Conjugative Transposons: Transmissible Resistance Islands

Abstract

The features of conjugative transposons are similar enough to those of pathogenicity islands (PAIs) to raise the question whether some PAIs are actually conjugative transposons. This chapter reviews the features associated with conjugative transposons and other transmissible integrated elements. Conjugative transposons do not fit neatly into any of the traditional categories of mobile elements such as transposons, bacteriophages, and plasmids; instead, they combine features of all of them. The mechanisms of integration and excision are less well established for Bacteroides conjugative transposons. The types of lambdoid integrase genes found so far on most conjugative transposons are not easy to spot in BLAST searches and must be checked at the amino acid level for the conserved amino acid signature. The transfer regions of conjugative transposons appear to be stripped-down versions of the transfer regions of conjugative plasmids. The authors found the same thing in the Bacteroides conjugative transposons: the transfer region is less than 16 kbp, and there are few homologs to known plasmid transfer genes. Excision and transfer of at least some of the conjugative transposons are stimulated by the antibiotic tetracycline. The Bacteroides conjugative transposons can also act in trans to trigger excision and circularization of unlinked and apparently unrelated integrated elements called nonreplicating Bacteroides units (NBUs).