Tn7 transposes into replicating DNA using an interaction with the processivity factor, facilitating genome evolution

Abstract

Transposons are mobile genetic elements that can move between locations in DNA that lack homology. Transposons play an important role in the evolution of genomes in every domain of life through activities such as horizontal gene transfer, gene disruption, gene expression modulation, and recombination. The bacterial transposon Tn7 maintains two distinct lifestyles, one in horizontally transferred DNA and the other in bacterial chromosomes. Access to these two DNA pools is mediated by two separate target selection pathways. The proteins involved in these pathways have evolved to specifically recognize their cognate target-sites using entirely different mechanisms but the same core transposition machinery. In this work, I analyze over 50 Tn7-like transposons and discuss how the molecular mechanisms of these genetic elements contribute to the success of both transposon and host. I focus particularly on the TnsE-pathway of transposition that is credited with optimizing transposition into DNA that is transported between bacterial hosts, and likely explains the presence of this transposon in phylogenetically diverse bacteria occupying a broad range of ecological niches. I show that the TnsE protein physically and functionally interacts with the processivity factor of the DNA replication machinery. I propose that this interaction allows Tn7 to identify insertion sites and to orient in one direction with active DNA replication by a process that is ubiquitous in bacteria. The TnsE interaction with an essential and conserved component of the replication machinery reveals a new mechanism by which Tn7, and possibly other elements, select target-sites associated with DNA replication. These results provide insight into various processes in eukaryotic and prokaryotic organisms involving processivity factors. I also analyze specific genetic pathways that affect the frequency of TnsE-mediated transposition, and correlate these genetic effects with protein-DNA complexes that may be recognized by TnsE and could be expected to be commonly found in these genetic backgrounds. The data presented here reveals ways in which Tn7 has directed the evolution of host genomes, and points to ways in which Tn7 might be used as a tool for understanding genetic phenomena.