Persistence of a micrococcal nuclease sensitive region spanning the promoter-coding region junction of a cell cycle regulated human H4 histone gene throughout the cell cycle.

We have examined the chromatin structure of the cell cycle regulated human H4 histone gene FO108A at various times during the cell cycle, by treating nuclei isolated from synchronized HeLa S3 cells with micrococcal nuclease. Purified DNA was fractionated electrophoretically, transferred to nitrocellulose, and hybridized to small (150-250 nucleotides) radiolabeled probes from various portions of the promoter and coding regions of the gene. Our results indicate the existence of a micrococcal nuclease sensitive region located between positions -60 and +90 base pairs (bp) from the start codon of the gene, which includes the TATA box. This nuclease-sensitive region persists at all the cell cycle times analyzed. Hybridization with a 250-bp probe containing only coding region sequences reveals a disrupted nucleosomal ladder during early S phase, when this H4 histone gene replicates and exhibits an enhanced level of transcription. By mid-S phase, the regular nucleosomal structure of the coding region is restored and persists during subsequent phases of the cell cycle. The disruption of a normal nucleosomal organization in the promoter and mRNA coding regions of this H4 histone gene is also supported by the sensitivity of these sequences to S1 nuclease.