Tn10 Transposase Mutants with Altered Transpososome Unfolding Properties are Defective in Hairpin Formation

H-NS binds with high affinity to the Tn10 transpososome and promotes transpososome stabilization

H-NS is a bacterial DNA-binding protein that regulates gene expression and DNA transposition. In the case of Tn10, H-NS binds directly to the transposition machinery (i.e. the transpososome) to influence the outcome of the reaction. In the current work we evaluated the binding affinity of H-NS for two forms of the Tn10 transpososome, including the initial folded form and a pre-unfolded form. These two forms differ in that IHF is bound to the former but not the latter. IHF binding induces a bend (or fold) in the transposon end that facilitates transpososome formation. However, the continued presence of IHF in the transpososome inhibits intermolecular transposition events. We show that H-NS binds particularly strongly to the pre-unfolded transpososome with an apparent K(d) of approximately 0.3 nM. This represents the highest affinity interaction between H-NS and a binding partner documented to date. We also show that binding of H-NS to the transpososome stabilizes this structure and propose that both high-affinity binding and stabilization result from the combined interaction between H-NS and DNA and H-NS and transposase within the transpososome. Mechanistic implications for tight binding of H-NS to the transpososome and transpososome stabilization are considered.

The nucleoid binding protein H-NS acts as an anti-channeling factor to favor intermolecular Tn10 transposition and dissemination

Dissemination of the bacterial transposon Tn10 is limited by target site channeling, a process wherein the transposon ends are forced to interact with and insert into a target site located within the transposon. Integration host factor (IHF) promotes this self-destructive event by binding to the transpososome and forming a DNA loop close to one or both transposon ends; this loop imposes geometric and topological constraints that are responsible for channeling. We demonstrate that a second 'host' protein, histone-like nucleoid structuring protein (H-NS), acts as an anti-channeling factor to limit self-destructive intramolecular transposition events in vitro. Evidence that H-NS competes with IHF for binding to the Tn10 transpososome to block channeling and that this event is relatively insensitive to the level of DNA supercoiling present in the Tn10-containing substrate plasmid are presented. This latter observation is atypical for H-NS, as H-NS binding to other DNA sequences, such as promoters, is generally affected by subtle changes in DNA structure.

Functional coupling between the two active sites during Tn 10 transposition buffers the mutation of sequences critical for DNA hairpin processing

DNA processing reactions often involve multiple components acting in concert to achieve the desired outcome. However, it is usually difficult to know how the components communicate and cooperate to orchestrate an ordered series of events. We address this question in the context of the Tn 10 transposition reaction, in which the DNA cleavage and joining events occur within a higher-order complex containing a transposase dimer, two transposon ends and the DNA-bending host-factor IHF (Integration Host Factor). Previously it was shown that the complex is asymmetric. The a side consists of an IHF protomer initially immobilized by a DNA-loop, but subsequently used to promote conformational changes required for the cleavage steps. The beta side of the complex was considered to fulfil a more passive role. Here we show that the a side of the complex promotes coupled conformational changes at both transposon ends, while the a and beta sides communicate and cooperate to dominate different phases of the transposition reaction. Together, these effects provide for a robust response to critical changes in the transposon end. These findings also explain the intriguing genetic phenotypes of a series of previously reported Tn10 mutants and have consequences for the evolution of new elements.

Transpososome dynamics and regulation in Tn10 transposition

Tn10 is a bacterial transposon that transposes through a non-replicative mechanism. This mode of DNA transposition is widely used in bacteria and is also used by "DNA-based" transposons in eukaryotes. Tn10 has served as a paradigm for this mode of transposition and continues to provide novel insights into how steps in transposition reactions occur and how these steps are regulated. A common feature of transposition reactions is that they require the formation of a higher order protein-DNA complex called a transpososome. A major objective in the last few years has been to better understand the dynamics of transpososome assembly and progression through the course of transposition reactions. This problem is particularly interesting in the Tn10 system because two important host proteins, IHF and H-NS, have been implicated in regulating transpososome assembly and/or function. Interestingly, H-NS is an integral part of stress response pathways in bacteria, and its function is known to be sensitive to changes in environmental conditions. Consequently, H-NS may provide a means of allowing Tn10 to responed to changing environmental conditions. The current review focuses on the roles of both IHF and H-NS on Tn10 transposition.

Maize response to acute arsenic toxicity as revealed by proteome analysis of plant shoots

Aerial parts (shoots) of maize seedlings fed hydroponically with 300 muM sodium arsenate [As(V)] or 250 muM sodium arsenite [As(III)] for 24 h were analyzed for differentially expressed proteins by 2-DE and digital image analysis. About 15% of total detected proteins (74 out of 500) were up- or, mainly, down-regulated by arsenic, among which 14 were selected as being those most affected by the metalloid. These proteins were analyzed by MALDI-TOF MS and 7 of them were identified: translation initiation factor eIF-5A, ATP synthase, cysteine synthase, malate dehydrogenase, protein kinase C inhibitor, Tn10 transposase-like protein, and guanine nucleotide binding protein. Each of these proteins was completely repressed by As(V) and/or As(III), except protein kinase C inhibitor, which was newly detected after exposure to As(V).

The global regulator H-NS acts directly on the transpososome to promote Tn10 transposition

The histone-like nucleoid structuring (H-NS) protein is a global transcriptional regulator that is known to regulate stress response pathways and virulence genes in bacteria. It has also been implicated in the regulation of bacterial transposition systems, including Tn10. We demonstrate here that H-NS promotes Tn10 transposition by binding directly to the transposition complex (or transpososome). We present evidence that, upon binding, H-NS induces the unfolding of the Tn10 transpososome and helps to maintain the transpososome in an unfolded state. This ensures that intermolecular (as opposed to self-destructive intramolecular) transposition events are favored. We present evidence that H-NS binding to the flanking donor DNA of the transpososome is the initiating event in the unfolding process. We propose that by recruiting H-NS as a modulator of transposition, Tn10 has evolved a means of sensing changes in host physiology, as the amount of H-NS in the cell, as well its activity, are responsive to changes in environmental conditions. Sensing of environmental changes through H-NS would allow transposition to occur when it is most opportune for both the transposon and the host.

The transpososome: control of transposition at the level of catalysis

Studies of several transposable genetic elements have pinpointed the importance of the transpososome, a nucleoprotein complex involving the transposon ends and a transposon-encoded enzyme--the transposase--as a key in regulating transposition. Transpososomes provide a precise architecture within which the chemical reactions involved in transposon displacement occur. Data are accumulating that suggest they are dynamic and undergo staged conformational changes to accommodate different steps in the transposition pathway. This has been underpinned by recent results obtained particularly with Tn5, Tn10 and bacteriophage Mu.

Cyclic changes in the affinity of protein-DNA interactions drive the progression and regulate the outcome of the Tn10 transposition reaction

The Tn10 transpososome is a DNA processing machine in which two transposon ends, a transposase dimer and the host protein integration host factor (IHF), are united in an asymmetrical complex. The transitions that occur during one transposition cycle are not limited to chemical cleavage events at the transposon ends, but also involve a reorganization of the protein and DNA components. Here, we demonstrate multiple pathways for Tn10 transposition. We show that one series of events is favored over all others and involves cyclic changes in the affinity of IHF for its binding site. During transpososome assembly, IHF is bound with high affinity. However, the affinity for IHF drops dramatically after cleavage of the first transposon end, leading to IHF ejection and unfolding of the complex. The ejection of IHF promotes cleavage of the second end, which is followed by restoration of the high affinity state which in turn regulates target interactions.