HI factor is a heat-stable protein found in large amounts in Escherichia coli. In vitro, this protein has been found to stimulate transcription of A templates by E. coli DNA-dependent RNA polymerase.The subunit molecular weight of this factor has been re-estimated and found to be 15500 IL-1000 in the presence of 2-mercaptoethanol. Crosslinking experiments performed with dimethylsuberimidate in the presence or absence of the DNA template indicate the presence of multiples of the 15 500-M, subunit up to the tetramer, the dimeric species being predominant. One cysteinyl residue per 15 000-M, subunit is labeled by 4-chloro-7-nitrobenzofurazan. This residue is not labeled if the factor is exposed to oxidizing conditions. In this case, three lysyl residues are titrated.HI factor behaves as a DNA-binding protein. We have detected binding to DNA by two independent methods : displacement of a fluorescently labeled factor by the native protein and retention of radioactive DNA on millipore filters in the presence of the factor. Under our experimental conditions (high ionic strength, abscnce of magnesium ions), the saturation function of A plac *DNA as well as of wild-type A DNA has been found to be non-cooperative. Saturation is reached when 300 k 30 molecules of dimeric factor are bound per R molecule, the average dissociation constant of the complex being 10 nM.The dissociation time of the HI -DNA complex is less than 5 s at 37 "C. The binding of this factor lowers the affinity of native DNA for ethidium bromide. In the presence of this intercalating dye, the solubility of the complex decreases drastically.In 1971, Jacquet et nl. described a heat-stable protein fraction from Esclzerichia coli which stimulated RNA synthesis directed by native , I or 480 DNA [l]. One year later, Cukier-Kahn el al. showed that this fraction could be resolved into several proteins. One of the stimulating factors, HI, was shown to be a low-molecular-weight protein having an isoelectric point close to 7. As the amount of HI required for maximal stimulation was proportional to the amount of DNA present in the assay in vitro, it was suggested that this factor stimulates DNA transcription by binding to the template and inducing a conformational change of the promotor [2]. It was later reported that promotors sensitive to catabolic repression were Abbreviutions. N bf-C1,4-chloro-7-nitroben70furazan (or 7-chloro-4-nitroben7o-2-oxa-1,3-diazole); fluorescamine, 4-phenylspiro-[furan-2(3),1 '-phthalan]-3,3-dione.Enzymes. RNA polymerase or nucleosidetriphosphatc : RNA nucleotidyltransferase (EC 2.7.7.6); beef pancreas deoxyribonuclease I (EC 3.1.4.5).better stimulated than others [3]. We have therefore undertaken a physico-chemical study of the interaction between this factor and two A DNA samples in order to investigate the structural basis of these observations.
MATERIALS AND METHODS
Prepration of D N A SamplesPhage DNAs, i plac imm 434 cS' S7 or A imm 434 cItS S7, were prepared as described elsewhere [4].Thermal induction of the lysogenic prophage wa...