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Histone fraction F1 has been isolated and purified from macronuclei of the ciliated protozoan, Tetrahymena pyriformis. In many respects, Tetrahymena F1 is similar to that of other organisms. It is the only Tetrahymena histone soluble in 5% perchloric acid or 5% trichloroacetic acid, has a higher molecular weight than any other Tetrahymena histone, is the histone most easily dissociated from Tetrahymena chromatin, and is susceptible to specific proteolytic cleavage. However, unlike F1 in all other organisms, Tetrahymena F1 is not the slowest-migrating histone fraction when analyzed by polyacrylamide gel electrophoresis at low pH. Tetrahymena F1 also exhibits unusual behavior in sodium dodecyl sulfate-containing polyacrylamide gels, migrating faster than calf thymus F1 at pH 10, and slower than calf thymus F1 at pH 7.6. Tetrahymena F1 was found to be highly phosphorylated in rapidly growing cells, suggesting that the relationship between cell replication and F1 phosphorylation previously observed in mammalian cells may extend to all eukaryotes. The observation that extensive F1 phosphorylation occurs in macronuclei, which divide amitotically, argues against a unique role for F1 phosphorylation in the process of chromosome condensation at mitosis.

Histone fraction F2A1 has been isolated and purified from macronuclei of the ciliate Tetrahymena pyriformis. It migrates as a single species on sodium dodecyl sulphate-acrylamide gel electrophoresis, with a molecular weight indistinguishable from that of calf thymus F2A1. The solubility properties of Tetrahymena F2A1 are also similar to those of calf thymus F2A1. Electrophoretic analyses on urea-acrylamide gels indicate that Tetrahymena F2A1 consists of four or five subspecies, the two fastest having electrophoretic mobilities identical with those of the two major electrophoretically separable forms of calf thymus F2A1. High resolution (long gel) electrophoresis coupled with incorporation of radioactive acetate both in vivo and in vitro suggest that, as in the case of calf thymus F2A1, differentical acetylation of a parent molecule can explain the observed electrophoretic heterogeneity of Tetrahymena F2A1. Electrophoretic analysis of histones isolated from the micronucleus, which is genetically less active than the macronucleus, indicates that it contains largely the relatively unacetylated (parent) form of histone F2A1.