Nonrandom utilization of acetylation sites in histones isolated from Tetrahymena. Evidence for functionally distinct H4 acetylation sites.

Macro- and micronuclei of the ciliated protozoan Tetrahymena thermophila afford a unique opportunity to study histone acetylation under conditions where postsynthetic "transcription"-related acetylation and synthetic "deposition"-related acetylation are nonoverlapping. Recent studies have demonstrated that at least two general systems of acetylation operate in Tetrahymena. One is postsynthetic, macronuclear specific, and may be related to gene expression in that nucleus (Vavra, K. J., Allis, C. D., and Gorovsky, M. A. (1982) J. Biol. Chem. 257, 2591-2598). The other is synthetic, common to macro- and micronuclei, and is likely related to histone deposition during replication (Allis, C. D., Chicoine, L. G., Richman, R., and Schulman, I. G. (1985a) Proc. Natl. Acad. Sci. U. S. A., 82, 8048-8052). A unique feature of H3 and H4 in Tetrahymena is that neither are blocked at their amino termini. We have exploited this fact as well as the resolving capability of acid-urea gel electrophoresis and current microsequencing techniques to examine whether utilization of different NH2-terminal acetylation sites is random or nonrandom during the progression toward high acetylation states. Of the four acetylation sites which have been identified in H4 (in Tetrahymena these are lysines at positions 4, 7, 11, and 15), we find that lysine 7 is the exclusive site of postsynthetic acetylation in populations of monoacetylated H4 isolated from macronuclei. This site is retained in populations of diacetylated H4, which are acetylated exclusively at lysines 4 and 7. Our data also suggest that there is some preference for using lysine 11 (as compared to 15) as the third site of acetylation in triacetylated molecules. The data demonstrate that the postsynthetic acetylation-deacetylation process is surprisingly nonrandom for H4 in Tetrahymena macronuclei. We have also investigated the same question with macronuclear H3 (which contains acetylation sites at lysines 9, 14, 18, and 23). Our data demonstrate that unlike H4, lysines at position 9 or 14 are likely to be utilized as sites of acetylation within a population of monoacetylated H3. Both of these acetylation sites are retained in diacetylated H3 which suggests that if site 9 is used initially as the site of monoacetylation, 14 is used secondarily (and vice versa). Our data show, moreover, that there is a preference for utilizing lysine 18 as the third acetylation site (in triacetylated H3). Thus, these data show that H3 is also acetylated in a nonrandom fashion in macronuclei. Finally, we have determined which acetylation sites are utilized in macro- or micronuclear H4 when it is undergoing active synthesis and deposition.(ABSTRACT TRUNCATED AT 400 WORDS)