The expression of genes coding for the four core histones (H2A, H2B, H3, and H4) was studied in tsAF8 cells. These baby hamster kidney-derived cells are a temperature-sensitive (ts) mutant of the cell cycle that arrest in G1 at the restrictive temperature. When serum-deprived tsAF8 cells are stimulated with serum, they enter the S phase at the permissive temperature of 34 °C, but are blocked in G1 at the nonpermissive temperature of 39.6 °C. Northern blot analysis using cloned human histone DNA probes detected only very low levels of histone RNA either in quiescent tsAF8 cells or in cells serum stimulated at the nonpermissive temperature for 24 h. Cellular levels of histone RNA were markedly increased in cells serum stimulated at 34 °C for 24 h. Temperature shift-up experiments after serum stimulation of quiescent populations showed that the amount of histone RNA was related to the number of cells that entered the S phase. Those cells that synthesized histone RNA and entered the S phase were capable of dividing. This is the first demonstration in a mammalian G1-specific ts mutant that the expression of H2A, H2B, H3, and H4 histone genes depends on the entry of cells into the S phase of the cell cycle.
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