Transcription of Mammalian Chromatin by Mammalian DNA‐Dependent RNA Polymerases

Abstract

The transcription of rat‐liver chromatin has been studied using partially purified from AI and highly purified form B DNA‐dependent RNA polymerases isolated from rat liver. Chromatin contains endogenous RNA polymerase activity. This activity is evident only when the RNA polymerase assays are carried out at high ionic strength and its appearance as the ionic‐strenght increases is thought to be due to the removal of chromatin‐associated proteins which block further transcription by the bound enzyme. This activity is insensitive to the rifampicin derivative AF/0‐13 which is shown to inhibit initiation of RNA synthesis by mammalian RNA polymerases. From AI polymerase is virtually inactive in the transcription of chromatin whereas the from B RNA polymerase (α‐amanitin sensitive) actively transcibes chromatin. This activity has two salt concentration optima: (a) 0.01 M (NH 4 ) 2 SO 4 or 0.03 M KCI; (b) 0.16 M (NH 4 ) 2 SO 4 or 0.25 MKCI. Both these activities are inhibited by rifampicin AF/0‐13. A comparison of the activity of the rat‐liver from B polymerase and the Micrococcus lysodeikticus RNA polymerase demonstrates that chromatin is a more efficent template for the rat‐liver enzyme. Evidence is presented that the rat‐liver form B and the Micrococcus lysodeikticus RNA polymerases bind to and transcribe from different sites on the chromatin DNA.