Characterisation of a Chromatin Fraction Bearing Pulse-Labelled RNA. 1. Nascent RNA, RNA Polymerase B and Transcribed DNA Sequence Content

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Abstract
A fraction of polynucleosomal chromatin from mouse [mastocytoma P815] cells which are enriched in transcribing regions was further characterized after separation by multistep partition in a dextran/poly(ethylene glycol) 2-phase mixture. This fraction contains an average of 16% of the total DNA and 63% of the 3-min pulse-labeled RNA in chromatin which, by a number of criteria, represents nascent RNA transcripts. Of the total transcribing RNA polymerase B molecules in chromatin, 52% are found in this fraction by titration with radioactive .alpha.-amanitin. About 28% of the single-copy DNA sequences in this fraction hybridize to nuclear polyadenylated RNA, compared with only 1.5% of the single-copy sequences in the remaining (nontranscribing) chromatin fraction.