Proteins of chromatin in genetic restriction: IV. Comparison of histone and nonhistone proteins of rat liver nucleolar and extranucleolar chromatin

Abstract

Nonhistone proteins from the condensed (nucleolar) and diffuse (extranucleolar) fractions of rat liver and kidney chromatin preparations were compared by electrophoresis in polyacrylamide gels. Although considerable differences were seen between the rat liver and kidney nonhistone chomatin proteins, the nucleolar and extranucleolar nonhistone chromatin proteins from the same source were found to be qualitatively similar. The tissue-specific electrophoretic distribution of nonhistone proteins was not affected by extraction of the liver and kidney chromatin preparations with 0.3 M NaCl. This was interpreted to indicate that the electrophoretic tissue specificity of nonhistone proteins was not caused by cytoplasmic contamination. The electrophoretic patterns of histones were practically identical in nucleolar and extranucleolar chromatin samples. The close similarity of these two types of chromatin was further substantiated by analysis of their derivative thermal denaturation profiles. The modified thermal stability of nucleolar chromatin was found to be caused by its content of RNA which partially destabilized the DNA-histone interactions. Nucleolar chromatin is substantially heterochromatic, as compared to the euchromatic extranucleolar chromatin, and the striking structural differences between these two chromatin types do not appear to be reflected in their protein contents at the present level of resolution.