Tissue-specific binding of testis nuclear proteins to a sequence element within the promoter of the testis-specific histone H1t gene

Use of assisted reproductive technologies (ARTs) to shorten the generational interval in ruminants: current status and perspectives

The challenges posed by climate change and increasing world population are stimulating renewed efforts for improving the sustainability of animal production. To meet such challenges, the contribution of genomic selection approaches, in combination with assisted reproductive technologies (ARTs), to spreading and preserving animal genetics is essential. The largest increase in genetic gain can be achieved by shortening the generation interval. This review provides an overview of the current status and progress of advanced ARTs that could be applied to reduce the generation time in both female and male of domestic ruminants. In females, the use of juvenile in vitro embryo transfer (JIVET) enables to generate offspring after the transfer of in vitro produced embryos derived from oocytes of prepubertal genetically superior donors reducing the generational interval and acceleration genetic gain. The current challenge is increasing in vitro embryo production (IVEP) from prepubertal derived oocytes which is still low and variable. The two main factors limiting IVEP success are the intrinsic quality of prepubertal oocytes and the culture systems for in vitro maturation (IVM). In males, advancements in ARTs are providing new strategies to in vitro propagate spermatogonia and differentiate them into mature sperm or even to recapitulate the whole process of spermatogenesis from embryonic stem cells. Moreover, the successful use of immature cells, such as round spermatids, for intracytoplasmic injection (ROSI) and IVEP could allow to complete the entire process in few months. However, these approaches have been successfully applied to human and mouse whereas only a few studies have been published in ruminants and results are still controversial. This is also dependent on the efficiency of ROSI that is limited by the current isolation and selection protocols of round spermatids. In conclusion, the current efforts for improving these reproductive methodologies could lead toward a significant reduction of the generational interval in livestock animals that could have a considerable impact on agriculture sustainability.

Germline-specific H1 variants: the "sexy" linker histones

The eukaryotic genome is packed into chromatin, a nucleoprotein complex mainly formed by the interaction of DNA with the abundant basic histone proteins. The fundamental structural and functional subunit of chromatin is the nucleosome core particle, which is composed by 146 bp of DNA wrapped around an octameric protein complex formed by two copies of each core histone H2A, H2B, H3, and H4. In addition, although not an intrinsic component of the nucleosome core particle, linker histone H1 directly interacts with it in a monomeric form. Histone H1 binds nucleosomes near the exit/entry sites of linker DNA, determines nucleosome repeat length and stabilizes higher-order organization of nucleosomes into the ∼30 nm chromatin fiber. In comparison to core histones, histone H1 is less well conserved through evolution. Furthermore, histone H1 composition in metazoans is generally complex with most species containing multiple variants that play redundant as well as specific functions. In this regard, a characteristic feature is the presence of specific H1 variants that replace somatic H1s in the germline and during early embryogenesis. In this review, we summarize our current knowledge about their structural and functional properties.

Transcription factor RFX2 is abundant in rat testis and enriched in nuclei of primary spermatocytes where it appears to be required for transcription of the testis-specific histone H1t gene

Previous work in our laboratory revealed upregulated transcription of the testis-specific linker histone H1t gene in pachytene primary spermatocytes during spermatogenesis. Using the H1t X-box as an affinity chromatography probe, we identified Regulatory Factor X2 (RFX2), a member of the RFX family of transcription factors, as a nuclear protein that binds the probe. We also showed that RFX2 activated the H1t promoter in transient expression assays. However, other RFX family members have the same DNA-binding domain and they also may regulate H1t gene expression. Therefore, in this study we examined the distribution of RFX2 and other RFX family members in rat testis germinal cells and in several tissues. Among tissues examined, RFX2 is most abundant in testis. Testis RFX2 is most abundant in spermatocytes where transcription of the H1t gene is upregulated and the steady-state H1t mRNA level is high. RFX2 levels decrease but RFX1 levels increase in early spermatids where H1t gene transcription is downregulated. Antibodies against RFX2 generate a shifted band in electrophoretic mobility shift assays (EMSA) using H1t or testisin X-box DNA probes with nuclear proteins from spermatocytes. These data support the hypothesis that RFX2 expression is upregulated in spermatocytes where it participates in activating transcription of the H1t gene and other testis genes. These data also support the possibility that other RFX family members may bind to the H1t promoter in other testis germinal cell types and in nongerminal cells to downregulate H1t gene transcription.

Transcriptional activation of the testis-specific histone H1t gene by RFX2 may require both proximal promoter X-box elements

The rat testis-specific linker histone H1t gene is transcribed in pachytene primary spermatocytes during spermatogenesis. Our previous work using transgenic mice demonstrated that spermatocyte-specific transcription of the H1t gene is dependent upon a proximal promoter element designated the TE element. TE is composed of two adjacent and inverted imperfect repeat sequences designated TE1 and TE2 and both of these palindromic elements are similar in sequence to the X-box, a DNA consensus sequence that binds regulatory factor X (RFX). RFX2 is the major enriched protein derived from rat testis nuclear extracts when using the TE1 element as an affinity chromatography probe. Co-expression of RFX2 together with an H1t promoted reporter vector in transient expression assays activates the H1t promoter in the GC-2spd germinal cell line, and mutation of either X-box significantly represses activity. However, RFX2 partially reactivates the promoter when either of the X-box elements is independently mutated. In order to totally block reactivation by RFX2, it is necessary to mutate both X-boxes simultaneously. Therefore, RFX2 appears to be able to bind to either X-box independently to partially activate the promoter of the testis-specific histone H1t gene, but simultaneous binding of RFX2 to both X-box elements may be required for maximal promoter activation.

Testis-specific transcriptional control

In the testis, tissue-specific transcription is essential for proper expression of the genes that are required for the reproduction of the organism. Many testis-specific genes are required for mitotic proliferation of spermatogonia, spermatocytes undergoing genetic recombination and meiotic divisions, and differentiation of haploid spermatids. In this article we describe some of the genes that are transcribed in male germinal cells and in non-germinal testis cells. Because significant progress has been made in examination of promoter elements and their cognate transcription factors that are involved in controlling transcription of the testis-specific linker histone H1t gene in primary spermatocytes, this work will be reviewed in greater detail. The gene is transcriptionally active in spermatocytes and repressed in all other germinal and non-germinal cell types and, therefore, it serves as a model for study of regulatory mechanisms involved in testis-specific transcription.

Regulatory factor X2 (RFX2) binds to the H1t/TE1 promoter element and activates transcription of the testis-specific histone H1t gene

Transcription of the mammalian testis-specific linker histone H1t gene occurs only in pachytene primary spermatocytes during spermatogenesis. Studies of the wild type (Wt) and mutant H1t promoters in transgenic mice show that transcription of the H1t gene is dependent upon the TE promoter element. We purified an 85 kDa protein from rat testis nuclear extracts using the TE1 subelement as an affinity chromatography probe and analysis revealed that the protein was RFX2. The TE1 element is essentially an X-box DNA consensus element and regulatory factor X (RFX) binds specifically to this element. Polyclonal antibodies directed against RFX2 supershift the low mobility testis nuclear protein complex formed in electrophoretic mobility shift assays (EMSA). RFX2 derived from primary spermatocytes, where the transcription factor is relatively abundant, binds with high affinity to the TE1 element. Coexpression of RFX2 together with an H1t promoter/reporter vector activates the H1t promoter in a cultured GC-2spd germinal cell line, but mutation of either the TE1 subelement or the TE2 subelements represses activity. These observations lead us to conclude that the TE1 and TE2 subelements of the testis-specific histone H1t promoter are targets of the transcription factor RFX2 and that this factor plays a key role in activating transcription of the H1t gene in primary spermatocytes. Published 2003 Wiley-Liss, Inc.

Location of promoter elements necessary and sufficient to direct testis-specific expression of the Hst70/Hsp70.2 gene

The rat Hst70 gene and its mouse counterpart Hsp70.2 are expressed specifically in pachytene primary spermatocytes and spermatids. Here we demonstrate that a 165 bp fragment of the Hst70 gene promoter, containing the T1 transcription start site region, entire exon 1 and 42 bp 5' region of the intron, is sufficient to drive testis-specific expression of the chloramphenicol acetyltransferase reporter gene in transgenic mice with the same developmentally regulated pattern as the endogenous Hsp70.2 gene. We show further that high-level tissue-specific gene expression requires additional sequences localized upstream of the T2 transcription start site. Electrophoretic mobility-shift assay analysis revealed that only testes of juvenile rats, when Hst70 gene expression is repressed, contain proteins that specifically bind to the Oct (octamer) sequence localized directly downstream of the T1 site.

RFX2 is a potential transcriptional regulatory factor for histone H1t and other genes expressed during the meiotic phase of spermatogenesis

H1t is a novel linker histone variant synthesized in mid- to late pachytene spermatocytes. Its regulatory region is of interest because developmentally specific expression has been impressed on an otherwise ubiquitously expressed promoter. Using competitive band-shift assays and specific antisera, we have now shown that the H1t-60 CCTAGG palindrome motif region binds members of the RFX family of transcriptional regulators. The testis-specific binding complex contains RFX2, probably as a homodimer. Other DNA-protein complexes obtained from testis as well as somatic organs contain RFX1, primarily as a heterodimer. Western blots confirmed that RFX2 expression is greatly enhanced in adult testis and that RFX2 is equally prominent in highly enriched populations of late pachytene spermatocytes and round spermatids. Immunohistochemistry carried out on mouse testis showed that RFX2 is strongly expressed in pachytene spermatocytes, remains high in early round spermatids, and declines only in advance of nuclear condensation. Maximum expression correlates well with the appearance of H1t. In contrast, RFX1 immunoreactivity in germ cells was only detected in late round spermatids. RFX-specific band complexes were also identified for both the mouse lamin C2 and Sgy promoters, using either testis nuclear extracts or in vitro-synthesized RFX2. These results call attention to RFX2 as a transcription factor with obvious potential for the regulation of gene expression during meiosis and the early development of spermatids.

Specific binding of nuclear proteins to a bifunctional promoter element upstream of the H1/AC box of the testis-specific histone H1t gene

The testis-specific histone H1t gene is transcribed exclusively in primary spermatocytes during spermatogenesis. Studies with transgenic mice show that 141 base pairs (bp) of the H1t proximal promoter accompanied with 800 bp of downstream sequence are sufficient for tissue-specific transcription. Nuclear proteins from testis and pachytene spermatocytes produce footprints spanning the region covering the repressor element (RE) from 100 to 125 nucleotides upstream of the H1t transcriptional initiation site. Only testis nuclear proteins bind to the 5'-end of the element and produce a unique, low-mobility complex in electrophoretic mobility shift assays. This testis complex is distinct from the complex formed by a repressor protein derived from several cell lines that binds to the 3'-end of the element. The testis complex band is formed when using nuclear proteins from primary spermatocytes, where the H1t gene is transcribed, and band intensity drops 70%-80% when using nuclear proteins from early spermatids, where H1t gene transcription ceases. Protein-DNA cross-linking experiments using testis nuclear proteins produce electrophoretic bands of 59, 52, and 50 kDa on SDS/PAGE gels.

H1t/GC-box and H1t/TE1 element are essential for promoter activity of the testis-specific histone H1t gene

The testis-specific linker histone H1t gene is transcribed exclusively in mid to late pachytene primary spermatocytes. Tissue-specific expression of the gene is mediated primarily through elements located within the proximal promoter. Previous work in transgenic animals identified a unique 40-base pair promoter element designated H1t/TE that is essential for spermatocyte-specific expression. The H1t/TE element contains three subelements designated TE2, GC-box, and TE1 based on in vitro footprinting and electrophoretic mobility shift assays. Because GC-box is a consensus site for binding of Sp transcription-factor family members, experiments were performed demonstrating that two Sp family members, Sp1 and Sp3, were present in testis cells from 9-day-old and adult rats and in pachytene primary spermatocytes and early spermatids. A 95- to 105-kDa form of Sp1 is most abundant in the tissues and cell lines examined, but a 60-kDa form of Sp1 is the most abundant species in spermatocytes and early spermatids. Further examination of Sp1 and Sp3 from adult testis, primary spermatocytes, and early spermatids showed that they can bind to the H1t/TE element. In order to determine the contributions of the subelements to H1t transcription, we mutated each of them in H1t promoter luciferase reporter vectors. Mutation of the GC-box and TE1 subelement reduced expression 77% and 49%, respectively, compared with the wild-type H1t promoter in transient expression assays in a testis GC-2spd cell line that was derived from germinal cells. These studies suggest that Sp transcription factors may be involved in transcription of the H1t gene and the GC-box and the TE1 subelement are required for activation of the H1t promoter.

Sp1 and Sp3 activate the testis-specific histone H1t promoter through the H1t/GC-box

The testis-specific linker histone H1t gene is transcribed exclusively in mid to late pachytene primary spermatocytes. Tissue specific expression of the gene is mediated in large part through elements located within the proximal promoter. Previous work in transgenic animals showed that a unique 40 bp promoter element designated H1t/TE is essential for spermatocyte-specific expression. The H1t/TE element contains a GC-box, which is a perfect consensus binding site for members of the Sp family of transcription factors. We have shown that Sp1 and Sp3 are present in testis cells from 9-day-old and adult rats and in pachytene primary spermatocytes and early spermatids and that they can bind to the H1t/GC-box. Mutagenesis of the GC-box reduced H1t promoter activity. Furthermore, a CpG dinucleotide within the GC-box was totally unmethylated in rat testis primary spermatocytes where the gene is transcribed but it was methylated in liver where the gene is silenced. These previous studies supported the importance of the GC-box and suggested that Sp transcription factors contribute to expression of the H1t gene. In this study, we show that co-transfection of Sp1 and Sp3 expression vectors leads to an upregulation of histone H1t promoter activity in several cell lines including testis GC-2spd cells. However, very low H1t promoter activity is seen in GC-2spd cells grown at 39 degrees C, which correlates with lower levels of Sp1 and Sp3 in these cells grown at this elevated temperature. Upregulation of the H1t promoter by Sp1 and Sp3 was also seen in cotransfected NIH3T3 and C127I cell lines. On the other hand, co-transfection of the Sp1 and Sp3 expression vectors does not lead to upregulation of activity of the cell-cycle dependent histone H1d promoter.

Characterization of the H1t promoter: role of conserved histone 1 AC and TG elements and dominance of the cap-proximal silencer

H1t is a testis-specific variant histone 1 gene transcribed in pachytene spermatocytes. As part of a program to understand its transcriptional control, we have investigated the effect of the cap-proximal, GC-rich silencer element in the context of various lengths of upstream sequence. By transient transfection of NIH 3T3 cells, we showed that a targeted mutation in the silencer has a large (>10-fold) effect on reporter gene expression, regardless of the length of upstream sequence present. No other discrete silencing activity was observed in the upstream region extending to nucleotide -1842. Similarly, when the silencer mutation was introduced into the natural gene, H1t expression was readily detected in permanently transfected cells by both RNase protection and Western blot analysis, regardless of the extent of 5' or 3' flanking genomic DNA. In constructs with the mutated silencer, we showed interdependence of the characteristic H1 AC and TG box regulatory elements. Promoter up-regulation occurred only when both were intact, and possibly identical binding factors were demonstrated for each by electrophoretic mobility shift assays. In view of its precisely regulated but limited expression, it is interesting that H1t retains all the promoter elements known to activate standard H1 genes, including the TG/AC unit, SP1 site, and CCAAT element. Their presence emphasizes the apparent dominance of the silencer element in most cells.

Testis-specific histone H1t gene is hypermethylated in nongerminal cells in the mouse

The testis-specific histone H1t gene is expressed only in pachytene primary spermatocytes during spermatogenesis. There is a correlation between the specific binding of testis nuclear proteins to a rat histone H1t promoter sequence, designated the H1t/TE element, and the onset of transcription in primary spermatocytes. Our laboratory has shown that mice bearing the rat gene with a deletion of the TE promoter element and replacement with a heterologous stuffer DNA fragment fail to express the rat H1t transgene in any tissue. In this study we report that five CpGs located within the H1t proximal promoter, including two CpGs located within the essential TE promoter element, contain unmethylated cytosines in vivo in genomic DNA derived from primary spermatocytes where the H1t gene is expressed. All seven CpGs are hypermethylated in vivo in genomic DNA derived from liver cells where gene expression is repressed. Further, in vitro methylation of an H1t promoter-driven reporter plasmid markedly reduced expression in a transient transfection assay system. These results suggest that cytosine methylation may contribute to the transcriptional silencing of the testis-specific histone H1t gene in nonexpressing tissues such as liver.

Elimination of male germ cells in transgenic mice by the diphtheria toxin A chain gene directed by the histone H1t promoter

Expression of the diphtheria toxin A-chain gene was directed to the male germ line by fusion to 1 kilobase of the 5'-flanking DNA of the rat histone H1t gene. Two independent lines of mice were established that expressed the toxic transgene. Female carriers were fertile; males were sterile although otherwise apparently normal. Adult transgenic males had very small testes that were virtually devoid of germ cells. A developmental study showed that germ cells survived until late fetal life but that testes of 3-day-old transgenic mice were severely depleted of prospermatogonia. During postnatal development of transgenic animals, remaining germ cells progressed to the pachytene stage of meiosis in 10% to 30% of tubular cross sections but degenerated before the completion of meiosis. By 3 mo of age the residual germ cells had almost completely disappeared. These transgenic lines demonstrate the complete tissue specificity of the H1t promoter and reveal a period of its activity just prior to formation of the definitive adult spermatogonial stem cell population. Whereas full expression of H1t occurs only in mid to late pachytene spermatocytes, one or more of the factors that impart tissue specificity to its expression must be transiently activated in the neonatal germ line. This report discusses the possibility that this genetic technique for eliminating germ cells may have practical application in making recipients for spermatogonial stem cell transplantation.

Normal spermatogenesis in mice lacking the testis-specific linker histone H1t

H1 histones bind to linker DNA and nucleosome core particles and facilitate the folding of chromatin into a more compact structure. Mammals contain seven nonallelic subtypes of H1, including testis-specific subtype H1t, which varies considerably in primary sequence from the other H1 subtypes. H1t is found only in pachytene spermatocytes and early, haploid spermatids, constituting as much as 55% of the linker histone associated with chromatin in these cell types. To investigate the role of H1t in spermatogenesis, we disrupted the H1t gene by homologous recombination in mouse embryonic stem cells. Mice homozygous for the mutation and completely lacking H1t protein in their germ cells were fertile and showed no detectable defect in spermatogenesis. Chromatin from H1t-deficient germ cells had a normal ratio of H1 to nucleosomes, indicating that other H1 subtypes are deposited in chromatin in place of H1t and presumably compensate for most or all H1t functions. The results indicate that despite the unique primary structure and regulated synthesis of H1t, it is not essential for proper development of mature, functional sperm.

Binding of nuclear proteins to an upstream element involved in transcriptional regulation of the testis-specific histone H1t gene

The testis-specific histone H1t is synthesized during spermatogenesis exclusively in late pachytene primary spermatocytes. Transcription of the H1t gene is repressed in every tissue except testis. Within the testis, transcription is repressed during development before the spermatocyte stage and in later stages of germinal cell maturation. Mechanisms involved in transcriptional repression of the H1t gene are unknown. To assess the contribution of upstream H1t promoter sequence to transcriptional silencing in nonexpressing cells, H1t-promoted reporter vectors were constructed using pGL3 Basic. Transient expression assays with these reporter vectors driven by H1t promoter deletions allowed us to identify a region from 948 to 780 bp upstream from the H1t transcriptional initiation site that functions as a silencer. Examination of nuclear protein binding to this DNA regulatory region by electrophoretic mobility shift assays using extracts from C127I cells, rat testis, and pachytene spermatocytes revealed a low mobility band produced only by nuclear proteins derived from nonexpressing cells that may contain proteins that repress H1t gene transcription.

Localization of upstream elements involved in transcriptional regulation of the rat testis-specific histone H1t gene in somatic cells

The testis-specific histone H1t is synthesized exclusively in late pachytene primary spermatocytes during spermatogenesis. The mechanisms involved in transcriptional repression of the H1t gene during development before the spermatocyte stage and in later stages of germinal cell maturation and in nonexpressing somatic tissues are unknown. To assess the contribution of the upstream DNA sequence to H1t transcriptional silencing in nonexpressing cells, a set of histone H1t-promoted reporter vectors was constructed. Transient transfection of mouse C127I cells with these reporter vectors allowed us to identify a transcriptional silencer located between 948 base pairs (bp) and 780 bp upstream from the H1t transcriptional initiation site. Histone H1t-promoted luciferase activity increased 4-fold when the region between 948 bp and 875 bp upstream from the transcriptional initiation site was eliminated. Addition of a 73-bp rat H1t promoter fragment (-948 to -875, containing the 5' portion of the silencer region) to a site immediately upstream from the histone H1d proximal promoter led to significantly reduced luciferase expression upon transient transfection (56% in C127I cells and 44% in HeLa cells). Nuclear proteins were found to bind to DNA within the H1t silencer region when assayed by in vitro deoxyribonuclease (DNase) I footprinting. Thus, our data suggest that an active transcriptional silencer mechanism involving a specific and autonomous H1t promoter element (nucleotides -948/-875) may be operative to minimize expression of the H1t gene in nontesticular cells.

Testis-specific TTF-D binds to single-stranded DNA in the c-mos and Odf1 promoters and activates Odf1

We recently identified testis-specific nuclear factor binding sites in the testis-specific promoters of the c-mos gene and the Odf1 gene, which are 80% identical. Here we characterize a testis-specific nuclear factor, TTF-D, which is able to complex with both binding sites and stimulates Odf1 promoter activity. TTF-D is detectable in mouse testis as early as day 11 postpartum and contains three peptides of 22, 25, and 35 kDa in size. Surprisingly, TTF-D binds specifically to its cognate double-stranded DNA binding site as well as to its single-stranded DNA binding site. Both double-stranded and single-stranded binding site oligonucleotide DNA can specifically repress Odf1 promoter activity. Our results suggest that TTF-D is involved in positive transcription regulation of a pre-meiotic and a post-meiotic gene in the testis.

DNA-protein interactions in the CCAAT box region of the murine lactate dehydrogenase C promoter

Electrophoretic Mobility (EMSA), using oligonucleotides containing CCAAT box sequences from the murine Idhc promoter show the presence of CCAAT binding proteins in nuclear extracts from liver and testis. In a liver extract, a single shifted band is seen. However, in the testis extract, two shifts are observed, one of which may be due to a testis specific isoform of CCAAT binding factor (CBF). Southwestern analysis with an oligonucleotide probe containing these sequences reveals the presence of a protein of approximately 120 kD in the testis extract. In the liver extract, a 70-kD protein binds the probe. An antibody against HeLa CBF causes a supershift in testis nuclear extract.

The TE promoter element of the histone H1t gene is essential for transcription in transgenic mouse primary spermatocytes

Transcriptional activation of the testis-specific histone H1t gene occurs in pachytene primary spermatocytes during spermatogenesis. Specific binding of testis nuclear proteins to a rat histone H1t promoter sequence, designated the H1t/TE element, correlates with the onset of transcription. This element, located between the H1t/AC box and the H1t/CCAAT box, contains inverted repeats of a shorter element. When the native rat H1t gene along with flanking sequences, including 2453 base pairs (bp) upstream and 3784 bp downstream from the coding region, was microinjected into mouse embryos, the offspring of the resulting transgenic mice transcribed the transgene in a tissue-specific manner and only in primary spermatocytes. In the present study the TE promoter element was deleted and replaced with a heterologous stuffer DNA fragment. When the mutant rat DNA fragment was used to create transgenic mice, offspring of the mice bearing the promoter mutation did not transcribe the rat H1t gene in any tissue. On the other hand, transcription of the rat H4t transgene, which is located approximately 1.5 kilobases downstream from the H1t gene, occurred in these animals. Therefore, these studies support the hypothesis that the TE element is essential for enhanced testis-specific transcription of the H1t gene in primary spermatocytes.

Regulation of transcription of the testis-specific histone H1t gene by multiple promoter elements

This is a review of mechanisms that contribute to testis-specific transcription of the histone H1t gene. The mammalian testis-specific histone H1t gene is transcribed only in primary spermatocytes during spermatogenesis. Linker histones bind to DNA and contribute to chromatin condensation by formation of the 30 nm chromatin fiber. Furthermore, linker histones contribute to regulation of transcription of specific genes. Histone H1t, which binds more weakly to DNA than the other six known linker histones, is expressed in cells that are involved in the essential processes of crossing over and mismatch repair of DNA and in cells that undergo a dramatic alteration in gene expression. However, contributions of this linker histone to these processes are unknown. Subtle differences are found in the H1t promoter compared to the other H1 promoters. Nevertheless, several lines of evidence support the hypothesis that a sequence element designated TE that is located within the H1t promoter is essential for enhanced testis-specific transcription of this gene. Transgenic mice bearing a rat H1t transgene which contains a replacement of the TE element with stuffer DNA fail to express rat H1t mRNA. In addition, an upstream sequence appears to function as a silencer element that leads to transcriptional repression of the H1t gene in nongerminal cells. Thus, multiple promoter elements appear to contribute to regulation of transcription of the histone H1t gene.

Regulation of Pdha-2 expression is mediated by proximal promoter sequences and CpG methylation

Spermatogenesis is a complex process requiring the coordinate expression of a number of testis-specific genes. One of these, Pdha-2, codes for the murine spermatogenesis-specific isoform of the E1a subunit of the pyruvate dehydrogenase complex. To begin to delineate the mechanisms regulating its expression in vivo, we have generated transgenic mice lines carrying Pdha-2 promoter deletion constructs. Here we report that transgenic mice harboring a construct containing only 187 bp of promoter and upstream sequences (core promoter) is sufficient for directing the testis-specific expression of a chloramphenicol acetyltransferase (CAT) reporter gene. Like the endogenous Pdha-2, the CAT gene is expressed in testis in a stage-specific manner. Our studies also show a correlation between CpG methylation within the core promoter and its capacity to regulate transcription. In NIH 3T3 cell lines stably transfected with the Pdha-2 core promoter-CAT construct, high levels of CAT reporter expression are observed, whereas the endogenous Pdha-2 gene is repressed. In these cells, the CpG dinucleotides residing within the transfected promoter are hypomethylated whereas those residing in the endogenous promoter are methylated. Furthermore, promoter activity can be abated by the in vitro methylation of its CpG dinucleotides. DNase I footprint analysis indicates that at least one site for the methylation-mediated repression may occur through the ATF/cyclic AMP response element binding element located within the core promoter. Mutations within this element reduces activity to approximately 50% of the wild-type promoter activity. These results suggest that tissue-specific gene expression may be modulated by other mechanisms in addition to specific transcription factor availability and cooperativity. We propose that methylation may be a mechanism by which repression of the testis-specific Pdha-2 gene is established in somatic tissue.

Binding of nuclear proteins to a conserved histone H1t promoter element suggests an important role in testis-specific transcription

The testis-specific histone H1t gene is transcribed only in primary spermatocytes during spermatogenesis. Recently, expression of the rat gene was shown to be limited to primary spermatocytes in transgenic mice, revealing that promoter elements sufficient for regulating tissue-specific transcription were present in the cloned rat gene. In this study the mouse histone H1t gene has been cloned, and sequenced and its promoter region has been compared to the rat H1t promoter with regard to conserved elements and protein binding activity. The amino acid sequence of each of the three H1t coding region domains is conserved when compared to the homologous domain in H1t derived from other species. H1t mRNA is found only in testis, where it accumulates to a high steady-state level, and examination of enriched testis cell populations shows that expression is limited to primary spermatocytes. Protein binding assays using nuclear extracts from various mouse tissues reveal testis-specific binding to TE1 and TE2, imperfect inverted repeat elements within the larger TE element. Although the H1t promoter contains an Sp1 consensus motif within the H1t/TE element, binding of testis Sp1 to the motif could not be detected using specific anti-Sp1 antibodies.

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.

Structure of genes for sperm-specific nuclear basic protein (SP4) in Xenopus laevis

Nuclear basic proteins in sperm of Xenopus laevis consist of 6 sperm-specific proteins (SPs1-6) in addition to somatic core histones. Using a cDNA for SP4 as a probe, we cloned genomic DNA containing SP4 genes from a genomic library constructed from recombinant lambda bacteriophage containing 12.0 kbp-EcoRI digests of J-strain X. laevis liver DNA. Construction of restriction maps based on Southern blot analysis revealed the existence of a total of five SP4 genes which are arranged in a tandemly repeated array forming a cluster of simple multigenes per haploid genome, over a range of 18 kbp. Among these genes, the one located at the most upstream position differed from others in possessing a single base substitution which gave rise to a replacement of one out of 78 amino acid residues. The DNA containing the second to the fourth SP4 genes, arranged at about 3 kbp intervals each, was totally sequenced for 10,165 bp. Each gene was found to contain one intron, typical TATA and CCAAT boxes in the 5'-flanking region, and a polyadenylation signal in the 3'-flanking region. Comparative sequence analyses revealed three regions of extensive homology within the upstream non-coding region among three genes, suggesting a possible relevance to their expression at a particular phase of spermatogenesis and/or in testis.

DNA-protein binding studies in the 5' flanking region of rat proacrosin gene which is transcribed in diploid germ cells

The proacrosin gene is transcribed in diploid spermatogenic cells and translated in haploid round spermatids. In order to evaluate sequences which are involved in proacrosin gene transcription, DNA-protein interactions were analyzed in 1.2 kb of the 5'flanking region of the rat gene. 13 protein binding sites were identified by DNase I footprinting using nuclear extracts from rat testis and brain, respectively. Five footprints (F1, F3, F7, TS2, TS3) which suggest an interaction with testis specific nuclear factors were further examined by gel retardation assays. Three testis specific binding sites (F1, F7, TS2, located 472bp, 697bp and 1004bp upstream of ATG, respectively) could be identified with both methods. The binding site F1 contains a motif which is similar to a testis specific footprint found in mouse protamine 1 gene. The nucleotide sequence of F7 contains the recognition motif of an isoform of the transcription factor GATA1, which is expressed in testis. Furthermore F1 and F7 are located in that part of the 5'flanking region of the proacrosin gene, which can direct proacrosin gene expression in germ cells of male transgenic mice.

Developmentally regulated expression of linker-histone variants in vertebrates

The identification of histone H1 variants in vertebrates suggests that these proteins may have specialized functions. During embryonic development, a correspondence between the expression of each of the linker-histone variants and the proliferative and transcriptional activity of embryonic cells can be observed. Analysis of the developmentally regulated expression of these variants leads to the subdivision of these variants into distinct classes. This subdivision may also provide insight into the significance of the differential expression of variants and the roles individual linker histones have in chromatin structure and function.

Primate testicular histone H1t genes are highly conserved and the human H1t gene is located on chromosome 6

The testis-specific histone H1t gene is known to be transcribed only in pachytene primary spermatocytes during spermatogenesis. Previous studies of the rat histone H1t gene revealed a unique promoter sequence element between the H1/GC box and the H1/CCAAT box. Proteins in crude nuclear extracts of rat testis bind specifically to this sequence element and a temporal correlation exists between the appearance of these DNA binding proteins and the onset of transcription. These discoveries led to a search for histone H1t genes in other mammalian species. The human and monkey histone H1t genes were amplified from genomic DNA using the polymerase chain reaction (PCR). The amplified genes were cloned and the genomic derived inserts were sequenced using linear PCR. Both proximal promoters contained the highly conserved H1/AC box, H1/CCAAT box, and H1/TATA box found in nongerminal H1 genes. Both promoters also contained the H1/GC box and the H1t/CCTAGG sequence element between the H1/GC box and H1/CCAAT box previously seen only in the H1t promoter. Specific amplification of the human H1t gene using template DNA samples from a NIGMS human/rodent somatic cell hybrid mapping panel has shown that the human histone H1t gene is located on chromosome 6.

Histone H1t: a tissue-specific model used to study transcriptional control and nuclear function during cellular differentiation

One of the most prominent and best studied family of genes is the histone gene family. In recent years, histone gene regulation during the cell cycle of somatic cells has been studied extensively. This paper is intended to highlight and emphasize recent data concerning the tissue-specific expression of histone H1t using spermatogenesis as a model system. In this article we describe a unique DNA element within the proximal promoter of the histone H1t gene. This element has been shown to bind exclusively to nuclear proteins from pachytene spermatocytes and early spermatids. Thus, there is a strong temporal correlation between the appearance of the testis-specific DNA-binding protein and the onset of transcription of the testis-specific histone H1t gene.