Restriction of template capacity of rat liver chromatin by a non-histone peptide from calf thymus

Abstract

WE have reported¹ the isolation of a thymic peptide which strongly inhibits DNA-directed RNA polymerase in vitro. We have shown that transcription is controlled at the level of initiation. The active factor was isolated from aqueous ultrafiltered thymus extracts, purified by chromatography, first on DEAE–cellulose and then on Dowex 50 WX2, and characterised as a peptide of low molecular weight (<5,000). Its biological activity cannot be attributed to the presence of a nuclease or of a histone fragment. The inhibition of RNA synthesis is due to an ionic interaction between the peptide and DNA. The physicochemical and thermodynamic properties of this interaction are described elsewhere². We report here the effect(s) of the thymic peptide on transcription of rat liver chromatin. With some obvious limitations³, the in vitro chromatin transcription system is closer to the in vivo state than the purified DNA. Our experiments were prompted by our previous results, which demonstrated that the thymic extract modifies the physicochemical state of DNA and the interactions between DNA and nuclear proteins in the liver of old rats^4,5.