Expression of the rat testis-specific histone H1t gene in transgenic mice. One kilobase of 5'-flanking sequence mediates correct expression of a lacZ fusion gene.
H1t is synthesized in mid to late pachytene spermatocytes of the male germ line and is the only tissue-specific member of the mammalian H1 histone family. As a step toward identifying DNA sequences that confer its tissue-specific expression, we have produced transgenic mice containing the intact rat H1t gene as well as a H1t-lacZ fusion gene. Transgenic mice carrying a 6.8-kilobase fragment of rat genomic DNA encompassing the H1t gene expressed rat H1t at high levels in the testis and in no other organ examined. H1t fragments truncated to within 141 base pairs (bp) of the gene in the 5' direction or within 837 bp in the 3' direction retained testis specificity. Expression of rat H1t protein was also evident in the testes of the transgenic mice, and in some lines the level of rat H1t exceeded that of the mouse protein. The stage of spermatogenesis of transgene expression was assessed by following appearance of transgenic mRNA in developing mice and by immunohistochemistry using an antiserum to rat H1t. In lines from three different constructs, expression was restricted to germinal cells, although in two strongly expressing lines the transgenes were expressed somewhat prematurely in preleptotene spermatocytes. An H1t(-948/+71)-lacZ fusion was also expressed specifically in the spermatocytes and round spermatids of a transgenic line, confirming that sequences sufficient for correct tissue and developmental expression lie within this 1,019-bp segment of the gene.