Fragile X syndrome is the result of transcriptional suppression of the gene FMR1 as a result of a trinucleotide repeat expansion mutation. The normal function of the FMR1 protein (FMRP) and the mechanism by which its absence leads to mental retardation are unknown. Ribonucleoprotein particle (RNP) domains were identified within FMRP, and RNA was shown to bind in stoichiometric ratios, which suggests that there are two RNA binding sites per FMRP molecule. FMRP was able to bind to its own message with high affinity (dissociation constant = 5.7 nM) and interacted with approximately 4 percent of human fetal brain messages. The absence of the normal interaction of FMRP with a subset of RNA molecules might result in the pleiotropic phenotype associated with fragile X syndrome.

A current model for transcription-coupled DNA repair is that RNA polymerase, arrested at a DNA lesion, directs the repair machinery to the transcribed strand of an active gene. To help elucidate this role of RNA polymerase, we constructed DNA templates containing the major late promoter of adenovirus and a cyclobutane pyrimidine dimer (CPD) at a specific site. CPDs, the predominant DNA lesions formed by ultraviolet radiation, are good substrates for transcriptioncoupled repair. A CPD located on the transcribed strand of the template was a strong block to polymerase movement, whereas a CPD located on the nontranscribed strand had no effect on transcription. Furthermore, the arrested polymerase shielded the CPD from recognition by photolyase, a bacterial DNA repair protein. Transcription elongation factor SIU (also called TFIIS) facilitates read-through of a variety of transcriptional pause sites by a process in which RNA polymerase II cleaves the nascent transcript before elongation resumes. We show that SU1 induces nascent transcript cleavage by RNA polymerase H stalled at a CPD. However, this cleavage does not remove the arrested polymerase from the site of the DNA lesion, nor does it facilitate translesional bypass by the polymerase. The arrested ternary complex is stable and competent to resume elongation, demonstrating that neither the polymerase nor the RNA product dissociates from the DNA template.Helix-distorting lesions are produced in cellular DNA by various endogenous and exogenous agents. Among such lesions is the cyclobutane pyrimidine dimer (CPD), the most prevalent lesion formed by short-wavelength UV radiation. CPDs can block DNA replication and transcription, leading to cell death. If unrepaired, the DNA damage can lead to mutagenesis, activation of protooncogenes, and ultimately carcinogenesis. One mechanism to remove these lesions is nucleotide excision repair (NER), common to a wide range of species from Escherichia coli to humans. The basic mechanism of NER is well understood in E. coli (1). Recognition of the damage is followed by incision of the damaged DNA strand on both sides of the lesion. The damage-containing oligonucleotide is removed, the resultant gap is filled in by DNA polymerase, and DNA ligase completes the process by joining the repair patch to the contiguous DNA strand. Although the detailed mechanism of NER in eukaryotes is not established as firmly, it appears to have the same essential features. A striking property of NER is the intragenomic heterogeneity of repair efficiency (2). Expressed genes are repaired more rapidly than the overall genome in rodent (3) and human (4) cells in culture. Furthermore, this preferential repair is largely due to efficient repair of the transcribed strand of an active gene compared to the nontranscribed strand or unexpressed DNA sequences (5). In addition to mammalian cells, preferential repair of transcribed DNA strands has been demonstrated in E. coli (6) and Saccharomyces cerevisiae (7-9), so it is likely to be a universal phenome...

Mutation of FMR1 results in fragile X mental retardation. The most common FMR1 mutation is expansion of a CGG repeat tract at the 5' end of FMR1, which leads to cytosine methylation and transcriptional silencing. Both DNA methylation and histone deacetylation have been associated with transcriptional inactivity. The finding that the methyl cytosine-binding protein MeCP2 binds to histone deacetylases and represses transcription in vivo supports a model in which MeCP2 recruits histone deacetylases to methylated DNA, resulting in histone deacetylation, chromatin condensation and transcriptional silencing. Here we demonstrate that the 5' end of FMR1 is associated with acetylated histones H3 and H4 in cells from normal individuals, but acetylation is reduced in cells from fragile X patients. Treatment of fragile X cells with 5-aza-2'-deoxycytidine (5-aza-dC) resulted in reassociation of acetylated histones H3 and H4 with FMR1 and transcriptional reactivation, whereas treatment with trichostatin A (TSA) led to almost complete acetylated histone H4 and little acetylated histone H3 reassociation with FMR1, as well as no detectable transcription. Our results represent the first description of loss of histone acetylation at a specific locus in human disease, and advance understanding of the mechanism of FMR1 transcriptional silencing.

IMP dehydrogenase (IMPDH) is the rate-limiting enzyme in the de novo synthesis of guanine nucleotides.It is a target of therapeutically useful drugs and is implicated in the regulation of cell growth rate. In the yeast Saccharomyces cerevisiae, mutations in components of the RNA polymerase II (Pol II) transcription elongation machinery confer increased sensitivity to a drug that inhibits IMPDH, 6-azauracil (6AU), by a mechanism that is poorly understood. This phenotype is thought to reflect the need for an optimally functioning transcription machinery under conditions of lowered intracellular GTP levels. Here we show that in response to the application of IMPDH inhibitors such as 6AU, wild-type yeast strains induce transcription of PUR5, one of four genes encoding IMPDH-related enzymes. Yeast elongation mutants sensitive to 6AU, such as those with a disrupted gene encoding elongation factor SII or those containing amino acid substitutions in Pol II subunits, are defective in PUR5 induction. The inability to fully induce PUR5 correlates with mutations that effect transcription elongation since 6AU-sensitive strains deleted for genes not related to transcription elongation are competent to induce PUR5. DNA encompassing the PUR5 promoter and 5 untranslated region supports 6AU induction of a luciferase reporter gene in wild-type cells. Thus, yeast sense and respond to nucleotide depletion via a mechanism of transcriptional induction that restores nucleotides to levels required for normal growth. An optimally functioning elongation machinery is critical for this response.

RNA chain elongation by RNA polymerase II (pol II) is a complex and regulated process which is coordinated with capping, splicing, and polyadenylation of the primary transcript. Numerous elongation factors that enable pol II to transcribe faster and/or more efficiently have been purified. SII is one such factor. It helps pol II bypass specific blocks to elongation that are encountered during transcript elongation. SII was first identified biochemically on the basis of its ability to enable pol II to synthesize long transcripts.(1) Both the high resolution structure of SII and the details of its novel mechanism of action have been refined through mutagenesis and sophisticated in vitro assays. SII engages transcribing pol II and assists it in bypassing blocks to elongation by stimulating a cryptic, nascent RNA cleavage activity intrinsic to RNA polymerase. The nuclease activity can also result in removal of misincorporated bases from RNA. Molecular genetic experiments in yeast suggest that SII is generally involved in mRNA synthesis in vivo and that it is one type of a growing collection of elongation factors that regulate pol II. In vertebrates, a family of related SII genes has been identified; some of its members are expressed in a tissue‐specific manner. The principal challenge now is to understand the isoform‐specific functional differences and the biology of regulation exerted by the SII family of proteins on target genes, particularly in multicellular organisms. BioEssays 22:327–336, 2000. © 2000 John Wiley & Sons, Inc.

Fragile X syndrome is caused by an expansion of a polymorphic CGG triplet repeat that results in silencing of FMR1 expression. This expansion triggers methylation of FMR1's CpG island, hypoacetylation of associated histones, and chromatin condensation, all characteristics of a transcriptionally inactive gene. Here, we show that there is a graded spectrum of histone H4 acetylation that is proportional to CGG repeat length and that correlates with responsiveness of the gene to DNA demethylation but not with chromatin condensation. We also identify alterations in patient cells of two recently identified histone H3 modifications: methylation of histone H3 at lysine 4 and methylation of histone H3 at lysine 9, which are marks for euchromatin and heterochromatin, respectively. In fragile X cells, there is a decrease in methylation of histone H3 at lysine 4 with a large increase in methylation at lysine 9, a change that is consistent with the model of FMR1's switch from euchromatin to heterochromatin in the disease state. The high level of histone H3 methylation at lysine 9 may account for the failure of H3 to be acetylated after treatment of fragile X cells with inhibitors of histone deacetylases, a treatment that fully restores acetylation to histone H4. Using 5-aza-2'-deoxycytidine, we show that DNA methylation is tightly coupled to the histone modifications associated with euchromatin but not to the heterochromatic mark of methylation of histone H3 at lysine 9, consistent with recent findings that this histone modification may direct DNA methylation. Despite the drug-induced accumulation of mRNA in patient cells to 35% of the wild-type level, FMR1 protein remained undetectable. The identification of intermediates in the heterochromatinization of FMR1 has enabled us to begin to dissect the epigenetics of silencing of a disease-related gene in its natural chromosomal context.

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Yeast RNA polymerase II enzymes containing single amino acid substitutions in the second largest subunit were analyzed in vitro for elongation-related defects. Mutants were chosen for analysis based on their ability to render yeast cells sensitive to growth on medium containing 6-azauracil. RNA polymerase II purified from three different 6-azauracil-sensitive yeast strains displayed increased arrest at well characterized arrest sites in vitro. The extent of this defect did not correlate with sensitivity to growth in the presence of 6-azauracil. The most severe effect resulted from mutation rpb2-10 (P1018S), which occurs in region H, a domain highly conserved between prokaryotic and eukaryotic RNA polymerases that is associated with nucleotide binding. The average elongation rate of this mutant enzyme is also slower than wild type. We suggest that the slowed elongation rate and an increase in dwell time of elongating pol II leads to rpb2-10's arrest-prone phenotype. This mutant enzyme can respond to SII for transcriptional read-through and carry out SII-activated nascent RNA cleavage.RNA polymerase II (pol II) 1 can encounter a variety of transcriptional blocks during the elongation phase of RNA synthesis. These include DNA sequences (reviewed in , DNA bound proteins (Deuschle et al., 1990;Izban and Luse, 1991;Kuhn et al., 1990;Reines and Mote, 1993), DNA binding drugs (Mote et al., 1994), and covalent modifications due to DNA damage (Donahue et al., 1994). In vitro studies have identified mechanisms by which elongating pol II can overcome some of these obstacles. Intrinsic DNA sequences that cause pol II to become arrested have been most extensively investigated and are frequently employed as a model in studies of transcriptional arrest.Stably arrested pol II can be reactivated for RNA chain extension by elongation factor SII. SII binds the enzyme and activates a ribonuclease activity thought to reside within pol II (reviewed in ). This ribonuclease removes a small number of nucleotides from the 3′ end of the nascent RNA prior to its re-extension through the blockage Luse, 1993a, 1993b;Gu and Reines, 1995b) in a reiterative cleavage and resynthesis process, allowing a pol II molecule multiple attempts at chain extension through an obstacle (Gu et al