gordon{at}anu.edu.au