STRUCTURE OF ACTIVE CHROMATIN - COVALENT MODIFICATIONS OF HISTONES INACTIVE AND INACTIVE GENES OF CONTROL AND HYPOTHYROID RAT-LIVER

Covalent modifications of histones in active and bulk chromatin it, fr actions were studied in liver tissue from control and hypo-thyroid rat s. The levels of acetylation and ubiquitination of histones were simil ar in the active and bulk chromatin fractions, and were not influenced by hypothyroidism. Histone H2A only was phosphorylated in control act ive and bulk chromatin fractions. The extent of this phosphorylation d id nor differ between the two fractions, but hypothyroidism greatly su ppressed it, indicating an association with tissue growth. ADP-ribosyl ation of histones was found to be mainly associated with transcription al inactivation of the chromatin, while histone methylation was correl ated with growth inhibition of the tissue, as observed with hypothyroi dism. The validity of these conclusions will, however, depend upon the similarity of the turnover rates of these covalent modifications betw een active and inactive chromatin and between control and hypothyroid states.