Structure of active chromatin: covalent modifications of histones in active and inactive genes of control and hypothyroid rat liver.

Covalent modifications of histones in active and bulk chromatin fractions were studied in liver tissue from control and hypothyroid rats. The levels of acetylation and ubiquitination of histones were similar in the active and bulk chromatin fractions, and were not influenced by hypothyroidism. Histone H2A only was phosphorylated in control active and bulk chromatin fractions. The extent of this phosphorylation did not differ between the two fractions, but hypothyroidism greatly suppressed it. Indicating an association with tissue growth. ADP-ribosylation of histones was found to be mainly associated with transcriptional inactivation of the chromatin, while histone methylation was correlated with growth inhibition of the tissue, as observed with hypothyroidism. The validity of these conclusions will, however, depend upon the similarity of the turnover rates of these covalent modifications between active and inactive chromatin and between control and hypothyroid states.