Absence of histone F1 in a mitotically dividing, genetically inactive nucleus.

Histones were extracted from macro- and micronuclear chromatin of the ciliated protozoan Tetrahymena pyriformis. Conditions that resulted in macronuclear chromatin containing large amounts of histone F1 yielded micronuclear chromatin in which this histone was absent. Evidence is presented indicating that the absence of F1 from micronuclei is not a preparative artifact and that histone F1 is replaced by other histone fractions. Since micronuclei divide mitotically, while macronuclei divide amitotically, these results suggest that histone F1 and its phosphorylation do not play an indispensable role in the process of mitotic chromosome condensation, in chromosome replication, or in the separation of newly synthesized chromatids.