H1t is an H1 histone variant unique to late spermatocytes and early spermatids. Using gene targeting and embryonic stem cell technologies, we have produced mice with a disrupted H1t gene. Homozygous H1t-null mice have normal fertility and show no obvious phenotypic consequence due to the lack of this histone. Biochemical and immunohistochemical approaches were used to show that normal changes in chromosomal proteins occurred during spermatid development, including the appearance and disappearance of transition proteins 1 and 2. Both protamines 1 and 2 are present in normal amounts in sonication-resistant spermatid nuclei from H1t-null mice. Analysis of H1 histones by quantitative gel electrophoresis in enriched populations of pachytene spermatocytes and round spermatids showed that the lack of H1t is only partially compensated for by somatic H1s, so that the chromatin of these cells is H1 deficient. Because H1t is thought to create a less tightly compacted chromatin environment, it may be that H1-deficient chromatin is functionally similar to chromatin with H1t present, at least with respect to permitting spermatogenesis to proceed.

Histone H1t is synthesized only in male germ cells during the late pachytene stage of meiosis and is retained in spermatids until the nucleus elongates. Transgenic experiments suggest that spermatocyte-directing sequences lie within 140 base pairs of the cap site. To study the mechanism of this specificity we compared the DNase I footprints made on the immediate promoter regions of H1t and H1d (a typical somatic H1) by testis and liver extracts and observed both common and differentially protected regions. The common footprints of H1t included an Sp1 consensus (GC box 1) and a CCAAT motif. Electrophoretic mobility shift assays (EMSA) identified ubiquitous binding factors for GC box 1 and a binding factor for the CCAAT element that we identified immunologically as H1TF2. H1t-specific footprints occurred over the palindrome CCTAGG and a GC-rich sequence downstream of the TATA box (GC box 2). EMSA analysis of the palindrome identified testis-specific as well as ubiquitous binding factors. UV irradiation of a palindrome-binding reaction generated a cross-linked doublet of about 50 kDa from both testis and liver. Protein factors that bound to the GC box 2 sequence were similar from testis and liver, and GC box 1 and an Sp1 consensus competed for them. In vitro transcription directed by H1t occurred at comparable levels in testis and liver extracts. The importance of both GC box 1 and CCAAT elements was demonstrated by deletion analysis and by oligonucleotide competition. No dependence on the H1t palindrome was observed for in vitro transcription.

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