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)
Summary
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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