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The uptake and turnover of acetate in HeLa cell histone fractions.

Histone acetylation was investigated in monolayer HeLa cell cultures in order to compare the acetylation activity among the histone proteins and its relation to the rate of ribonucleic acid synthesis. Techniques were developed for the quantitative determination of the specific activities of the histone protein fractions resolved by acrylamide gel electrophoresis.

The rate of RNA synthesis was found to vary in the HeLa cell cultures according to the time elapsed from the initial medium change. Refeeding the cultures at 12 hr when the rate of RNA synthesis was maximal did not alter this rate. If cultures were maintained for 24 hr, at which time the rate of RNA synthesis was only 55% of the maximum, then a medium change induced a 40% increase in the rate of RNA synthesis. Since a medium renewal elevated the rate of histone acetylation at both times, it was not possible from these studies to relate directly changes in histone acetylation with altered rates of RNA synthesis. There were no obvious alterations in the pattern of histone acetylation with any of the imposed growth conditions. It was concluded therefore that although histone acetylation could be induced, no specificity was demonstrated.

The arginine-rich F3a and F2a1 histones achieved the highest specific activity during a 90-min pulse of acetate-1-14C. After a 90-min pulse followed by a 90-min chase, the major portion of the acetate-1-14C incorporated into the F3a histone was deacetylated, whereas 25 to 30% of the activity remained in F2a1 with only small amounts in the other histone fractions. The turnover of the acetyl groups did not appear to be the result of histone protein degradation, since amino acid pulse-labeled histones were stable under these conditions. The data presented are consistent with the assumption that the acetate species turning over most rapidly were those containing only internal e-amino nitrogens of lysine and/or O -acetyl linkages (as F3a, b) whereas histones containing only the amino-terminal acetyl groups (F1) were metabolically the most stable. Histone fractions which contain both amino-terminal and internal acetylated amino acids (as do F2a1 and F2a2) exhibit intermediate metabolic stability.