Proteins from the prokaryotic nucleoid Interaction of nucleic acids with the 15 kDa Escherichia coli histone‐like protein H‐NS

The interaction between nucleic acids and Escherichia coli 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 × SSC binds DNA with an apparent K _a−1.1 × 10⁴ M⁻¹· bp⁻¹. 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.