Control of IS911 target selection: how OrfA may ensure IS dispersion.
ABSTRACT IS911 transposition involves a closed circular insertion sequence intermediate (IS-circle) and two IS-encoded proteins: the transposase OrfAB and OrfA which regulates IS911 insertion. OrfAB alone promotes insertion preferentially next to DNA sequences resembling IS911 ends while the addition of OrfA strongly stimulates insertion principally into DNA targets devoid of the IS911 end sequences. OrfAB shares its N-terminal region with OrfA. This includes a helix-turn-helix (HTH) motif and the first three of four heptads of a leucine zipper (LZ). OrfAB binds specifically to IS911 ends via its HTH whereas OrfA does not. We show here: that OrfA binds DNA non-specifically and that this requires the HTH; that OrfA LZ is required for its multimerization; and that both motifs are essential for OrfA activity. We propose that these OrfA properties are required to assemble a nucleoprotein complex committed to random IS911 insertion. This control of IS911 insertion activity by OrfA in this way would assure its dispersion.
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ABSTRACT: Tight regulation of transposition activity is essential to limit damage transposons may cause by generating potentially lethal DNA rearrangements. Assembly of a bona fide protein-DNA complex, the transpososome, within which transposition is catalysed, is a crucial checkpoint in this regulation. In the case of IS911, a member of the large IS3 bacterial insertion sequence family, the transpososome (synaptic complex A; SCA) is composed of the right and left inverted repeated DNA sequences (IRR and IRL) bridged by the transposase, OrfAB (the IS911-encoded enzyme that catalyses transposition). To characterise further this important protein-DNA complex in vitro, we used different tagged and/or truncated transposase forms and analysed their interaction with IS911 ends using gel electrophoresis. Our results allow us to propose a model in which SCA is assembled with a dimeric form of the transposase. Furthermore, we present atomic force microscopy results showing that the terminal inverted repeat sequences are probably assembled in a parallel configuration within the SCA. These results represent the first step in the structural description of the IS911 transpososome, and are discussed in comparison with the very few other transpososome examples described in the literature.Mobile DNA. 01/2010; 1(1):16.
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ABSTRACT: Insertion sequences (ISs), arguably the smallest and most numerous autonomous transposable elements (TEs), are important players in shaping their host genomes. This review focuses on prokaryotic ISs. We discuss IS distribution and impact on genome evolution. We also examine their effects on gene expression, especially their role in activating neighboring genes, a phenomenon of particular importance in the recent upsurge of bacterial antibiotic resistance. We explain how ISs are identified and classified into families by a combination of characteristics including their transposases (Tpases), their overall genetic organization and the accessory genes which some ISs carry. We then describe the organization of autonomous and non-autonomous IS-related elements. This is used to illustrate the growing recognition that the boundaries between different types of mobile element are becoming increasingly difficult to define as more are being identified. We review the known Tpase types, their different catalytic activities used in cleaving and rejoining DNA strands during transposition, their organization into functional domains and the role of this in regulation. Finally we consider examples of prokaryotic IS domestication. In a more speculative section, we discuss the necessity of constructing more quantitative dynamic models to fully appreciate the continuing impact of TEs on prokaryotic populations. This article is protected by copyright. All rights reserved.FEMS microbiology reviews 02/2014; · 10.96 Impact Factor
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ABSTRACT: Transposition in IS3, IS30, IS21 and IS256 insertion sequence (IS) families utilizes an unconventional two-step pathway. A figure-of-eight intermediate in Step I, from asymmetric single-strand cleavage and joining reactions, is converted into a double-stranded minicircle whose junction (the abutted left and right ends) is the substrate for symmetrical transesterification attacks on target DNA in Step II, suggesting intrinsically different synaptic complexes (SC) for each step. Transposases of these ISs bind poorly to cognate DNA and comparative biophysical analyses of SC I and SC II have proven elusive. We have prepared a native, soluble, active, GFP-tagged fusion derivative of the IS2 transposase that creates fully formed complexes with single-end and minicircle junction (MCJ) substrates and used these successfully in hydroxyl radical footprinting experiments. In IS2, Step I reactions are physically and chemically asymmetric; the left imperfect, inverted repeat (IRL), the exclusive recipient end, lacks donor function. In SC I, different protection patterns of the cleavage domains (CDs) of the right imperfect inverted repeat (IRR; extensive in cis) and IRL (selective in trans) at the single active cognate IRR catalytic center (CC) are related to their donor and recipient functions. In SC II, extensive binding of the IRL CD in trans and of the abutted IRR CD in cis at this CC represents the first phase of the complex. An MCJ substrate precleaved at the 3' end of IRR revealed a temporary transition state with the IRL CD disengaged from the protein. We propose that in SC II, sequential 3' cleavages at the bound abutted CDs trigger a conformational change, allowing the IRL CD to complex to its cognate CC, producing the second phase. Corroborating data from enhanced residues and curvature propensity plots suggest that CD to CD interactions in SC I and SC II require IRL to assume a bent structure, to facilitate binding in trans. Different transpososomes are assembled in each step of the IS2 transposition pathway. Recipient versus donor end functions of the IRL CD in SC I and SC II and the conformational change in SC II that produces the phase needed for symmetrical IRL and IRR donor attacks on target DNA highlight the differences.Mobile DNA. 01/2012; 3(1):1.