Involvement of H-NS in Transpositional Recombination Mediated by IS 1

Open Access
Publisher Website
Google Scholar
Abstract
IS 1 , the smallest active transposable element in bacteria, encodes a transposase that promotes inter- and intramolecular transposition. Host-encoded factors, e.g., histone-like proteins HU and integration host factor (IHF), are involved in the transposition reactions of some bacterial transposable elements. Host factors involved in the IS 1 transposition reaction, however, are not known. We show that a plasmid with an IS 1 derivative that efficiently produces transposase did not generate miniplasmids, the products of intramolecular transposition, in mutants deficient in a nucleoid-associated DNA-binding protein, H-NS, but did generate them in mutants deficient in histone-like proteins HU, IHF, Fis, and StpA. Nor did IS 1 transpose intermolecularly to the target plasmid in the H-NS-deficient mutant. The hns mutation did not affect transcription from the indigenous promoter of IS 1 for the expression of the transposase gene. These findings show that transpositional recombination mediated by IS 1 requires H-NS but does not require the HU, IHF, Fis, or StpA protein in vivo. Gel retardation assays of restriction fragments of IS 1 -carrying plasmid DNA showed that no sites were bound preferentially by H-NS within the IS 1 sequence. The central domain of H-NS, which is involved in dimerization and/or oligomerization of the H-NS protein, was important for the intramolecular transposition of IS 1 , but the N- and C-terminal domains, which are involved in the repression of certain genes and DNA binding, respectively, were not. The SOS response induced by the IS 1 transposase was absent in the H-NS-deficient mutant strain but was present in the wild-type strain. We discuss the possibility that H-NS promotes the formation of an active IS 1 DNA-transposase complex in which the IS 1 ends are cleaved to initiate transpositional recombination through interaction with IS 1 transposase.