Comparison of the structural organization and expression of germinal and somatic rat histone H4 genes.

A rat somatic histone H4 gene was isolated by screening a rat genomic library using a cloned cell-cycle-regulated human histone H4 gene as a probe. The somatic histone H4 gene was subcloned and the nucleotide sequence was determined. The structural organization and expression of the somatic histone H4 gene and the rat germinal histone H4t gene were compared. Although the predicted amino-acid sequences of the two histones were identical, 49 out of 102 codons differed. The leader sequence of the germinal histone H4t mRNA was 17 bases compared to 40 bases for the somatic histone H4 mRNA, and the 3' terminal sequence of the germinal histone H4t mRNA was 52 bases compared to 75 bases for the somatic histone H4 mRNA. The germinal histone H4 gene also lacked a consensus purine-rich motif which was present in the 5' noncoding region of the somatic histone H4 gene. Northern blot analyses and S1-nuclease protection analyses revealed that the germinal histone H4t and H1t genes were expressed during spermatogenesis in rat pachytene spermatocytes, and the somatic histone H4 gene was expressed only in nongerminal rat cells and tissues. The histone H4t gene was also expressed in some other rat cell types. The differences in expression of the histone H4t and H1t genes may reflect differences in transcription, differences in turnover rates of the mRNAs, or a combination of these factors.