ACCURATE TRANSCRIPTION OF SIMIAN VIRUS-40 CHROMATIN IN A HELA-CELL EXTRACT

During SV40 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 SV40 virions exhibits properties that are similar to those of transcriptionally active eukaryotic chromatin. The influence of this chromatin structure on specific RNA initiation by RNA polymerase II was examined by using the in vitro [human cervical carcinoma] HeLa cell extract of Manley et al. The 5'' ends of RNA transcripts were positioned by the runoff 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 5240 .+-. 10 and 5145 .+-. 10, and 2 major late RNA transcripts, which orginated 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. Evidenlty, extracellular SV40 virion chromatin modifications do not regulate SV40 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.