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.Macromolecular metabolism is strikingly altered in eukaryotic cells during mitosis. RNA synthesis virtually ceases (1-4), and the rate of protein synthesis is significantly reduced (3, 5). Johnson and Holland (6) reported that mitotic chromatin has a reduced template activity for RNA synthesis. We recently confirmed this observation and have shown that this restriction of chromatin template activity may be produced by the chromosomal protein fraction specific for mitotic chromatin (7). The template activity of chromatin reconstituted by gradient dialysis from purified HeLa cell DNA and the chromosomal proteins isolated from Colcemidarrested metaphase cells was considerably less than that of chromatin reconstituted from S-phase chromosomal proteins. The degree of template restriction was proportional to the amount of mitotic chromosomal proteins in the reconstituted chromatin. This finding led us to suggest that the inhibition of template activity of mitotic chromatin might be accounted for by a constituent of the chromosomal protein fraction that binds to DNA during mitosis and thereby prevents transcription by RNA polymerase (7).Recent studies from several laboratories have indicated that the nonhistone chromosomal proteins may play a significant role in the control of gene expression in eukaryotic cells (8,9). These proteins possess tissue specificity (10-17) and are capable of interacting with DNA to selectively modify transcription (18,19 Fla. 32601 in the nonhistone proteins have been described in various phases of the HeLa cell cycle (20,22).In this report we show that nonhistone chromosomal proteins may be responsible for the inhibition of template activity of mitotic chromatin.
MATERIALS AND METHODSExponentially growing HeLa S3 cells were maintained in suspension culture in Joklik-modified Eagle's Minimal Essential Medium supplemented with 3.5% (each) of calf and fetal-calf serum. Synchronization of the cells was produced (20) by 2 cycles of 2 mM thymidine block...