Interactions of the histone octamer with single-stranded DNA. Sedimentation analysis and low-angle X-ray diffraction

A complex between 140-160 nucleotide single-stranded DNA and the octamer of histones was formed and analyzed by electron microscopy and X-ray low angle diffraction. The morphology of the complex is very similar to that of the nucleosome; the diffraction pattern appears less defined than for chromatin showing broader maxima in the same positions. These results strongly suggest that this particle has a geometry very similar to that of the fundamental subunit of chromatin. The possibility of artifacts due to renaturation reaction promoted by histones is ruled out by the analysis of the complex with S1 nuclease and by the formation of a 'nucleosome like' particle using poly(dT) instead of DNA. Association of the histone octamer with either the 140-160 nucleotide single-stranded DNA or the 140-160 bp double-stranded DNA was evaluated at different histone/DNA input ratios. In both cases, the formation of the complex appears to be regulated by comparable association constants, and in both cases the trend of the complexation reaction in function of the temperature is almost the same. These results suggest that an alternative binding of the histone octamer to double-stranded or to single-stranded DNA requires low energy charge and may be involved in the processes of replication and transcription of the 'active chromatin'.