Histone phosphorylation in macro- and micronuclei of Tetrahymena thermophila

The patterns of histone phosphorylation in amitotically dividing, transcriptionally active macronuclei and in mitotically dividing, transcriptionally inert micronuclei of the ciliated protozoan Tetrahymena thermophila have been analyzed. Taken together, the major phosphorylation events in these two nuclei and their dependence on cell growth and/or division are remarkably similar to those in mammalian cells. Phosphorylation of H1-type proteins occurs in both nuclei and is positively correlated with growth and/or division. Phosphorylation of histone H3 is also positively correlated with growth and/or division but occurs only in micronuclei. Phosphorylation of histone H2A is relatively independent of growth state and occurs largely, if not exclusively, in macronuclei. Given the unique partition of nuclear functions between macro- and micronuclei, these results, coupled with previously reported temporal correlations between specific histone phosphorylations and cell cycle events in mammalian cells [Gurley, L. R., Tobey, R. A., Walters, R. A., Hildebrand, C. E., Hohmann, P. G., D'Anna, J. A., Barham, S. S., & Deaven, L. L. (1978a) in Cell Cycle Regulation (Jeter, J. R., Cameron, J. L., Padilla, G. M. & Zimmerman, A. M. Eds.) pp 37-60, Academic Press, New York], allow insights into the functions of histone phosphorylations. Specifically, a nonmitotic function for extensive H1 phosphorylation and a unique mitotic function for H3 phosphorylation are clearly indicated. A new role for H2A phosphorylation in the regulation of transcriptional activity is also proposed.