Structural and functional analysis of the rat testis-specific histone H1t gene.

A 6.86 kb rat genomic DNA fragment containing the testis-specific histone H1t gene and the histone H4t gene has been sequenced. S1-nuclease protection analyses of total cellular RNA from rat liver and testis showed that histone H1t mRNA was present only in testis. Examination of various highly enriched populations of rat testis cell types revealed that H1t mRNA was found exclusively in a fraction enriched in pachytene spermatocytes. When protein, DNA interactions within the proximal promoter region of the histone H1t gene were examined by electrophoretic mobility shift assays, only minor differences were found in mobility shift patterns of the H1t promoter in assays comparing binding of nuclear proteins from pachytene spermatocytes and early spermatids. However, major differences in binding were observed upon comparing nuclear proteins from rat pachytene spermatocytes to liver. Comparison of binding patterns of rat testis, rat hepatoma H4 cells, HeLa cells, and COS-1 cells also revealed dramatic differences. Transcriptional activity of the histone H1t promoter was examined by measuring H1t promoted chloramphenicol acetyltransferase (CAT) mRNA levels in transient expression assays in transfected rat hepatoma H4 cells, HeLa cells, and COS-1 cells. These assays revealed that the histone H1t promoted CAT gene functioned poorly in HeLa cells and COS-1 cells compared to expression with the parent SV40 promoted vector pSV2CAT. The H1t promoted CAT gene apparently did not work at all in transfected rat hepatoma H4 cells, which is consistent with testis germinal cell specific expression of the histone H1t gene.