Histones and other basic nuclear proteins in genetically active and genetically inactive nuclei of the ciliate, Oxytricha sp.

Abstract

Genetically active DNA in the macronucleus of the ciliated protozoan, Oxytricha sp., is associated with histones f1, f2a1, f2a2, f2b and f3 ∗ ∗ In alternative nomenclature, histone f1 is H1; f2a2, H2a; f2b, H2b; f3, H3; and f2a1, H4. . Only the f1 fraction differs significantly from vertebrate histones. Oxytricha macronuclear f1 histone comprises 34% of total histone, and in polyacrylamide-urea electrophoretic gels, this fraction migrates faster than all other histones. Genetically inactive DNA in the micronucleus of Oxytricha is associated with basic proteins that are unlike histones both in electrophoretic behavior and amino acid composition. Micronuclei divide mitotically, yet micronuclear basic proteins contain no component comparable to f1 histone, suggesting that f1 histone and its phosphorylation are not always a prerequisite for chromosome condensation in mitosis. Micronuclear basic proteins are similar in their electrophoretic pattern and extreme lysine-richness to a class of basic proteins found in sperm of the mollusc Spisula solidissima; these sperm proteins are intermediate between histones and protamines in basicity. Histones and active macronuclear DNA are organized into beaded chromatin structures (nu bodies). Micronuclear DNA, however, may be compacted into configurations that are inaccessible to RNA polymerase when associated with a class of basic proteins that are similar to those found in S. solidissima sperm.