Characterisation of a chromatin fraction bearing pulse-labelled RNA. 1. Nascent RNA, RNA polymerase B and transcribed DNA sequence content

Distribution of carbon tetrachloride-metabolite(s) to DNase I-sensitive and -resistant chromatin

The distribution of carbon tetrachloride (CCl4) metabolite(s) to DNase I-sensitive and -resistant regions of mouse hepatic chromatin was determined following incubation of whole chromatin with [14C]CCl4 in the presence of hepatic microsomes and an NADPH-regenerating system. At 2 h of incubation, [14C]CCl4 metabolite(s) bound essentially to the same extent to DNase I-sensitive and -resistant chromatin regions. Binding of [14C]CCl4 metabolite(s) to DNase I-resistant regions, however, increased significantly over the DNase I-sensitive regions when incubation was prolonged to 4 h. This result suggests that CCl4 does not bind preferentially to DNase I-sensitive regions of chromatin.

On the structure of active chromatin. A flow linear dichroism study of chromatin fractionated by nuclease digestion

Nuclei from Ehrlich ascites cells were treated with micrococcal nuclease or DNase I and extracted with 1 mM EDTA. The chromatin fraction released by this procedure showed positive flow linear dichroism (LD) at low salt (2 mM NaCl) while the non-released fraction had negative LD. Furthermore, the chromatin structure responsible for the positive LD was found to be labile: The LD was reduced by heat (37 degrees C) or RNase treatment and inverted to a negative LD by electric fields (10 kV/cm) and by the presence of DNA binding dyes.

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.