Characterisation of a chromatin fraction bearing pulse-labelled RNA. 2. Quantification of histones and high-mobility-group proteins.

The histone variants and high-mobility-group (HMG) proteins of a transcribing fraction of chromatin, described in the preceding paper of this journal, have been analysed qualitatively and quantitatively by a combination of one-dimensional and two-dimensional gel electrophoresis. The stoichiometry of the four core histones (all variants included) in this fraction is equimolar and is not detectably different from that in the nontranscribing fraction or in total chromatin. The molar ratio of histone H1 to the core histones is markedly lower, by approximately 72%, than that in the nontranscribing fraction. A minor histone variant identified as M1 (an H2A variant) is detected only in the transcribing fraction, while variant H3.1 is found only in the non-transcribing fraction. Proteins A24, HMG1 and HMG2 are essentially absent from the transcribing fraction; HMG14 is found uniquely in this fraction, while HMG17 occurs at a relatively lower level.