Proteins from the prokaryotic nucleoid

Abstract

The H-NMR spectra of the 2 E. coli basic, low-MW (.apprxeq. 9000) DNA-binding proteins NS1 and NS2 and of their native complex NS were studied at 400 MHz and a number of resonances and resonance peaks were assigned. As in the case of some eukaryotic histones, the presence of a large number of high-field perturbed Phe resonances, several shielded and deshielded methyl resonances and backbone NH protons quite inaccessible to the solvent clearly indicate the existence of extensive tertiary and, even more so, quaternary structures involving hydrophobic interactions. These structures are lost upon heating, but readily reform upon cooling. Spectral differences between NS1, NS2 and NS and the greater thermal stability of NS indicate that molecules of the heterologous subunits (NS1 and NS2) aggregate (dimerize) preferentially in comparison to the self-aggregation of the homologous subunits. Unlike those of the eukaryotic histones, the tertiary and quaternary structures of NS are insensitive to extensive variations of the ionic strength.