Localization of the HU Protein on the Escherichia coli Nucleoid

We recently described the isolation and characterization of a low-molecular-weight DNA-binding protein, HU, from Escherichia coli (Rouvière-Yaniv and Gros 1975). In some of its properties, as well as in its amino acid composition, this protein resembles the eukaryotic histone H2B. Like histones, this protein is highly conserved in prokaryotes. Closely related proteins showing immunological cross-reactivity with the anti-HU serum were isolated from several strains of blue-green algae (Haselkorn and Rouvière-Yaniv 1976; Rouvière-Yaniv et al. 1977) as well as from Bacillus subtilis (J. Rouvière-Yaniv and F. Le Hegarat, unpubl.). Analysis of the N-terminal portions of both E. coli HU (B. Laine et al., unpubl.) and algal HU (A. Aitken et al., unpubl.) showed a high degree of sequence conservation between these two proteins. Among the first 25 amino acids, ten residues are identical, six are conservative changes, and three can be related by a single base change in the DNA sequence...