Abstract
Chromatin was isolated from synchronized HeLa cells at different stages of the cell division cycle and fractionated into DNA, histones, and nonhistone proteins. Electrophoresis of the nonhistone proteins in sodium dodecyl sulfate-polyacrylamide gels revealed a highly reproducible pattern of 22 bands, having estimated molecular weights of 15,000-180,000, with 85% (by mass) over 40,000. The amounts of some nonhistone proteins varied during the cell cycle by as much as 50%, while others remained at a constant level. One group of nonhistone proteins (molecular weight 75,000) was greatly reduced just before the start of DNA replication (S-phase), then returned to normal levels in the mid-S phase. These results are discussed with regard to the possible role of nonhistone proteins in regulating chromosome structure and function.