Evolution and spread of bacterial transposons

Abstract

Transposons are mobile genetic elements (MGE) that can carry additional genetic cargo nonessential for their own transposition. This cargo can include antibiotic or heavy metal resistance genes or those increasing metabolic plasticity. In addition to transposing across or between the chromosome and other replicons in a single cell, they can transfer between bacteria as passengers on conjugative plasmids that are capable of intercellular transfer. Transposons can be grouped on mechanisms of movement and by lengths of the bounding inverted repeats or of target site duplication created by transposition. Class II transposons, such as Tn3 and Tn501 utilise two-step replicative transposition involving a transposase, tnpA, and a resolvase, tnpR, gene. The Tn402/Tn5053 family has four genes, tniABQR, required for transposition, while Tn7 and relatives contain five (tnsABCDE). Despite their role in antimicrobial resistance dissemination and a detailed mechanistic view of transposition there have been no studies aimed at revealing the evolutionary history of transposon families by interrogating global genome sequence datasets. This is largely because the ability to search all existent bacterial sequences is non-trivial and only recently realised. We took a selection of 37 representative variants of the above transposon families and queried their nature and distribution across sequence space using bigsi (http://www.bigsi.io/), a searchable index of the bacterial ENA (455 632 datasets, Dec 2016) and Shovill (https://github.com/tseemann/shovill). Constrained by the sequence data that exists and the biases this may engender, this analysis provides broad insights into the prevalence and spread of important MGE.