The Relationship of Histone Phosphorylation to Deoxyribonucleic Acid Replication and Mitosis during the HeLa S-3 Cell Cycle

Abstract

Abstract There are two distinct kinds of histone phosphorylation in continuously dividing HeLa S-3 cells. Histones f22a1 and f22b (or f22a2, or both) molecules incorporate [32P]orthophosphate throughout the cell cycle at fairly uniform rates which are proportional to the amounts of these polypeptides. These phosphorylations are neither coupled with DNA replication in S phase nor obviously related to increased RNA synthesis in G1 of the cell cycle. The function of this kind of histone phosphorylation is unknown. The second kind of phosphorylation begins in S phase and ultimately converts over 90 % of all histone f1 molecules into one of two principal phosphorylated forms by the end of G2 phase of the cell cycle. After mitosis histone f1 molecules are extensively dephosphorylated during G1 until DNA replication begins, at which time the phosphorylation cycle commences once again. This simultaneous phosphorylation of both old and newly synthesized histone f1 molecules is partially coupled with DNA replication and increases more than 10-fold in rate as the cells proceed from G1 into S phase. Continuous labeling, pulse-chase, and inhibitor experiments indicate that the phosphorylation of histone f1 polypeptides is neither correlated with gene activation (increases in RNA synthesis) nor does it appear to be related to histone transport from the cytoplasm to the nucleus. The results of these experiments do suggest that the phosphorylation of histone f1 molecules may be related to the separation and condensation of sister chromatids prior to and during mitosis.