Role of nonhistone chromosomal proteins in the restriction of mitotic chromatin template activity.

RNA synthesis virtually ceases during mitosis in eukaryotic cells. This phenomenon is explainable, at least in part, by the reduced template activity for RNA synthesis of mitotic chromatin. The restriction in template activity can be accounted for by the presence in the chromatin of mitotic-specific nonhistone proteins. Chromatin reconstituted by gradient dialysis from the pooled histones of S-phase and mitotic chromatin and the nonhistone proteins of mitotic chromatin had a template activity similar to that of native mitotic chromatin, and lower than native S-phase chromatin or chromatin reconstituted with the pooled histones and the S-phase and nonhistone proteins. The template activity was identical for chromatin reconstituted from the pooled nonhistone proteins and either the histones from S-phase or mitotic chromatin. These results are supported with data showing that the procedure of reconstitution produces a chromatin indistinguishable in its protein composition both qualitatively and quantitatively from its native counterpart.