Effect of growth stage on histone H1 metabolism in human diploid fibroblasts

Metabolism of histone H1 at different stages of cell growth was investigated in order to get a better understanding of the role of histone H1 in the cell growth of human diploid fibroblasts. Histone H1 content exhibited some fluctuation during the culture stage of cell growth. When cells entered confluent phase, the ratio of histone H1 to total histones decreased significantly. Histone H1 had a turnover half-life of 80 hours whereas nucleosomal histones did not significantly turn-over regardless of the growth stage. DNA synthesis was drastically diminished with increased cell density whereas histone synthesis was less sensitive to contact-inhibition. The gradual decline of histone H1 content with increased cell density suggests that its degradation is slightly superior to its residual synthesis. When the confluent cells were seeded at low density, cell proliferation resumed and histone H1 was synthesized and deposited to chromatin in a greater amount than nucleosomal histones, thus resulting in an abrupt increase of histone H1 content. The possible role of histone H1 metabolism in normal cells is discussed.