The synthesis of the various classes of core particle histones was determined in human diploid fibroblast-like cells of different in vitro ages as they were stimulated to enter the cell cycle. Histone H2A synthesis in older populations was lower during G1 but was similar to younger cells at G0/G1 and S phases. The reduced synthesis during G1 was primarily the result of a decline in the synthesis of the H2A.1/.2 component.
Histone variant synthesis patterns from human diploid fibroblast-like cells of different in vitro ages were determined during exponential growth, at confluence, and during low serum arrest. The results are reported as the ratios of H2A variant synthesis (H2A.1 and H2A.2/H2A.x and H2A.z) and H3 variant synthesis (H3.1 and H3.2/H3.3) that have been used to characterize individual cell cycle states. Hydroxyurea was employed in some experiments to reduce S phase cells. The results indicate that high population doubling level (PDL) cells move through the G1 phase of the division cycle during exponential growth and exist in the G0 cell cycle state at confluence and during low serum arrest. Low PDL cells, however, exist in the G1 cell cycle state at confluence and revert to a G0 state only after maintenance as quiescent populations. This would suggest that when stimulated high PDL cells cannot enter into S phase, they revert to a GO cell cycle state.
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