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

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.