Abstract

IS607-family transposons are unusual because they do not have terminal inverted repeats or generate target site duplications. They encode two protein-coding genes, but only tnpA is required for transposition. Our X-ray structures confirm that TnpA is a member of the serine recombinase (SR) family, but the chemically-inactive quaternary structure of the dimer, along with the N-terminal location of the DNA binding domain, are different from other SRs. TnpA dimers from IS1535 cooperatively associate with multiple subterminal repeats, which together with additional nonspecific binding, form a nucleoprotein filament on one transposon end that efficiently captures a second unbound end to generate the paired-end complex (PEC). Formation of the PEC does not require a change in the dimeric structure of the catalytic domain, but remodeling of the C-terminal α-helical region is involved. We posit that the PEC recruits a chemically-active conformer of TnpA to the transposon end to initiate DNA chemistry.

Highlights

  • Sometimes thought of as DNA parasites, transposable elements (TE) are widely recognized as playing prominent roles in the evolution of genomes (Biemont, 2010; Brunet and Doolittle, 2015; Volff, 2006)

  • Transposition onto l was measured after phage induction in a recA E. coli l lysogen, and the resulting l lysates were used for transduction selecting tetracycline resistance. l::IS607-tet transpositions were obtained for IS607orfA at a frequency of 1 Â 10À7/pfu (Figure 1C), but no confirmed transposition events were obtained with IS607orfAB

  • We note that the relative expressions of orfB and orfA in the IS607 constructs are likely to be different than in the native element; these results indicate that OrfA is sufficient for promoting transposition and that OrfB is inhibitory, as concluded earlier (Kersulyte et al, 2000)

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Summary

Introduction

Sometimes thought of as DNA parasites, transposable elements (TE) are widely recognized as playing prominent roles in the evolution of genomes (Biemont, 2010; Brunet and Doolittle, 2015; Volff, 2006). Biochemistry and Chemical Biology Chromosomes and Gene Expression recombination reactions, promote DNA transposition These enzymes splice DNA through a sequential pair of single-strand exchanges or through double strand breaks, generating a transient covalent linkage between the cleaved DNA end and a tyrosine or serine on the protein (Rubio-Cosials et al, 2018; Stark, 2014; Wood and Gardner, 2015). The SRs found in IS607-family transposable elements, are distinguished by the location of their DBDs at their N-termini (experimentally confirmed below) This domain architecture is paradoxical because studies on smSRs imply that an N-terminally located DBD would be incompatible with the formation of active tetramers, which is the critical regulatory step of these reactions (Johnson, 2015; Rice, 2015; Stark, 2014).

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