METABOLISM OF HISTONE FRACTIONS .2. CONSERVATION AND TURNOVER OF HISTONE FRACTIONS IN MAMMALIAN CELLS

During exponential growth of Chinese hamster cells in suspension culture, histone f1 was observed to turn over with a half-life of 74 hr, while histones f2a1, f2a2, f2b, and f3 did not turn over but were conserved for three cell generations. However, when synchronized cultures were blocked at the G1 S boundary by inhibiting DNA synthesis with thymidine, histones f2a1, f2a2, f2b, and f3 were observed to turn over with halflives of 60, 58, 55, and 68 hr, respectively. This turnover indicated that prelabeled histones were being replaced by newly synthesized, nonlabeled histones under these conditions. Therefore, histone synthesis was not completely inhibited when net DNA synthesis was inhibited by thymidine. The turnover of histone f1 during thymidine block was more complex than the turnover of the other four histones. Early in thymidine block, when stable RNA synthesis was still occurring, f1 turned over with a half-life of 31 hr, but later in thymidine block, when stable RNA synthesis had stopped, the half-life was reduced to 47 hr (a rate somewhat similar to the turnover of the other histones during thymidine block). It appears that the turnover of f1 during exponential growth continued after thymidine block was applied and was added to the f1 turnover observed in late thymidine block, resulting in the rapid f1 turnover observed early in block. This rapid f1 turnover continued during block only as long as stable RNA synthesis continued, suggesting that the f1 turnover during exponential growth and part of the f1 turnover during early block may be functionally involved in some aspect of stable RNA synthesis requiring removal of f1 from the chromatin. © 1969.