Reversible changes in the nucleosomal organization of a human H4 histone gene during the cell cycle

Abstract

The organization of nucleosomes associated with a cell cycle regulated human H4 histone gene was examined in synchronized HeLa S3 cells. At various times during the cell cycle, nuclei were digested with micrococcal nuclease, and the nucleosomal pattern of the gene was obtained by Southern blot analysis using radiolabeled human histone H4 gene probes. We have detected reversible changes during the cell cycle in the chromatin structure of this gene, as reflected by the shortening of the nucleosomal spacing after replication and the peak of transcription. This variation is also observed when DNA and protein syntheses are inhibited. By using a probe that comprises 250 base pairs (bp) of the coding region and 240 bp of the 5'' end of the gene, containing the promoter and DNase I sensitive sequences, we also have observed a general disruption of the nucleosomal organization, which is reflected by a degeneration of the characteristic nucleosomal ladder produced by micrococcal nuclease digestion. This modification coincides with the replication and active transcription of the gene (early S phase), which recovers its regular nucleosomal appearance when both processes have been completed, although the nucleosome linker length is shortened. When the probe utilized comprises the distal 3'' end of the gene, there is no disruption of the nucleosomal pattern, but the linker region also exhibits a shortened length. A non-cell cycle regulated gene (.beta.-globin) does not exhibit such modifications in any of the situations analyzed. The observed variations in nucleosomal spacing and in the regular nucleosomal pattern may be functionally related to modifications in DNAase I and S1 sensitivity in the 5'' end of the H4 gene during S phase [Chrysogelos, S., Riley, D. E., Stein, G., and Stein, J. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 7535]. Together, these results suggest that there is a sequential remodeling of chromatin structure of a cell cycle dependent human H4 histone gene that correlates with expression.