Regulation of histone acetylation during macronuclear differentiation in Tetrahymena: evidence for control at the level of acetylation and deacetylation.

During the postzygotic period of the sexual cycle (conjugation) in the ciliated protozoan, Tetrahymena, daughter products from a single micronuclear mitotic division develop into new macronuclei (anlagen) or new micronuclei depending upon their cytoplasmic location. In this study we have monitored the status of histone acetylation in synchronous populations of developing nuclei isolated from conjugating cells. Particular attention has been paid to the level of histone acetylation in new macronuclei following their differentiation from micronuclei. Like micronuclei isolated from vegetative cells (Vavra et al., 1982), micronuclei from conjugating cells (5 hr, 10-12 hr, and 15-16 hr) contain little if any acetylated histone and incorporate little postsynthetic acetate under any of our experimental conditions. In contrast, young new macronuclei (4C, 10-12 hr) incorporate significant amounts of acetate in vitro and in vivo provided that sodium butyrate is included during the labeling period. These results suggest that 4C anlagen contain both active acetylase and deacetylase activities even though the actual steady state level of acetylation found in these nuclei is low, more like that of micronuclei. At later stages of macronuclear maturation (8C, 15-16 hr), inner histones are hyperacetylated in a manner similar to parental, fully differentiated macronuclei. Furthermore, 8C anlagen incorporate acetate well even in the absence of sodium butyrate. Taken together these results suggest that endogenous deacetylase enzymes become either down-regulated and/or the rate of histone acetylases increases markedly during macronuclear differentiation.