Involvement of H-NS in transpositional recombination mediated by IS1

IS1 the smallest active transposable element in bacteria, encodes a transpo sase that promotes inter- and intramolecular transposition. Host-encoded fa ctors, e.g., histone-like proteins HU and integration host factor (IHF), ar e involved in the transposition reactions of some bacterial transposable el ements. Host factors involved in the IS1 transposition reaction, however, a re not known. We show that a plasmid with an IS1 derivative that efficientl y produces transposase did not generate miniplasmids, the products of intra molecular transposition, in mutants deficient in a nucleoid-associated DNA- binding protein, H-NS, but did generate them in mutants deficient in histon e-like proteins HU, IHF, Fis, and StpA, Nor did IS1 transpose intermolecula rly to the target plasmid in the H-NS-deficient mutant. The hns mutation di d not affect transcription from the indigenous promoter of IS1 for the expr ession of the transposase gene. These findings show that transpositional re combination mediated by IS1 requires II-NS but does not require the HU, IHF Fis, or StpA protein in vivo. Gel retardation assays of restriction fragme nts of IS1-carrying plasmid DNA showed that no sites were bound preferentia lly by H-NS within the IS1 sequence. The central domain of R-NS, which is i nvolved in dimerization and/or oligomerization of the H-NS protein, was imp ortant for the intramolecular transposition of ISI, but the N- and e-termin al domains, which are involved in the repression of certain genes and DNA b inding, respectively, were not. The SOS response induced by the IS1 transpo sase 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 formati on of an active IS1 DNA-transposase complex in which the IS1 ends are cleav ed to initiate transpositional recombination through interaction with IS1 t ransposase.