Evidence for the formation of nucleosome-like histone complexes on single-stranded DNA

Using histones reconstituted with RNA and DNA celluloses, we have shown elsewhere that histones elute identically with salt from single- and double-stranded DNA, but differently from RNA (Palter and Alberts, 1979). In this paper we characterize further the suspected specific binding interactions between histones and single-stranded DNA. Nuclease digestion of complexes of histone reconstituted with single-stranded DNA generates only a small yield of discrete (approximately 9S) particles. We can, however, efficiently obtain such 9S "nucleosome-like" complexes when nuclease treatment is avoided and histones are reconstituted directly with short single-stranded DNA pieces. Strikingly, these 9S subunits contain an equimolar composition of the four nucleosomal histones. When these subunits are visualized in the electron microscope, they appear as globular particles which are morphologically indistinguishable from normal mononucleosomes. Based on their sedimentation properties, histone-to-DNA ratio, histone composition and particle diameter, we conclude that they represent an octamer of the four histones (containing two molecules of each histone) associated with single-stranded DNA. These data, viewed in the context of other information concerning chromatin, suggest that nucleosome cores may become transiently bound to single strands of DNA as DNA and RNA polymerases pass.