Differential expression of two clusters of mouse histone genes

Mutation in the mouse histone gene Hist2h3c1 leads to degeneration of the lens vesicle and severe microphthalmia

During an ENU (N-ethyl-N-nitrosourea) mutagenesis screen, we observed a dominant small-eye mutant mouse with viable homozygotes. A corresponding mutant line was established and referred to as Aey69 (abnormality of the eye #69). Comprehensive phenotyping of the homozygous Aey69 mutants in the German Mouse Clinic revealed only a subset of statistically significant alterations between wild types and homozygous mutants. The mutation causes microphthalmia without a lens but with retinal hyperproliferation. Linkage was demonstrated to mouse chromosome 3 between the markers D3Mit188 and D3Mit11. Sequencing revealed a 358 A-> C mutation (Ile120Leu) in the Hist2h3c1 gene and a 71 T-> C (Val24Ala) mutation in the Gja8 gene. Detailed analysis of eye development in the homozygous mutant mice documented a perturbed lens development starting from the lens vesicle stage including decreasing expression of crystallins as well as of lens-specific transcription factors like PITX3 and FOXE3. In contrast, we observed an early expression of retinal progenitor cells characterized by several markers including BRN3 (retinal ganglion cells) and OTX2 (cone photoreceptors). The changes in the retina at the early embryonic stages of E11.5-E15.5 happen in parallel with apoptotic processes in the lens at the respective stages. The excessive retinal hyperproliferation is characterized by an increased level of Ki67. The hyperproliferation, however, does not disrupt the differentiation and appearance of the principal retinal cell types at postnatal stages, even if the overgrowing retina covers finally the entire bulbus of the eye. Morpholino-mediated knock-down of the hist2h3ca1 gene in zebrafish leads to a specific perturbation of lens development. When injected into zebrafish zygotes, only the mutant mouse mRNA leads to severe malformations, ranging from cyclopia to severe microphthalmia. The wild-type Hist2h3c1 mRNA can rescue the morpholino-induced defects corroborating its specific function in lens development. Based upon these data, it is concluded that the ocular function of the Hist2h3c1 gene (encoding a canonical H3.2 variant) is conserved throughout evolution. Moreover, the data highlight also the importance of Hist2h3c1 in the coordinated formation of lens and retina during eye development.

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A dominant small-eye mutant mouse is caused by a mutation in the histone gene Hist2H3c1.

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Morpholino-mediated knock-down of hist2h3ca1 in the zebrafish validated this finding.

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The mutation leads to degeneration of the lens vesicle and retina hyperproliferation.

A subset of replication-dependent histone mRNAs are expressed as polyadenylated RNAs in terminally differentiated tissues

Histone proteins are synthesized in large amounts during S-phase to package the newly replicated DNA, and are among the most stable proteins in the cell. The replication-dependent (RD)-histone mRNAs expressed during S-phase end in a conserved stem-loop rather than a polyA tail. In addition, there are replication-independent (RI)-histone genes that encode histone variants as polyadenylated mRNAs. Most variants have specific functions in chromatin, but H3.3 also serves as a replacement histone for damaged histones in long-lived terminally differentiated cells. There are no reported replacement histone genes for histones H2A, H2B or H4. We report that a subset of RD-histone genes are expressed in terminally differentiated tissues as polyadenylated mRNAs, likely serving as replacement histone genes in long-lived non-dividing cells. Expression of two genes, HIST2H2AA3 and HIST1H2BC, is conserved in mammals. They are expressed as polyadenylated mRNAs in fibroblasts differentiated in vitro, but not in serum starved fibroblasts, suggesting that their expression is part of the terminal differentiation program. There are two histone H4 genes and an H3 gene that encode mRNAs that are polyadenylated and expressed at 5- to 10-fold lower levels than the mRNAs from H2A and H2B genes, which may be replacement genes for the H3.1 and H4 proteins.

PLAZA: a comparative genomics resource to study gene and genome evolution in plants

The number of sequenced genomes of representatives within the green lineage is rapidly increasing. Consequently, comparative sequence analysis has significantly altered our view on the complexity of genome organization, gene function, and regulatory pathways. To explore all this genome information, a centralized infrastructure is required where all data generated by different sequencing initiatives is integrated and combined with advanced methods for data mining. Here, we describe PLAZA, an online platform for plant comparative genomics (http://bioinformatics.psb.ugent.be/plaza/). This resource integrates structural and functional annotation of published plant genomes together with a large set of interactive tools to study gene function and gene and genome evolution. Precomputed data sets cover homologous gene families, multiple sequence alignments, phylogenetic trees, intraspecies whole-genome dot plots, and genomic colinearity between species. Through the integration of high confidence Gene Ontology annotations and tree-based orthology between related species, thousands of genes lacking any functional description are functionally annotated. Advanced query systems, as well as multiple interactive visualization tools, are available through a user-friendly and intuitive Web interface. In addition, detailed documentation and tutorials introduce the different tools, while the workbench provides an efficient means to analyze user-defined gene sets through PLAZA's interface. In conclusion, PLAZA provides a comprehensible and up-to-date research environment to aid researchers in the exploration of genome information within the green plant lineage.

Chapter 2. Cell-cycle regulation of histone mRNA degradation in Mammalian cells: role of translation and oligouridylation

Replication-dependent histone mRNAs are coordinately regulated in parallel with DNA replication. Histone mRNAs accumulate to high levels only in S-phase cells and are degraded rapidly at the end of S phase or when DNA replication is inhibited in S-phase cells. The unique 3' end on histone mRNAs is the cis element responsible for the regulation of histone mRNA degradation. This chapter describes the approaches used to demonstrate the connection between translation of histone mRNA and its degradation as well as the pathway of histone mRNA degradation in mammalian cells. In particular, the initial step in histone mRNA degradation is attachment of an oligo(U) tail to the 3' end of histone mRNA, providing a platform for binding factors that trigger mRNA degradation.

Translation termination is involved in histone mRNA degradation when DNA replication is inhibited

The levels of replication-dependent histone mRNAs are coordinately regulated with DNA synthesis. A major regulatory step in histone mRNA metabolism is regulation of the half-life of histone mRNAs. Replication-dependent histone mRNAs are the only metazoan mRNAs that are not polyadenylated. Instead, they end with a conserved stem-loop structure, which is recognized by the stem-loop binding protein (SLBP). SLBP is required for histone mRNA processing, as well as translation. We show here, using histone mRNAs whose translation can be regulated by the iron response element, that histone mRNAs need to be actively translated for their rapid degradation following the inhibition of DNA synthesis. We also demonstrate the requirement for translation using a mutant SLBP which is inactive in translation. Histone mRNAs are not rapidly degraded when DNA synthesis is inhibited or at the end of S phase in cells expressing this mutant SLBP. Replication-dependent histone mRNAs have very short 3' untranslated regions, with the stem-loop located 30 to 70 nucleotides downstream of the translation termination codon. We show here that the stability of histone mRNAs can be modified by altering the position of the stem-loop, thereby changing the distance from the translation termination codon.

Nuclear export of metazoan replication-dependent histone mRNAs is dependent on RNA length and is mediated by TAP

Replication-dependent histone mRNAs are the only metazoan mRNAs that are not polyadenylated, ending instead in a conserved stem-loop sequence. Histone pre-mRNAs lack introns and are processed in the nucleus by a single cleavage step, which produces the mature 3' end of the mRNA. We have systematically examined the requirements for the nuclear export of a mouse histone mRNA using the Xenopus oocyte system. Histone mRNAs were efficiently exported when injected as mature mRNAs, demonstrating that the process of 3' end cleavage is not required for export factor binding. Export also does not depend on the stem-loop binding protein (SLBP) since mutations of the stem-loop that prevent SLBP binding and competition with a stem-loop RNA did not affect export. Only the length of the region upstream of the stem-loop, but not its sequence, was important for efficient export. Histone mRNA export was blocked by competition with constitutive transport element (CTE) RNA, indicating that the mRNA export receptor TAP is involved in histone mRNA export. Consistent with this observation, depletion of TAP from Drosophila cells by RNAi resulted in the restriction of mature histone mRNAs to the nucleus.

SLBP is associated with histone mRNA on polyribosomes as a component of the histone mRNP

The stem-loop binding protein (SLBP) binds the 3' end of histone mRNA and is present both in nucleus, and in the cytoplasm on the polyribosomes. SLBP participates in the processing of the histone pre-mRNA and in translation of the mature message. Histone mRNAs are rapidly degraded when cells are treated with inhibitors of DNA replication and are stabilized by inhibitors of translation, resulting in an increase in histone mRNA levels. Here, we show that SLBP is a component of the histone messenger ribonucleoprotein particle (mRNP). Histone mRNA from polyribosomes is immunoprecipitated with anti-SLBP. Most of the SLBP in cycloheximide-treated cells is present on polyribosomes as a result of continued synthesis and transport of the histone mRNP to the cytoplasm. When cells are treated with inhibitors of DNA replication, histone mRNAs are rapidly degraded but SLBP levels remain constant and SLBP is relocalized to the nucleus. SLBP remains active both in RNA binding and histone pre-mRNA processing when DNA replication is inhibited.

The oligo(A) tail on histone mRNA plays an active role in translational silencing of histone mRNA during Xenopus oogenesis

Metazoan replication-dependent histone mRNAs end in a stem-loop sequence. The one known exception is the histone mRNA in amphibian oocytes, which has a short oligo(A) tail attached to the stem-loop sequence. Amphibian oocytes also contain two proteins that bind the 3' end of histone mRNA: xSLBP1, the homologue of the mammalian SLBP, and xSLBP2, which is present only in oocytes. xSLBP2 is an inhibitor of histone mRNA translation, while xSLBP1 activates translation. The short A tail on histone mRNAs appears at stage II to III of oogenesis and is present on histone mRNAs throughout the rest of oogenesis. At oocyte maturation, the oligo(A) tail is removed and the xSLBP2 is degraded, resulting in the activation of translation of histone mRNA. Both SLBPs bind to the stem-loop with the oligo(A) tail with similar affinities. Reporter mRNAs ending in the stem-loop with or without the oligo(A) tail are translated equally well in a reticulocyte lysate, and their translation is stimulated by the presence of xSLBP1. In contrast, translation of the reporter mRNA with an oligo(A) tail is not activated in frog oocytes in response to the presence of xSLBP1. These results suggest that the oligo(A) tail is an active part of the translation repression mechanism that silences histone mRNA during oogenesis and that its removal is part of the mechanism that activates translation.

The stem-loop binding protein is required for efficient translation of histone mRNA in vivo and in vitro

Metazoan replication-dependent histone mRNAs end in a conserved stem-loop rather than in the poly(A) tail found on all other mRNAs. The 3' end of histone mRNA binds a single class of proteins, the stem-loop binding proteins (SLBP). In Xenopus, there are two SLBPs: xSLBP1, the homologue of the mammalian SLBP, which is required for processing of histone pre-mRNA, and xSLBP2, which is expressed only during oogenesis and is bound to the stored histone mRNA in Xenopus oocytes. The stem-loop is required for efficient translation of histone mRNAs and substitutes for the poly(A) tail, which is required for efficient translation of other eucaryotic mRNAs. When a rabbit reticulocyte lysate is programmed with uncapped luciferase mRNA ending in the histone stem-loop, there is a three- to sixfold increase in translation in the presence of xSLBP1 while xSLBP2 has no effect on translation. Neither SLBP affected the translation of a luciferase mRNA ending in a mutant stem-loop that does not bind SLBP. Capped luciferase mRNAs ending in the stem-loop were injected into Xenopus oocytes after overexpression of either xSLBP1 or xSLBP2. Overexpression of xSLBP1 in the oocytes stimulated translation, while overexpression of xSLBP2 reduced translation of the luciferase mRNA ending in the histone stem-loop. A small region in the N-terminal portion of xSLBP1 is required to stimulate translation both in vivo and in vitro. An MS2-human SLBP1 fusion protein can activate translation of a reporter mRNA ending in an MS2 binding site, indicating that xSLBP1 only needs to be recruited to the 3' end of the mRNA but does not need to be directly bound to the histone stem-loop to activate translation.

Developmental control of histone mRNA and dSLBP synthesis during Drosophila embryogenesis and the role of dSLBP in histone mRNA 3' end processing in vivo

In metazoans, the 3' end of histone mRNA is not polyadenylated but instead ends with a stem-loop structure that is required for cell cycle-regulated expression. The sequence of the stem-loop in the Drosophila melanogaster histone H2b, H3, and H4 genes is identical to the consensus sequence of other metazoan histone mRNAs, but the sequence of the stem-loop in the D. melanogaster histone H2a and H1 genes is novel. dSLBP binds to these novel stem-loop sequences as well as the canonical stem-loop with similar affinity. Eggs derived from females containing a viable, hypomorphic mutation in dSLBP store greatly reduced amounts of all five histone mRNAs in the egg, indicating that dSLBP is required in the maternal germ line for production of each histone mRNA. Embryos deficient in zygotic dSLBP function accumulate poly(A)(+) versions of all five histone mRNAs as a result of usage of polyadenylation signals located 3' of the stem-loop in each histone gene. Since the 3' ends of adjacent histone genes are close together, these polyadenylation signals may ensure the termination of transcription in order to prevent read-through into the next gene, which could possibly disrupt transcription or produce antisense histone mRNA that might trigger RNA interference. During early wild-type embryogenesis, ubiquitous zygotic histone gene transcription is activated at the end of the syncytial nuclear cycles during S phase of cycle 14, silenced during the subsequent G(2) phase, and then reactivated near the end of that G(2) phase in the well-described mitotic domain pattern. There is little or no dSLBP protein provided maternally in wild-type embryos, and zygotic expression of dSLBP is immediately required to process newly made histone pre-mRNA.

Assembly of U7 small nuclear ribonucleoprotein particle and histone RNA 3' processing in Xenopus egg extracts

In animals, replication-dependent histone genes are expressed in dividing somatic cells during S phase to maintain chromatin condensation. Histone mRNA 3'-end formation is an essential regulatory step producing an mRNA with a hairpin structure at the 3'-end. This requires the interaction of the U7 small nuclear ribonucleoprotein particle (snRNP) with a purine-rich spacer element and of the hairpin-binding protein with the hairpin element, respectively, in the 3'-untranslated region of histone RNA. Here, we demonstrate that bona fide histone RNA 3' processing takes place in Xenopus egg extracts in a reaction dependent on the addition of synthetic U7 RNA that is assembled into a ribonucleoprotein particle by protein components available in the extract. In addition to reconstituted U7 snRNP, Xenopus hairpin-binding protein SLBP1 is necessary for efficient processing. Histone RNA 3' processing is not affected by addition of non-destructible cyclin B, which drives the egg extract into M phase, but SLBP1 is phosphorylated in this extract. SPH-1, the Xenopus homologue of human p80-coilin found in coiled bodies, is associated with U7 snRNPs. However, this does not depend on the U7 RNA being able to process histone RNA and also occurs with U1 snRNPs; therefore, association of SPH1 cannot be considered as a hallmark of a functional U7 snRNP.

Stem-loop binding protein, the protein that binds the 3' end of histone mRNA, is cell cycle regulated by both translational and posttranslational mechanisms

The expression of the replication-dependent histone mRNAs is tightly regulated during the cell cycle. As cells progress from G(1) to S phase, histone mRNA levels increase 35-fold, and they decrease again during G(2) phase. Replication-dependent histone mRNAs are the only metazoan mRNAs that lack polyadenylated tails, ending instead in a conserved stem-loop. Much of the cell cycle regulation is posttranscriptional and is mediated by the 3' stem-loop. A 31-kDa stem-loop binding protein (SLBP) binds the 3' end of histone mRNA. The SLBP is necessary for pre-mRNA processing and accompanies the histone mRNA to the cytoplasm, where it is a component of the histone messenger RNP. We used synchronous CHO cells selected by mitotic shakeoff and HeLa cells synchronized at the G(1)/S or the M/G(1) boundary to study the regulation of SLBP during the cell cycle. In each system the amount of SLBP is regulated during the cell cycle, increasing 10- to 20-fold in the late G(1) and then decreasing in the S/G(2) border. SLBP mRNA levels are constant during the cell cycle. SLBP is regulated at the level of translation as cells progress from G(1) to S phase, and the protein is rapidly degraded as they progress into G(2). Regulation of SLBP may account for the posttranscriptional component of the cell cycle regulation of histone mRNA.

Formation of the 3' end of histone mRNA

All metazoan messenger RNAs, with the exception of the replication-dependent histone mRNAs, terminate at the 3' end with a poly(A) tail. Replication-dependent histone mRNAs end instead in a conserved 26-nucleotide sequence that contains a 16-nucleotide stem-loop. Formation of the 3' end of histone mRNA occurs by endonucleolytic cleavage of pre-mRNA releasing the mature mRNA from the chromatin template. Cleavage requires several trans-acting factors, including a protein, the stem-loop binding protein (SLBP), which binds the 26-nucleotide sequence; and a small nuclear RNP, U7 snRNP. There are probably additional factors also required for cleavage. One of the functions of the SLBP is to stabilize binding of the U7 snRNP to the histone pre-mRNA. In the nucleus, both U7 snRNP and SLBP are present in coiled bodies, structures that are associated with histone genes and may play a direct role in histone pre-mRNA processing in vivo. One of the major regulatory events in the cell cycle is regulation of histone pre-mRNA processing, which is at least partially mediated by cell-cycle regulation of the levels of the SLBP protein.

Stem-loop binding protein facilitates 3'-end formation by stabilizing U7 snRNP binding to histone pre-mRNA

The 3' end of histone mRNA is formed by an endonucleolytic cleavage of the primary transcript after a conserved stem-loop sequence. The cleavage reaction requires at least two trans-acting factors: the stem-loop binding protein (SLBP), which binds the stem-loop sequence, and the U7 snRNP that interacts with a sequence downstream from the cleavage site. Removal of SLBP from a nuclear extract abolishes 3'-end processing, and the addition of recombinant SLBP restores processing activity of the depleted extract. To determine the regions of human SLBP necessary for 3' processing, various deletion mutants of the protein were tested for their ability to complement the SLBP-depleted extract. The entire N-terminal domain and the majority of the C-terminal domain of human SLBP are dispensable for processing. The minimal protein that efficiently supports cleavage of histone pre-mRNA consists of 93 amino acids containing the 73-amino-acid RNA-binding domain and 20 amino acids located immediately next to its C terminus. Replacement of these 20 residues with an unrelated sequence in the context of the full-length SLBP reduces processing >90%. Coimmunoprecipitation experiments with the anti-SLBP antibody demonstrated that SLBP and U7 snRNP form a stable complex only in the presence of pre-mRNA substrates containing a properly positioned U7 snRNP binding site. One role of SLBP is to stabilize the interaction of the histone pre-mRNA with U7 snRNP.

The mouse histone H2a gene contains a small element that facilitates cytoplasmic accumulation of intronless gene transcripts and of unspliced HIV-1-related mRNAs

Histone mRNAs are naturally intronless and accumulate efficiently in the cytoplasm. To learn whether there are cis-acting sequences within histone genes that allow efficient cytoplasmic accumulation of RNAs, we made recombinant constructs in which sequences from the mouse H2a gene were cloned into a human beta-globin cDNA. By using transient transfection and RNase protection analysis, we demonstrate here that a 100-bp sequence within the H2a coding region permits efficient cytoplasmic accumulation of the globin cDNA transcripts. We also show that this sequence appears to suppress splicing and can functionally replace Rev and the Rev-responsive element in the cytoplasmic accumulation of unspliced HIV-1-related mRNAs. Like the Rev-responsive element, this sequence acts in an orientation-dependent manner. We thus propose that the sequence identified here may be a member of the cis-acting elements that facilitate the cytoplasmic accumulation of naturally intronless gene transcripts.

The mouse histone H1 genes: gene organization and differential regulation

There are six mouse histone H1 genes present in the histone gene cluster on mouse chromosome 13. These genes encode five histone H1 variants expressed in somatic cells, H1a to H1e, and the testis-specific H1t histone. Two of the genes that have not been assigned previously to the five somatic H1 subtypes have been identified as encoding the H1b and H1d subtypes. Three of the H1 genes, H1a, H1c and H1t, are present on an 80 kb segment of DNA that contains nine core histone genes. Two others, H1d and H1e, are present in a second patch, while the H1b gene is at least 500 kb away in a patch containing 14 core histone genes. The histone H1 genes are differentially expressed. All five genes for the somatic histone H1 proteins are expressed in exponentially growing cells. However, the levels of H1a, H1b and H1d mRNAs are greatly reduced in cells that are terminally differentiated or arrested in G0, while the H1c and H1e mRNAs continue to be expressed. In addition to the major RNA that ends at the stem-loop, the H1c gene expresses a longer, polyadenylated mRNA in differentiated cells, although in varying amounts. None of the other histone H1 genes encodes detectable amounts of polyadenylated mRNAs.

Increasing the distance between the snRNA promoter and the 3' box decreases the efficiency of snRNA 3'-end formation

Chimeric genes which contained the mouse U1b snRNA promoter, portions of the histone H2a or globin coding regions and the U1b 3'-end followed by a histone 3'-end were constructed. The distance between the U1 promoter and the U1 3' box was varied between 146 and 670 nt. The chimeric genes were introduced into CHO cells by stable transfection or into Xenopus oocytes by microinjection. The efficiency of utilization of the U1 3' box, as measured by the relative amounts of transcripts that ended at the U1 3' box and the histone 3'-end, was dependent on the distance between the promoter and 3'-end box. U1 3'-ends were formed with >90% efficiency on transcripts shorter than 200 nt, with 50-70% efficiency on transcripts of 280-400 nt and with only 10-20% efficiency on transcripts >500 nt. Essentially identical results were obtained after stable transfection of CHO cells or after injecting the genes into Xenopus oocytes. The distance between the U1 promoter and the U1 3' box must be <280 nt for efficient transcription termination at the U1 3' box, regardless of the sequence transcribed.

Efficient expression of protein coding genes from the murine U1 small nuclear RNA promoters

Few promoters are active at high levels in all cells. Of these, the majority encode structural RNAs transcribed by RNA polymerases I or III and are not accessible for the expression of proteins. An exception are the small nuclear RNAs (snRNAs) transcribed by RNA polymerase II. Although snRNA biosynthesis is unique and thought not to be compatible with synthesis of functional mRNA, we have tested these promoters for their ability to express functional mRNAs. We have used the murine U1a and U1b snRNA gene promoters to express the Escherichia coli lacZ gene and the human alpha-globin gene from either episomal or integrated templates by transfection, or infection into a variety of mammalian cell types. Equivalent expression of beta-galactosidase was obtained from < 250 nucleotides of 5'-flanking sequence containing the complete promoter of either U1 snRNA gene or from the 750-nt cytomegalovirus promoter and enhancer regions. The mRNA was accurately initiated at the U1 start site, efficiently spliced and polyadenylylated, and localized to polyribosomes. Recombinant adenovirus containing the U1b-lacZ chimeric gene transduced and expressed beta-galactosidase efficiently in human 293 cells and airway epithelial cells in culture. Viral vectors containing U1 snRNA promoters may be an attractive alternative to vectors containing viral promoters for persistent high-level expression of therapeutic genes or proteins.

Characterization of the mouse histone gene cluster on chromosome 13: 45 histone genes in three patches spread over 1Mb

The histone gene cluster on mouse chromosome 13 has been isolated and characterized. Using overlapping YAC clones containing histone genes from chromosome 13, a contig of approximately 2 Mb has been defined. It contains 45 histone genes, organized in three patches containing tightly clustered genes. An 80-kb patch (patch III) containing 12 histone genes is near one end of the contig, and a similar-sized patch (patch I) containing 15 histone genes is near the other end of the contig, located at least 500 kb from the central patch (patch II) of histone genes. The entire cluster contains six histone H1 genes, including the testis-specific histone H1t gene that maps to the middle of the cluster. All nine histone H3 genes in this cluster have been sequenced, and their level of expression determined. Each histone H3 gene is distinct, with five genes encoding the H3.2 protein subtype and four genes encoding the H3.1 protein. They are all expressed, with each histone H3 gene accounting for a small proportion of the total histone H3 mRNA.

Characterization of the 55-kb mouse histone gene cluster on chromosome 3

The histone gene cluster on mouse chromosome 3 has been isolated as a series of overlapping P1 clones, covering 110-120 kb, by probing with the histone H3-614 gene that had been mapped previously to mouse chromosome 3. There are genes for 10 core histone proteins present in a 55-kb cluster within this contig. There are three histone H3 genes, two of which are identical; four histone H2a genes, two of which are identical, one histone H4 gene; and two histone H2b genes. These histone H3 and H2a genes encode approximately 40% of the total H3 and H2a mRNA, whereas the histone H4 and histone H2b genes encode < 10% of the total H4 and H2b mRNA. There are no histone H1 genes present in this cluster. All of the histone H2a genes encode histone H2a.2 proteins (or variants of H2a.2), and account for all the H2a.2 genes in the mouse genome. All three histone H3 genes encode the histone H3.2 protein. A 21-kb region containing the adjacent H3-614 and H2a-614 genes has been duplicated and is present in an inverted repeat separated by 4.5 kb. The other two H2a genes are adjacent, with the 3' ends of their mRNAs separated by only 49 nucleotides in the DNA and the U7 snRNP binding sites separated by only 20 nucleotides. One of the histone H2b genes has lost the stem-loop sequence characteristic of the replication-dependent histone mRNAs and encodes only polyadenylated mRNAs.

Structure of a cluster of mouse histone genes

The structure of a 25 kilobase region of mouse DNA containing 6 functional histone genes and an H2a pseudogene has been determined. The sequences and levels of expression of the H3 and H2b gene as well as the sequence of the H2a pseudogene have been determined.

The coding sequences of mouse H2A and H3 histone genes contains a conserved seven nucleotide element that interacts with nuclear factors and is necessary for normal expression

Expression of replication-dependent histone genes of all classes is up-regulated coordinately at the onset of DNA synthesis. The cellular signals involved in coordinate regulation of these genes are not known. Here we report identification of an alpha element, present within the mouse histone coding region activating sequence (CRAS). We show evidence that this element is present in histone genes from two classes, H2a and H3, in the mouse. This element has two biological functions in histone gene expression, i.e. the element interacts with nuclear proteins in regulation of gene expression, as well as encoding the amino acids of the histone proteins. We present both in vivo and in vitro evidence that interaction of nuclear proteins with this element is required for normal expression. The binding site for nuclear protein(s) has been precisely defined by means of synthetic oligonucleotides, as well as DNase I protection and methylation interference. It is interesting to note that the histone CRAS alpha element is mutated in a replication-independent H3.3 gene; 5 of 7 nt in the CRAS alpha box are changed in this gene.

Isolation of two murine H1 histone genes and chromosomal mapping of the H1 gene complement

The mammalian H1 histone gene complement consists of at least seven H1 protein isoforms. These include five S-phase-dependent H1 protein subtypes and two more distantly related proteins, which are expressed upon terminal differentiation (H1o) or during the pachytene stage of spermatogenesis (H1t). In the past, three replication-dependent murine H1 genes plus the H1o and H1t genes have been isolated and characterized. In this report, we describe the sequences of two more H1 genes, and we show that all five murine replication-dependent H1 genes and the H1t gene map to the region A2-3 on Chromosome (Chr) 13. This is in agreement with our previous finding that the human H1 histone gene complement maps to 6p21.3, which corresponds to the A2-3 region on the murine Chr 13. Previous reports have shown that the replication-independent H1o genes map to syntenic regions on Chrs 22 (human H1o) and 15 (murine H1o).

Differential expression of the two types of histone H2A genes in wheat

Five histone H2A cDNA clones have been isolated from a wheat cDNA library. They were divided into two groups, termed type 1 and type 2, based on their deduced amino acid sequences and their gene expression patterns. Three type 1 clones had ORFs encoding proteins similar to angiosperm histone H2As known so far, whereas two type 2 clones encoded an identical protein, which was more similar to Norway spruce (gymnosperm) H2A than to the angiosperm H2As. The C-terminus of the type 2 H2A was shorter than that of the type 1 H2As and lacked the characteristic SPKK motif that is conserved in angiosperm H2As. Northern analysis revealed that the mRNA levels of the type 1 H2A genes were high in proliferating cells during germination and in various tissues of young seedlings, while the mRNA levels of the type 2 genes were high in non-proliferating cells in which the type 1 gene was poorly expressed. This result suggests that the expression of these two groups of H2A genes is differently regulated during development in wheat.

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.

Point mutations in the stem-loop at the 3' end of mouse histone mRNA reduce expression by reducing the efficiency of 3' end formation

Mammalian histone mRNAs end in a highly conserved stem-loop structure, with a six-base stem and a four-base loop. We have examined the effect of mutating the stem-loop on the expression of the histone mRNA in vivo by introducing the mutated histone genes into CHO cells by stable transfection. Point mutations have been introduced into the loop sequence and into the UA base pair at the top of the stem. Changing either the first or the third base of the conserved UYUN sequence in the loop to a purine greatly reduced expression, while changing both U's to purines abolished expression. A number of alterations in the stem sequence, including reversing the stem sequence, reversing the two base pairs at the base of the stem, or destroying the UA base pair at the top of the stem, also abolished expression. Changing the UA base pair to a CG or a UG base pair also reduced expression. The loss of expression is due to inefficient processing of the pre-mRNA, as judged by the efficiency of processing in vitro. Addition of a polyadenylation site or the wild-type histone processing signal downstream of a mutant stem-loop resulted in rescuing the processing of the mutant pre-histone mRNA. These results suggest that if the histone pre-mRNA is not rapidly processed, then it is degraded.

Isolation and characterization of the gene encoding histone H2A from Trypanosoma cruzi

In the present paper we report the isolation and characterization of the sequence of two genomic DNA fragments coding for the histone H2A of Trypanosoma cruzi. An analysis of the predicted amino acid sequence shows the presence of the amino-terminal motif characteristic of the H2A histones proteins and the Lys-Lys motif reported to be the site for the ubiquitin attachment. Southern blots of total parasite DNA probed with the H2A sequence suggested that the T. cruzi histone H2A gene is encoded in two independent gene clusters. The molecular karyotyping of the parasite indicated that these two clusters locate in a single chromosome of about 700 kb in length. The T. cruzi H2A mRNA is polyadenylated as are the basal histone mRNAs of higher eukaryotes and the histone mRNAs of yeast. By polymerase chain reaction amplification and sequencing and by S1 mapping we determined respectively the 5' and 3' end of the gene showing that the miniexon is added to the mRNA 71 nucleotides upstream of the ATG initiation codon and that the polyadenylation site locates in nucleotide position 773-775 close to invert repeats.

Point mutations in the stem-loop at the 3' end of mouse histone mRNA reduce expression by reducing the efficiency of 3' end formation

Mammalian histone mRNAs end in a highly conserved stem-loop structure, with a six-base stem and a four-base loop. We have examined the effect of mutating the stem-loop on the expression of the histone mRNA in vivo by introducing the mutated histone genes into CHO cells by stable transfection. Point mutations have been introduced into the loop sequence and into the UA base pair at the top of the stem. Changing either the first or the third base of the conserved UYUN sequence in the loop to a purine greatly reduced expression, while changing both U's to purines abolished expression. A number of alterations in the stem sequence, including reversing the stem sequence, reversing the two base pairs at the base of the stem, or destroying the UA base pair at the top of the stem, also abolished expression. Changing the UA base pair to a CG or a UG base pair also reduced expression. The loss of expression is due to inefficient processing of the pre-mRNA, as judged by the efficiency of processing in vitro. Addition of a polyadenylation site or the wild-type histone processing signal downstream of a mutant stem-loop resulted in rescuing the processing of the mutant pre-histone mRNA. These results suggest that if the histone pre-mRNA is not rapidly processed, then it is degraded.

Variable effects of the conserved RNA hairpin element upon 3' end processing of histone pre-mRNA in vitro

We have studied the requirements for efficient histone-specific RNA 3' processing in nuclear extract from mammalian tissue culture cells. Processing is strongly impaired by mutations in the pre-mRNA spacer element that reduce the base-pairing potential with U7 RNA. Moreover, by exchanging the hairpin and spacer elements of two differently processed H4 genes, we find that this difference is exclusively due to the spacer element. Finally, processing is inhibited by the addition of competitor RNAs, if these contain a wild-type spacer sequence, but not if their spacer element is mutated. Conversely, the importance of the hairpin for histone RNA 3' processing is highly variable: A hairpin mutant of the H4-12 gene is processed with almost wild-type efficiency in extract from K21 mouse mastocytoma cells but is strongly affected in HeLa cell extract, whereas an identical hairpin mutant of the H4-1 gene is affected in both extracts. The hairpin defect of H4-12-specific RNA in HeLa cells can be overcome by a compensatory mutation that increases the base complementarity to U7 snRNA. Very similar results were also obtained in RNA competition experiments: processing of H4-12-specific RNA can be competed by RNA carrying a wild-type hairpin element in extract from HeLa, but not K21 cells, whereas processing of H4-1-specific RNA can be competed in both extracts. With two additional histone genes we obtained results that were in one case intermediate and in the other similar to those obtained with H4-1. These results suggest that hairpin binding factor(s) can cooperatively support the ability of U7 snRNPs to form an active processing complex, but is(are) not directly involved in the processing mechanism.

Expression of histone and alkaline phosphatase genes in UMR 106-01 rat osteoblast-like cells exposed to the Hoechst dye H33342

The fluorescent Dye H33342 (H342) is a bis-benzimidazole used for intravital fluorescent staining. In this report, we found that H342 completely abolished histone 2a mRNA but had no effect on alkaline phosphatase gene expression and protein synthesis in UMR 106-01 rat osteoblast-like cells. The complete loss of histone 2a mRNA occurred after only 20 min of treatment with H342. This effect is unlikely to be a result of inhibition of DNA synthesis, which was only partly suppressed. The mechanism of the action of H342 on histone 2a mRNA is presently unknown.

The histone mRNA 3' end is required for localization of histone mRNA to polyribosomes

The final step in mRNA biosynthesis is transport of the mRNA from the nucleus to the cytoplasm. Histone genes from which the 3' stem-loop has been deleted are transcribed to give RNAs with heterogeneous 3' ends. These RNAs are localized in the nucleus and are stable. Addition of the histone 3' processing signal either on short (< 250 nts) or long (> 1000 nts) transcripts restores 3' processing and transport of the mRNA to the cytoplasm. In addition chimeric histone-U1 snRNA genes which produced RNAs with either histone or U1 3' ends were analyzed. Transcripts which ended with U1 snRNA 3' ends were not efficiently localized to polyribosomes. However, transcripts containing the same sequences including the snRNA 3' end followed by the histone 3' end were present in the cytoplasm on polyribosomes. Taken together these results suggest that the histone 3' end is required for export of histone mRNA to the cytoplasm and association of the mRNA with polyribosomes.

Multiple processing-defective mutations in a mammalian histone pre-mRNA are suppressed by compensatory changes in U7 RNA both in vivo and in vitro

To study the role of base-pairing between the mammalian U7 snRNA and the highly variable histone downstream element (HDE) during the 3'-end maturation of mammalian histone pre-mRNAs, we mutated the HDE of the mouse H2A-614 gene and assayed processing in HeLa cells both in vivo and in vitro. Either a 9-nucleotide deletion or a block substitution of pyrimidines for 6 purines within the HDE abolished all 3'-end processing. Compensatory changes were introduced into a synthetic human U7 gene, whose transcripts assemble into Sm snRNPs in vivo. Suppression of the 6-purine substitution as well as a 3-purine substitution within the HDE was obtained in vivo by coexpressing the corresponding U7 suppressor RNAs and in vitro by using nuclear extracts prepared from HeLa cells containing U7 suppressor genes. Our results not only provide genetic evidence for base-pairing between the U7 snRNP and the HDE of mammalian histone pre-mRNAs but reveal an unexpected tolerance to drastic changes in the nature of the base-paired region.

A common transcriptional activator is located in the coding region of two replication-dependent mouse histone genes

There is a region in the mouse histone H3 gene protein-encoding sequence required for high expression. The 110-nucleotide coding region activating sequence (CRAS) from codons 58 to 93 of the H3.2 gene restored expression when placed 520 nucleotides 5' of the start of transcription in the correct orientation. Since identical mRNA molecules are produced by transcription of the original deletion gene and the deletion gene with the CRAS at -520, effects of the deletions on mRNA stability or other posttranscriptional events are completely ruled out. Inversion of the CRAS sequence in its proper position in the H3 gene resulted in only a threefold increase in expression, and placing the CRAS sequence 5' of the deleted gene in the wrong orientation had no effect on expression. In-frame deletions in the coding region of an H2a.2 gene led to identification of a 105-nucleotide sequence in the coding region between amino acids 50 and 85 necessary for high expression of the gene. Additionally, insertion of the H3 CRAS into the deleted region of the H2a.2 gene restored expression of the H2a gene. Thus, the CRAS element has an orientation-dependent, position-independent effect. Gel mobility shift competition studies indicate that the same proteins interact with both the H3 and H2a CRAS elements, suggesting that a common factor is involved in expression of histone genes.

Single-copy flanking sequences in human histone gene clusters map to chromosomes 1 and 6

Two single-copy sequences flanking two different human histone gene clusters were used as probes to map these clusters by in situ hybridization. pFF435B, a unique sequence subclone derived from a lambda genomic clone (lambda HHG55) containing H2A, H2B, H3, and H4 genes, mapped to chromosome 1q21 (chi 2 = 120.99, P less than 0.001). pST519E, a single-copy sequence derived from a lambda genomic clone (lambda HHG17) containing only H3 and H4 genes, mapped to chromosome 6p21 (chi 2 = 112.62, P less than 0.001). These findings agree with previous assignments of human histone genes to chromosomes 1 and 6 and demonstrate that the single-copy flanking sequences in different human histone gene clusters are unique for different chromosomes.

A common transcriptional activator is located in the coding region of two replication-dependent mouse histone genes

There is a region in the mouse histone H3 gene protein-encoding sequence required for high expression. The 110-nucleotide coding region activating sequence (CRAS) from codons 58 to 93 of the H3.2 gene restored expression when placed 520 nucleotides 5' of the start of transcription in the correct orientation. Since identical mRNA molecules are produced by transcription of the original deletion gene and the deletion gene with the CRAS at -520, effects of the deletions on mRNA stability or other posttranscriptional events are completely ruled out. Inversion of the CRAS sequence in its proper position in the H3 gene resulted in only a threefold increase in expression, and placing the CRAS sequence 5' of the deleted gene in the wrong orientation had no effect on expression. In-frame deletions in the coding region of an H2a.2 gene led to identification of a 105-nucleotide sequence in the coding region between amino acids 50 and 85 necessary for high expression of the gene. Additionally, insertion of the H3 CRAS into the deleted region of the H2a.2 gene restored expression of the H2a gene. Thus, the CRAS element has an orientation-dependent, position-independent effect. Gel mobility shift competition studies indicate that the same proteins interact with both the H3 and H2a CRAS elements, suggesting that a common factor is involved in expression of histone genes.

Regulation of histone mRNA in the unperturbed cell cycle: evidence suggesting control at two posttranscriptional steps

The levels of histone mRNA increase 35-fold as selectively detached mitotic CHO cells progress from mitosis through G1 and into S phase. Using an exogenous gene with a histone 3' end which is not sensitive to transcriptional or half-life regulation, we show that 3' processing is regulated as cells progress from G1 to S phase. The half-life of histone mRNA is similar in G1- and S-phase cells, as measured after inhibition of transcription by actinomycin D (dactinomycin) or indirectly after stabilization by the protein synthesis inhibitor cycloheximide. Taken together, these results suggest that the change in histone mRNA levels between G1- and S-phase cells must be due to an increase in the rate of biosynthesis, a combination of changes in transcription rate and processing efficiency. In G2 phase, there is a rapid 35-fold decrease in the histone mRNA concentration which our results suggest is due primarily to an altered stability of histone mRNA. These results are consistent with a model for cell cycle regulation of histone mRNA levels in which the effects on both RNA 3' processing and transcription, rather than alterations in mRNA stability, are the major mechanisms by which low histone mRNA levels are maintained during G1.

Polyadenylated and 3' processed mRNAs are transcribed from the mouse histone H2A.X gene

We have isolated a cDNA clone encoding a mouse histone H2A.X from a cDNA library of teratocarcinoma F9 cells. The predicted amino acid sequence of this clone is 97% identical to human histone H2A.X. The first 119 residues of the mouse H2A.X were very similar (96-97%) to those of the major H2A histones (H2A.1 and H2A.2) of mouse and the long carboxy terminal sequence of H2A.X was homologous with those of several lower eukaryotes. Northern blot analysis revealed that this cDNA hybridized with two mRNAs in different sizes, 0.5 kb and 1.4 kb. The two mRNAs were present in tissue culture cells, and in spleen, thymus and testes of mice, but the ratio of abundance of the two transcripts differed in different cells and tissues. The shorter mRNA contained the highly conserved palindromic sequence typical of the 3' end of replication-dependent histone genes. The amount of this transcript was coupled to DNA synthesis and rapidly decreased in culture cells. It was synthesized just after the beginning of S-phase and degraded just after the end of S-phase. On the other hand, the longer mRNA was polyadenylated at 0.9 kb downstream from the palindromic sequence. This transcript was very stable when compared with the shorter one. These results indicate that these two mRNAs are transcribed from a single gene and maintained differently during the cell cycle, perhaps to maintain a partially replication-dependent level of histone H2A.X.

Regulation of histone mRNA in the unperturbed cell cycle: evidence suggesting control at two posttranscriptional steps

The levels of histone mRNA increase 35-fold as selectively detached mitotic CHO cells progress from mitosis through G1 and into S phase. Using an exogenous gene with a histone 3' end which is not sensitive to transcriptional or half-life regulation, we show that 3' processing is regulated as cells progress from G1 to S phase. The half-life of histone mRNA is similar in G1- and S-phase cells, as measured after inhibition of transcription by actinomycin D (dactinomycin) or indirectly after stabilization by the protein synthesis inhibitor cycloheximide. Taken together, these results suggest that the change in histone mRNA levels between G1- and S-phase cells must be due to an increase in the rate of biosynthesis, a combination of changes in transcription rate and processing efficiency. In G2 phase, there is a rapid 35-fold decrease in the histone mRNA concentration which our results suggest is due primarily to an altered stability of histone mRNA. These results are consistent with a model for cell cycle regulation of histone mRNA levels in which the effects on both RNA 3' processing and transcription, rather than alterations in mRNA stability, are the major mechanisms by which low histone mRNA levels are maintained during G1.

An intact histone 3'-processing site is required for transcription termination in a mouse histone H2a gene

A transcription termination site has been characterized between the mouse histone H2a-614 and H3-614 genes. There is a poly(A)- RNA present in small amounts in the nucleus which ends 600 nucleotides 3' to the H2a-614 gene. Nuclear transcription studies demonstrate that transcription extends at least 600 nucleotides 3' to the gene but is greatly reduced 700 nucleotides 3' to the gene. If all or part of the normal 3'-processing signal, consisting of the stem-loop and the U7 small nuclear ribonucleoprotein binding site, is deleted, transcription then continues past the putative termination site and RNAs which end at the 3' end of the downstream H3-614 gene accumulate. Insertion of a 150-nucleotide fragment containing the termination site between the histone 3' end and downstream polyadenylation sites reduces usage of polyadenylation sites 85 to 90%. Taken together these results suggest there is a transcription termination site which requires an intact histone 3'-processing signal to function.

An intact histone 3'-processing site is required for transcription termination in a mouse histone H2a gene

A transcription termination site has been characterized between the mouse histone H2a-614 and H3-614 genes. There is a poly(A)- RNA present in small amounts in the nucleus which ends 600 nucleotides 3' to the H2a-614 gene. Nuclear transcription studies demonstrate that transcription extends at least 600 nucleotides 3' to the gene but is greatly reduced 700 nucleotides 3' to the gene. If all or part of the normal 3'-processing signal, consisting of the stem-loop and the U7 small nuclear ribonucleoprotein binding site, is deleted, transcription then continues past the putative termination site and RNAs which end at the 3' end of the downstream H3-614 gene accumulate. Insertion of a 150-nucleotide fragment containing the termination site between the histone 3' end and downstream polyadenylation sites reduces usage of polyadenylation sites 85 to 90%. Taken together these results suggest there is a transcription termination site which requires an intact histone 3'-processing signal to function.

Introns in histone genes alter the distribution of 3' ends

Chimeric genes were constructed which contained either a histone or globin promoter, a human alpha-globin coding region as a cDNA or containing one or both intervening sequences, and the 3' end of a mouse histone H2a gene. The genes were introduced into mouse L cells or Chinese Hamster Ovary cells. The genes containing at least one intervening sequence produced two mRNAs in about equal amounts, one which ended at a cryptic polyadenylation site 33 nucleotides 3' to the normal histone mRNA 3' end and one which ended at the normal histone 3' end. In contrast, the same construct containing a globin cDNA yielded mRNA ending only at the correct histone 3' end. Similar proportions of polyadenylated and non-polyadenylated mRNA were obtained when the cryptic polyadenylation signal was replaced with the globin polyadenylation signal. More than 90% of the transcripts were accurately spliced. All of the unspliced transcripts had histone 3' ends.

Age-related increase in methylation of ribosomal genes and inactivation of chromosome-specific rRNA gene clusters in mouse

An age-related increase in DNA methylation of the multicopy 18S and 28S ribosomal RNA genes was found in CBA/Ca mice beginning between 6 and 18 months of age at the 5' end of these genes in liver, brain and spleen. The highest level of age-associated hypermethylation was mapped to the proximal 5' spacer domain. Silver staining of actively transcribing ribosomal genes in metaphase chromosomes from stimulated spleen cells provided cytological evidence that these mice have 3 rRNA cistrons located on chromosomes 15, 16, and 18. The ribosomal gene cluster located on chromosome 16 was preferentially inactivated in older animals. Exposure of spleen cells from older individuals to 5-azacytidine appeared to both reactivate ribosomal gene clusters and reduce rRNA gene methylation.

Physical mapping of mouse histone gene clusters

A chicken histone H2A probe has been used to isolate five different clones from a mouse genomic library in lambda Charon 4A. The size of the inserts in these clones ranged from 11 to 16 kb. The full complement of histone genes was not present in any of the five isolated clones. A physical map, for each of the five clones, has been constructed using partial- and double-restriction digests.

Modifications in molecular mechanisms associated with control of cell cycle regulated human histone gene expression during differentiation

Histone proteins are preferentially synthesized during the S-phase of the cell cycle, and the temporal and functional coupling of histone gene expression with DNA replication is mediated at both the transcriptional and posttranscriptional levels. The genes are transcribed throughout the cell cycle, and a 3-5-fold enhancement in the rate of transcription occurs during the first 2 h following initiation of DNA synthesis. Control of histone mRNA stability also accounts for some of the 20-100fold increase in cellular histone mRNA levels during S-phase and for the rapid and selective degradation of the mRNAs at the natural completion of DNA replication or when DNA synthesis is inhibited. Two segments of the proximal promoter, designated Sites I and II, influence the specificity and rate of histone gene transcription. Occupancy of Sites I and II during all periods of the cell cycle by three transacting factors (HiNF-A, HiNF-C, and HiNF-D) suggests that these protein-DNA interactions are responsible for the constitutive transcription of histone genes. Binding of HiNF-D in Site II is selectively lost, whereas occupancy of Site I by HiNF-A and -C persists when histone gene transcription is down regulated when cells terminally differentiate. These results are consistent with a primary role for interactions of HiNF-D with a proximal promoter element in rendering cell growth regulated human histone genes transcribable in proliferating cells.

The efficiency of 3'-end formation contributes to the relative levels of different histone mRNAs

Sequences at both the 5' and 3' ends of mouse histone genes contribute to the expression of individual genes. The 3' sequences required for high expression of the mouse H2a-614 gene are the same as the sequences required for 3'-end formation. When these sequences were substituted for the 3' end of the poorly expressed H2a-291 gene, expression of the H2a-291 gene was increased fivefold. A 65-nucleotide fragment containing the H2a-614 3' processing signal increased expression of the H2a-291 gene when it was placed in the proper orientation downstream of the H2a-291 3' end. The only mRNAs that accumulated from this gene ended at the H2a-291 3' end, which suggests that the transcript is sequentially processed. In an in vitro processing system, the different histone 3' ends showed different processing efficiencies, which correlated with their expression in cells. These results suggest that the efficiency of processing is important in determining the steady-state levels of individual mouse histone mRNAs.

The efficiency of 3'-end formation contributes to the relative levels of different histone mRNAs

Sequences at both the 5' and 3' ends of mouse histone genes contribute to the expression of individual genes. The 3' sequences required for high expression of the mouse H2a-614 gene are the same as the sequences required for 3'-end formation. When these sequences were substituted for the 3' end of the poorly expressed H2a-291 gene, expression of the H2a-291 gene was increased fivefold. A 65-nucleotide fragment containing the H2a-614 3' processing signal increased expression of the H2a-291 gene when it was placed in the proper orientation downstream of the H2a-291 3' end. The only mRNAs that accumulated from this gene ended at the H2a-291 3' end, which suggests that the transcript is sequentially processed. In an in vitro processing system, the different histone 3' ends showed different processing efficiencies, which correlated with their expression in cells. These results suggest that the efficiency of processing is important in determining the steady-state levels of individual mouse histone mRNAs.

Mapping of genes for inhibin subunits alpha, beta A, and beta B on human and mouse chromosomes and studies of jsd mice

Inhibin (INH) is a gonadal glycoprotein hormone that regulates pituitary FSH secretion and may also play a role in the regulation of androgen biosynthesis. There are two forms of inhibin that strongly inhibit pituitary FSH secretion. These share the same alpha subunit that is covalently linked to one of two distinct beta subunits (beta A or beta B). However, dimers of two beta subunits are potent stimulators of FSH synthesis and release in vitro. The beta subunits share extensive sequence similarity with transforming growth factor beta. Recently isolated cDNAs for all three inhibin subunits have been used to map their cognate loci on human and mouse chromosomes by Southern blot analysis of somatic cell hybrid DNAs and by in situ hybridization. INH alpha and INH beta B genes were assigned to human chromosome 2, regions q33----qter and cen----q13, respectively, and to mouse chromosome 1. The INH beta A locus was mapped to human chromosome 7p15----p14 and mouse chromosome 13. The region of mouse chromosome 1 that carries other genes known to have homologs on human chromosome 2q includes the jsd locus (for juvenile spermatogonial depletion). Adult jsd/jsd mice have elevated levels of serum FSH and their testes are devoid of spermatogonial cells. The possibility that the mutation in jsd involves the INH alpha or INH beta B gene was investigated by Southern blotting of DNA from jsd/jsd mice, and no major deletions or rearrangements were detected.