Transcription, processing and structure of chromatin in SV40-transformed cell

Accurate transcription of simian virus 40 chromatin in a HeLa cell extract

During simian virus 40 viral maturation, a series of modifications occur which alter the composition of viral nucleoprotein complexes. As a consequence, the chromatin that is extracted from extracellular simian virus 40 virions exhibits properties that are similar to those of transcriptionally active eucaryotic chromatin. The influence of this chromatin structure on specific RNA initiation by RNA polymerase II was examined by using the in vitro HeLa cell extract of Manley et al. (Proc. Natl. Acad. Sci. U.S.A. 77:3855-3859, 1980). The 5' ends of RNA transcripts were positioned by the "run-off" assay, in which transcripts extend from the initiation site to termination sites created by restriction cleavage and by S1 nuclease analysis, using DNA probes labeled at their 5' termini. Two major early RNA transcripts, which originated at map positions 5,240 +/- 10 and 5,145 +/- 10, and two major late RNA transcripts, which originated at map positions 325 +/- 10 and 185 +/- 10, were identified. Transcripts were initiated with comparable relative efficiencies at the same 5' site when either purified DNA or chromatin was used as the template. Our results suggest that extracellular simian virus 40 virion chromatin modifications do not regulate simian virus 40 promoter selection but function to increase the accessibility of RNA promoter sequences to RNA polymerase II and allow efficient elongation of the RNA chain after the initiation event.

Enhanced myocardial RNA synthesis in spontaneously hypertensive rats possible role of high-mobility group non-histone proteins

Cardiac hypertrophy in spontaneously hypertensive rats is associated with increased nuclear RNA polymerase activity. In order to explore mechanisms facilitating the interaction of the enzyme with its endogenous template, we compared the structure of nuclear chromatin from myocytes of 20-week-old spontaneously hypertensive rats and normotensive Wistar-Kyoto controls. Enhanced RNA synthesis in hypertensive rats was accompanied by increased susceptibility to digestion by deoxyribonuclease I. Nick translation of nuclei also resulted in higher nucleotide incorporation in hypertensive rats. Salt-extraction abolished the differences in deoxyribonuclease I sensitivity between the two animal groups. Reconstitution with either 0.35 M NaCl-extract or high mobility group (HMG) non-histone proteins restored digestion susceptibility but did not equalize SHR and WKY cells. SDS-polyacrylamide gel electrophoresis of 0.35 M NaCl-extracts and supernatants from deoxyribonuclease I digestion revealed the presence of HMG proteins which were preferentially released in hypertensive rats. There was a small but statistically significant increase in nuclear HMG protein content in hypertensive rats (0.12 +/- 0.02 mg/mg DNA vs. 0.09 +/- 0.02 mg/mg DNA in Wistar-Kyotos, P less than 0.05) but no difference in their electrophoretic appearance. These results indicate that chromatin structure is altered in the hypertrophied myocardium with resultant increase in deoxyribonuclease I susceptibility. This increase appears to be partly dependent on the high-mobility group non-histone proteins.

Subnuclear fractionation by mild micrococcal-nuclease treatment of nuclei of different transcriptional activities causes a partition of expressed and non-expressed genes

Extremely mild treatment with micrococcal nuclease of isolated nuclei yields subnuclear fractions in which the majority of RNA polymerase II transcriptional complexes formed in vivo are segregated [Tata & Baker (1978) J. Mol. Biol. 118, 249-272]. We now describe different approaches followed to established whether or not the nuclei are thus resolved into transcribed and non-transcribed DNA. First, we have compared the sensitivity to deoxyribonuclease I, which is known to digest preferably expressed genes as present in nuclei or chromatin, of three micrococcal-nuclease-derived fractions from nuclei of different transcriptional activities. In transcriptionally active nuclei (rat liver, hen liver and oviduct, and Xenopus liver), the DNA in a polynucleosomal fraction comprising 6-15% of DNA and the majority of template-engaged RNA polymerase II (fraction P2) was 10-50 times as sensitive to deoxyribonuclease I as the DNA in the other two fractions (fractions P1 and S, comprising 78-88% of total nuclear DNA as large polynucleosomal aggregates and 2-6% of DNA mostly as mononucleosomes, respectively). In transcriptionally inactive nuclei obtained from hen erythrocytes, micrococcal nuclease did not separate DNA into fractions exhibiting such differential sensitivities. Second, we have monitored the partition of an expressed gene. Hybridization of complementary DNA to Xenopus albumin mRNA revealed a 5-10-fold enrichment of the albumin (but not the globin) gene in the P2 fraction of nuclei from Xenopus liver in which this gene is fully expressed. Third, a large part of the nascent rapidly labelled RNA synthesized in vivo in rat liver nuclei was recovered in the micrococcal-nuclease-derived fraction that is more susceptible to digestion with deoxyribonuclease I. It is concluded that mild micrococcal-nuclease treatment of nuclei causes their separation into transcribed and non-transcribed DNA as determined by a number of very different criteria.