A low-molecular-weight (7000), heat-stable protein-HU-that stimulates transcription of bacteriophage X DNA by E. coli RNA polymerase was purified from E. coli extracts using affinity chromatography on DNA-cellulose. HU binds to native DNA, resulting in an apparent thickening of the DNA chains as revealed by electron microscopy. Contrary to DNA unwinding proteins, it causes no destabilization of the double helix. HU differs from previously described transcription factors (HI, D, etc.) and from the lowmolecular-weight w subunit of the RNA polymerase. By its amino-acid composition and characteristics, HU displays an interesting. resemblance to some eukaryotic histones, such as H2B and Hi. A variety of low-molecular-weight proteins from Escherichia coli that stimulate RNA synthesis in vitro have been characterized (1-8). The heat-stable protein, HI (1, 2), was shown to enhance X-lac DNA transcription by E. colt RNA polymerase (3) while causing reduction of ribosomal RNA synthesis in an E. coli DNA-dependent system (4). A heatstable protein, the D factor, was reported to increase the specificity of X-DNA transcription by the E. colt polymerase (5). Another class of small, heat-stable proteins has also been described which stimulates in vitro the replication of RNA bacteriophage (6, 7). That these small protein factors could act by locally affecting the stability of nucleic acid secondary structure, hence favoring or inhibiting the action of polymerases, has already been suggested (1, 2, 5, 8), and it has been proposed that some of these entities, such as the H1 and the D factors, could represent the prokaryotic counterpart of eukaryotic nuclear proteins (3,(5)(6)(7)(8).In. the frame of this hypothesis, we have undertaken a more systematic analysis of DNA binding proteins by means of affinity chromatography on DNA-cellulose columns. We report, here, the purification from E. coli extracts of a small, heat-stable protein-HU-that stimulates transcription of bacteriophage X-DNA and displays by its amino-acid composition and physicochemical behavior some properties characteristic of eukaryotic histones. Fig. 1
MATERIALS AND METHODS
Bacterial