Proteolytic processing of histone H3 in chromatin: a physiologically regulated event in tetrahymena micronuclei

Abstract

Micronuclei of Tetrahymena thermophila contain two electrophoretically distinct forms of histone H3. The slower migrating micronuclear species, H3 S, is indistinguishable from macronuclear H3 by electrophoretic analyses in three gel systems and by partial proteolytic peptide mapping. The faster species, H3 F, is unique to micronuclei. Pulse-chase experiments with radioactive amino acids show that H3 S is a precursor to H3 F. We present evidence that the in vivo processing of H3 S into H3 F requires cell growth and/or division and may occur regularly each generation at a specific point in the cell cycle. The processing event must occur after H3 F is deposited on micronuclear chromatin, since both H3 S and H3 F can be isolated from sucrose gradient-purified mononucleosomes ( Allis, Glover and Gorovsky, 1979). Partial proteolytic peptide mapping coupled with 3H-N-ethylmaleimide labeling suggest that the processing event involves a proteolytic cleavage from the amino terminal end of H3 F. Automated sequence analyses of 14C-lysine-labeled macronuclear H3 together with either 3H-lysine-labeled H3 S or H3 F demonstrated that H3 F is derived from H3 S by a proteolytic cleavage which removes six residues from the amino terminus. These observations represent the first demonstration of a physiologically regulated proteolytic processing event in histone metabolism.