The interaction between nucleic acids and Escherichia coil H-NS, an abundant 15 kDa histone-like protein, has been studied by affinity chromatography, nitrocellulose filtration and fluorescence spectroscopy. Intrinsic fluorescence studies showed that the single Trp residue of H-NS (position 108) has a restricted mobility and is located within an hydrophobic region inaccessible to both anionic and cationic quenchers. Binding of H-NS to nucleic acids, however, results in a change of the microenvironment of the Trp residue and fluorescence quenching; from the titration curves obtained with addition of increasing amounts of poly(dA)-poly(dT) and poly(dC)-poly(dG) it can be estimated that an H-NS dimer in 1.5 x SSC binds DNA with an apparent Ka" 1.1 x 104 M -~ -bp-L H-NS binds to double-stranded DNA with a higher affinity than the more abundant histone-like protein NS(HU) and, unlike NS, prefers double-stranded to single-stranded DNA and DNA to RNA; both monovalent and divalent cations are required for optimal binding.
The promoter region of Escherichia coli hns, the structural gene for the DNA-binding protein H-N& has been identified by use of a promoter search vector and the in vivo transcriptional start point by primer extension analysis. The homologous hns genes of two other Enterobacteriaceae, Proteus vulgaris and Serratia marcescens, were identified by heterologous hybridization with a DNA probe derived from E. coli hns, cloned and sequenced. Taking into account only the invariant nucleotides and amino acids, the homology of H-NS among the three organisms was found to be >70% at the DNA level and > 75% at the protein level. The three hns genes were also found to have nearly identical transcriptional and translational signals.
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