Modification of histones during the mitotic and meiotic cycle of yeast

Abstract

Core histones (H2a ,H2b ,H3 ,H4) of yeast have been shown to be similar to histones from higher organisms [ 1 ] whereas a protein corresponding to histone Hl seems to be absent from this simple unicellular eukaryote [2]. Also the organization of yeast chromatin into nucleosomes closely resembles that of higher eukaryotes [ 11. Histones of other organisms have been found to be subject to various modifications. Phosphorylation, especially that of histones Hl and H3, occurs predominantly during chromosome condensation for mitosis in higher organisms [3] as well as in the slime mold, Physarum polycephalum [4]. Phosphorylation of histone H2a was suggested to be involved in interphase heterochromatin structure in a cell cycle-independent mamrer [5-71. Highly acetylated histone H4 was discovered specifically in transcriptionally active, DNase I-sensitive, chromatin [8-lo], its concentration is inversely correlated with the initiation of chromosome condensation in Physarum [II]. We asked whether such modifications, probably being devices for the modulation of DNA-histone interactions, also occurred in yeast and whether appearance of modified histone proteins varied during the vegetative cell cycle or during the sporulation process. We found incorporation of radioactive phosphate into yeast histone H2a and to a much smaller amount into H2b, but not into H3 and H4. The degree of incorporation did not change substantially during the vegetative growth cycle but increased considerably in cells preparing for sporulation. Radioactive sodium acetate was found to be incorporated into all core histone-bands. Analysis of unlabeled histones in acidurea gels, in which the acetylated variants of histones H4 and H3 can be separated, showed that these 2 histones in yeast exist as several subspecies acetylated to different degrees. Variations in the relative amounts of these subspecies were found during the mitotic cell cycle and less pronounced during the process of sporulation induction.