Analysis of the promoter for the gene encoding the testis-specific histone H1t in a somatic cell line: evidence for cell-cycle regulation and modulation by distant upstream sequences

Delta6-desaturase (FADS2) deficiency unveils the role of omega3- and omega6-polyunsaturated fatty acids

Mammalian cell viability is dependent on the supply of the essential fatty acids (EFAs) linoleic and alpha-linolenic acid. EFAs are converted into omega3- and omega6-polyunsaturated fatty acids (PUFAs), which are essential constituents of membrane phospholipids and precursors of eicosanoids, anandamide and docosanoids. Whether EFAs, PUFAs and eicosanoids are essential for cell viability has remained elusive. Here, we show that deletion of delta6-fatty acid desaturase (FADS2) gene expression in the mouse abolishes the initial step in the enzymatic cascade of PUFA synthesis. The lack of PUFAs and eicosanoids does not impair the normal viability and lifespan of male and female fads2 -/- mice, but causes sterility. We further provide the molecular evidence for a pivotal role of PUFA-substituted membrane phospholipids in Sertoli cell polarity and blood-testis barrier, and the gap junction network between granulosa cells of ovarian follicles. The fads2 -/- mouse is an auxotrophic mutant. It is anticipated that FADS2 will become a major focus in membrane, haemostasis, inflammation and atherosclerosis research.

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.

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.

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.

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.

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.

The testis-specific histone H1t gene is strongly repressed by a G/C-rich region just downstream of the TATA Box

H1t is a testis-specific histone 1 variant restricted to the male germ line and expressed only in pachytene spermatocytes. Understanding the regulation of the H1t gene is an interesting challenge as its promoter shares all of the recognized control elements of standard somatic H1 genes, yet H1t is not expressed in somatic or in early spermatogenic cells. To investigate the mechanism of this apparent repression, we exchanged three promoter subregions between H1t and a major somatic H1 gene (H1d) by introduction of suitable restriction sites just 5' of the TATA box and 3' of the conserved H1 AC box. Hybrid promoters were joined to a lacZ reporter gene and assayed by transient transfection in NIH3T3 fibroblasts. In this system the wild type H1d promoter was 20-fold stronger than the H1t promoter. Much of this difference in activity was traced to inhibitory sequences immediately downstream of the TATA box in H1t, although sequences upstream of the H1t AC box and within the H1t 5'-untranslated region played some role as well. A series of deletions and short oligonucleotide mutations scanned across the region between the TATA box and cap site identified two tracts of C (GC box 2) as the inhibitory sequences. While both Sp1 and Sp3 bind to this region weakly in vitro, they are unlikely to be responsible for the inhibitory effect of GC box 2, and additional binding proteins (CTB-4 and CTB-5) were identified by electrophoretic mobility shift assays as better candidates for mediating the repressive effect. When repression of the H1t promoter was relieved by mutation of GC box 2, additional mutations introduced into GC box 1 upstream of the CAAT box led to a large decrease in activity, indicating that these two G/C-rich elements have opposite effects on promoter activity.

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.

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.

A H1 histone gene-specific AC-box-related element influences transcription from a major chicken H1 promoter

In comparing several histone H1 promoters, we have identified a highly conserved sequence element, 5'-TGTGTTA, located approx. 450-480 bp upstream from the cap site. This TG-box is a near perfect inverted repeat of the previously characterized AC-box (5'-AAACACA). The distance between these elements is also highly conserved. We performed transient transfection assays with cat gene reporter constructs which indicated that both the presence and correct position of the TG-box were essential for maximal expression of the chicken 02 H1 promoter. To the best of our knowledge, this study represents the first demonstration of an effect by the TG-box on transcription of a major histone-encoding H1 gene.

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.

Cell cycle controlled histone H1, H3, and H4 genes share unusual arrangements of recognition motifs for HiNF-D supporting a coordinate promoter binding mechanism

Cell cycle and growth control of the DNA binding and transactivation functions of regulatory factors provides a direct mechanism by which cells may coordinate transcription of a multitude of genes in proliferating cells. The promoters of human DNA replication dependent histone H4, H3, and H1 genes interact with at least seven distinct proteins. One of these proteins is a proliferation-specific nuclear factor, HiNF-D, that interacts with a key cis-regulatory element (H4-Site II; 41 bp) present in H4 genes. Here we describe binding sites for HiNF-D in the promoters of H3 and H1 genes using cross-competition, deletion analysis, and methylation interference assays, and we show that HiNF-D recognizes intricate arrangements of at least two sequence elements (CA- and AG-motifs). These recognition motifs are irregularly dispersed and distantly positioned in the proximal promoters (200 bp) of both the H3 and H1 genes. In all cases, these motifs either overlap or are in close proximity to other established transcriptional elements, including ATF and CCAAT sequences. Although HiNF-D can interact with low affinity to a core recognition domain, auxiliary elements in both the distal and proximal portions of each promoter cooperatively enhance HiNF-D binding. Thus, HiNF-D appears to bridge remote regulatory regions, which may juxtapose additional trans-activating proteins interacting within histone gene promoters. Consistent with observations in many cell culture systems, the interactions of HiNF-D with the H4, H3, and H1 promoters are modulated in parallel during the cessation of proliferation in both osteosarcoma cells and normal diploid osteoblasts, and these events occur in conjunction with concerted changes in histone gene expression. Thus, HiNF-D represents a candidate participant in coordinating transcriptional control of several histone gene classes.

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.

Basal level transcription of the histone H1(0) gene is mediated by a 80 bp promoter fragment

The replacement histone H1(0) of the H1 group, known to interact with general transcription factors, has been found associated with transcriptionally repressed chromatin. Transcription of the gene in F9 stem cells is low but can be stimulated by treating the cells with retinoic acid. Using mutant deletions, we now demonstrate that basal level transcription in F9 cells is mediated by an 80 bp DNA fragment, located 430 bp upstream of the TATA box, which does not include the retinoic acid responsive element (RARE) known to bind retinoic acid receptors and stimulate transcription from an heterologous promoter after retinoic acid treatment. By footprinting, DMS interference, site-directed mutagenesis and UV-cross linking techniques we demonstrate that at least two nuclear factors, with MW of 90,000 and 30,000, bind to the 80 bp fragment and that this binding is necessary for transcription. Furthermore, positioning of this fragment upstream of the HSV-tk gene promoter stimulates transcription 2-3 times over control values, far less than the activity observed for this fragment in the homologous promoter, indicating that full activity of this fragment requires sequences located in the proximal part of the promoter.

Functional study of the germinal angiotensin I-converting enzyme promoter

Polymerase chain amplification experiments indicate that the germinal specific promoter of the angiotensin I-converting enzyme (ACE) is completely extinguished in somatic tissues. Despite this very strict specificity of expression, the germinal ACE promoter is active in transient transfection experiments in two somatic cell lines and one cell line of germinal origin. The analysis of the promoter shows the existence two regulatory elements within the first 350 bp: a proximal positive element and a distal negative element.

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

The rat histone H1t gene is transcribed only in testis germinal cells. This testis-specific chromosomal protein is first synthesized during spermatogenesis in pachytene spermatocytes and the entire complement of testis histones is replaced during the midspermatid stage of spermiogenesis by positively charged transition nuclear proteins TP1 and TP2. Mobility shift assays conducted using crude nuclear protein extracts from different tissues and an 18-bp DNA sequence element within the H1t promoter as a probe reveal binding only with nuclear proteins from testis. The binding is specifically competed with an excess of the same unlabeled DNA fragment but not with heterologous competitors. A larger oligonucleotide corresponding to the same sequence element plus 18 bp of the adjacent downstream H1/CCAAT element binds nuclear proteins from all tissues tested, but a unique low mobility band is formed only with testis extracts. Protein-DNA crosslinking experiments reveal that two major polypeptides with molecular weights of approximately 13 and 30 kDa bind to the 18-bp H1t promoter sequence element. This strong correlation between the tissue where the H1t gene is transcribed and the presence of testis-specific nuclear proteins that bind to a sequence element within the testis histone H1t promoter supports the possibility that these DNA-binding proteins may participate in formation of an active transcription initiation complex with the testis H1t promoter.

Temporal correlation between the appearance of testis-specific DNA-binding proteins and the onset of transcription of the testis-specific histone H1t gene

The histone H1t gene is transcribed only in testis. Northern blot analyses reveal that transcription of the H1t gene occurs first in pachytene primary spermatocytes. Thus, there is a temporal correlation between onset of transcription of the gene and synthesis of histone H1t in primary spermatocytes during spermatogenesis. Previous studies revealed that replacement of most H1t and core histones occurs during the midspermatid stage of spermiogenesis by transition proteins TP1 and TP2. In this paper we extend our study of the specific binding of testis nuclear proteins to a unique sequence element within the H1t promoter. The relatively tight binding is competed with an excess of homologous DNA but not with a mutated element. Testis proteins from prepubertal animals do not bind to the 18-bp promoter element out proteins from enriched populations of primary spermatocytes do bind. Therefore, the temporal correlation between onset of transcription of the H1t gene and the time when the specific H1t promoter-binding proteins are detected in primary spermatocytes suggests that the DNA-binding proteins might be germinal cell-specific transcription factors that participate in formation of an active H1t transcription initiation complex. These studies present the first analysis of binding sites for testis nuclear proteins from primary spermatocytes within the promoter of a gene expressed only during this stage of spermatogenesis.