Identification of a transcript release activity acting on ternary transcription complexes containing murine RNA polymerase I

Targeting Immunometabolism in Glioblastoma

We have only recently begun to understand how cancer metabolism affects antitumor responses and immunotherapy outcomes. Certain immunometabolic targets have been actively pursued in other tumor types, however, glioblastoma research has been slow to exploit the therapeutic vulnerabilities of immunometabolism. In this review, we highlight the pathways that are most relevant to glioblastoma and focus on how these immunometabolic pathways influence tumor growth and immune suppression. We discuss hypoxia, glycolysis, tryptophan metabolism, arginine metabolism, 2-Hydroxyglutarate (2HG) metabolism, adenosine metabolism, and altered phospholipid metabolism, in order to provide an analysis and overview of the field of glioblastoma immunometabolism.

Extensive length variation in the ribosomal DNA intergenic spacer of yellow perch (Perca flavescens)

The intergenic spacer (IGS) is located between ribosomal RNA (rRNA) gene copies. Within the IGS, regulatory elements for rRNA gene transcription are found, as well as a varying number of other repetitive elements that are at the root of IGS length heterogeneity. This heterogeneity has been shown to have a functional significance through its effect on growth rate. Here, we present the structural organization of yellow perch (Perca flavescens) IGS based on its entire sequence, as well as the IGS length variation within a natural population. Yellow perch IGS structure has four discrete regions containing tandem repeat elements. For three of these regions, no specific length class was detected as allele size was seemingly normally distributed. However, for one repeat region, PCR amplification uncovered the presence of two distinctive IGS variants representing a length difference of 1116 bp. This repeat region was also devoid of any CpG sites despite a high GC content. Balanced selection may be holding the alleles in the population and would account for the high diversity of length variants observed for adjacent regions. Our study is an important precursor for further work aiming to assess the role of IGS length variation in influencing growth rate in fish.

Annotation of Differential Gene Expression in Small Yellow Follicles of a Broiler-Type Strain of Taiwan Country Chickens in Response to Acute Heat Stress

This study investigated global gene expression in the small yellow follicles (6-8 mm diameter) of broiler-type B strain Taiwan country chickens (TCCs) in response to acute heat stress. Twelve 30-wk-old TCC hens were divided into four groups: control hens maintained at 25°C and hens subjected to 38°C acute heat stress for 2 h without recovery (H2R0), with 2-h recovery (H2R2), and with 6-h recovery (H2R6). Small yellow follicles were collected for RNA isolation and microarray analysis at the end of each time point. Results showed that 69, 51, and 76 genes were upregulated and 58, 15, 56 genes were downregulated after heat treatment of H2R0, H2R2, and H2R6, respectively, using a cutoff value of two-fold or higher. Gene ontology analysis revealed that these differentially expressed genes are associated with the biological processes of cell communication, developmental process, protein metabolic process, immune system process, and response to stimuli. Upregulation of heat shock protein 25, interleukin 6, metallopeptidase 1, and metalloproteinase 13, and downregulation of type II alpha 1 collagen, discoidin domain receptor tyrosine kinase 2, and Kruppel-like factor 2 suggested that acute heat stress induces proteolytic disintegration of the structural matrix and inflamed damage and adaptive responses of gene expression in the follicle cells. These suggestions were validated through gene expression, using quantitative real-time polymerase chain reaction. Functional annotation clarified that interleukin 6-related pathways play a critical role in regulating acute heat stress responses in the small yellow follicles of TCC hens.

Roadblock termination by reb1p restricts cryptic and readthrough transcription

Widely transcribed compact genomes must cope with the major challenge of frequent overlapping or concurrent transcription events. Efficient and timely transcription termination is crucial to control pervasive transcription and prevent transcriptional interference. In yeast, transcription termination of RNA polymerase II (RNAPII) occurs via two possible pathways that both require recognition of termination signals on nascent RNA by specific factors. We describe here an additional mechanism of transcription termination for RNAPII and demonstrate its biological significance. We show that the transcriptional activator Reb1p bound to DNA is a roadblock for RNAPII, which pauses and is ubiquitinated, thus triggering termination. Reb1p-dependent termination generates a class of cryptic transcripts that are degraded in the nucleus by the exosome. We also observed transcriptional interference between neighboring genes in the absence of Reb1p. This work demonstrates the importance of roadblock termination for controlling pervasive transcription and preventing transcription through gene regulatory regions.

Binding of the termination factor Nsi1 to its cognate DNA site is sufficient to terminate RNA polymerase I transcription in vitro and to induce termination in vivo

Different models have been proposed explaining how eukaryotic gene transcription is terminated. Recently, Nsi1, a factor involved in silencing of ribosomal DNA (rDNA), was shown to be required for efficient termination of rDNA transcription by RNA polymerase I (Pol I) in the yeast Saccharomyces cerevisiae. Nsi1 contains Myb-like DNA binding domains and associates in vivo near the 3' end of rRNA genes to rDNA, but information about which and how DNA sequences might influence Nsi1-dependent termination is lacking. Here, we show that binding of Nsi1 to a stretch of 11 nucleotides in the correct orientation was sufficient to pause elongating Pol I shortly upstream of the Nsi1 binding site and to release the transcripts in vitro. The same minimal DNA element triggered Nsi1-dependent termination of pre-rRNA synthesis using an in vivo reporter assay. Termination efficiency in the in vivo system could be enhanced by inclusion of specific DNA sequences downstream of the Nsi1 binding site. These data and the finding that Nsi1 blocks efficiently only Pol I-dependent RNA synthesis in an in vitro transcription system improve our understanding of a unique mechanism of transcription termination.

Suppression of PTRF alleviates the polymicrobial sepsis induced by cecal ligation and puncture in mice

BACKGROUND

Sepsis and sepsis-associated organ failure are devastating conditions. Understanding the detailed cellular/molecular mechanisms involved in sepsis should lead to the identification of novel therapeutic targets.

METHODS

Cecal ligation and puncture (CLP) was used as a polymicrobial sepsis model in vivo to determine mortality and end-organ damage. Macrophages were adopted as the cellular model in vitro for mechanistic studies.

RESULTS

PTRF+/- mice survived longer and suffered less organ damage after CLP. Reductions in nitric oxide (NO) and iNOS biosynthesis were observed in plasma, macrophages, and vital organs in the PTRF+/- mice. Using an acute sepsis model after CLP, we found that iNOS-/- mice had a comparable level of survival as the PTRF+/- mice. Similarly, polymerase I transcript release factor (PTRF) deficiency resulted in decreased iNOS and NO/ROS production in macrophages in vitro. Mechanistically, lipopolysaccharide (LPS) enhanced the co-localization and interaction between PTRF and TLR4 in lipid rafts. Deletion of PTRF blocked formation of the TLR4/Myd88 complex after LPS. Consistent with this, lack of PTRF impaired the TLR4 signaling, as shown by the decreased p-JNK, p-ERK, and p-p38, which are upstream factors involved in iNOS transcription.

CONCLUSION

PTRF is a crucial regulator of TLR4 signaling in the development of sepsis.

RNA polymerase I termination: Where is the end?

The synthesis of ribosomal RNA (rRNA) precursor molecules by RNA polymerase I (Pol I) terminates with the dissociation of the protein-DNA-RNA ternary complex. Based on in vitro results the mechanism of Pol I termination appeared initially to be rather conserved and simple until this process was more thoroughly re-investigated in vivo. A picture emerged that Pol I termination seems to be connected to co-transcriptional processing, re-initiation of transcription and, possibly, other processes downstream of Pol I transcription units. In this article, our current understanding of the mechanism of Pol I termination and how this process might be implicated in other biological processes in yeast and mammals is summarized and discussed. This article is part of a Special Issue entitled: Transcription by Odd Pols.

The Reb1-homologue Ydr026c/Nsi1 is required for efficient RNA polymerase I termination in yeast

Several DNA cis-elements and trans-acting factors were described to be involved in transcription termination and to release the elongating RNA polymerases from their templates. Different models for the molecular mechanism of transcription termination have been suggested for eukaryotic RNA polymerase I (Pol I) from results of in vitro and in vivo experiments. To analyse the molecular requirements for yeast RNA Pol I termination, an in vivo approach was used in which efficient termination resulted in growth inhibition. This led to the identification of a Myb-like protein, Ydr026c, as bona fide termination factor, now designated Nsi1 (NTS1 silencing protein 1), since it was very recently described as silencing factor of ribosomal DNA. Possible Nsi1 functions in regard to the mechanism of transcription termination are discussed.

Polymerase I and transcript release factor (PTRF)/cavin-1 is a novel regulator of stress-induced premature senescence

According to the "free radical theory" of aging, premature senescence induced by oxidative stress contributes to organismal aging. Polymerase I and transcript release factor (PTRF)/cavin-1 is a structural protein component of caveolae, invaginations of the plasma membrane involved in signal transduction. We show that oxidative stress up-regulates PTRF/cavin-1 protein expression and promotes the interaction between PTRF/cavin-1 and caveolin-1, another structural protein component of caveolae. Consistent with these data, the number of caveolae is dramatically increased in cells subjected to oxidative stress. We demonstrate that down-regulation of PTRF/cavin-1 by shRNA significantly inhibits oxidative stress-induced premature senescence. Mechanistically, we found that PTRF/cavin-1 expression is necessary for the oxidant-induced sequestration of Mdm2, a negative regulator of p53, into caveolar membranes, away from p53, and activation of the p53/p21(Waf1/Cip1) pathway. Expression of a mutant form of PTRF/cavin-1, which fails to localize to caveolar membranes after oxidative stress, inhibits oxidative stress-induced activation of p53 and induction of premature senescence. Thus, PTRF/cavin-1 is a novel regulator of oxidative stress-induced premature senescence by acting as a link between free radicals and activation of the p53/p21(Waf1/Cip1) pathway.

MURC/Cavin-4 and cavin family members form tissue-specific caveolar complexes

Polymerase I and transcript release factor (PTRF)/Cavin is a cytoplasmic protein whose expression is obligatory for caveola formation. Using biochemistry and fluorescence resonance energy transfer–based approaches, we now show that a family of related proteins, PTRF/Cavin-1, serum deprivation response (SDR)/Cavin-2, SDR-related gene product that binds to C kinase (SRBC)/Cavin-3, and muscle-restricted coiled-coil protein (MURC)/Cavin-4, forms a multiprotein complex that associates with caveolae. This complex can constitutively assemble in the cytosol and associate with caveolin at plasma membrane caveolae. Cavin-1, but not other cavins, can induce caveola formation in a heterologous system and is required for the recruitment of the cavin complex to caveolae. The tissue-restricted expression of cavins suggests that caveolae may perform tissue-specific functions regulated by the composition of the cavin complex. Cavin-4 is expressed predominantly in muscle, and its distribution is perturbed in human muscle disease associated with Caveolin-3 dysfunction, identifying Cavin-4 as a novel muscle disease candidate caveolar protein.

Transcription termination by nuclear RNA polymerases

Gene transcription in the cell nucleus is a complex and highly regulated process. Transcription in eukaryotes requires three distinct RNA polymerases, each of which employs its own mechanisms for initiation, elongation, and termination. Termination mechanisms vary considerably, ranging from relatively simple to exceptionally complex. In this review, we describe the present state of knowledge on how each of the three RNA polymerases terminates and how mechanisms are conserved, or vary, from yeast to human.

Complete sequence of the 45-kb mouse ribosomal DNA repeat: analysis of the intergenic spacer

DNA from a single bacterial artificial chromosome clone was used to sequence the mouse ribosomal DNA intergenic spacer from the 3' end of the 45S pre-RNA to the spacer promoter (Accession No. AF441733). This made possible the assembly of a complete mouse ribosomal DNA repeat unit (45309 bp long, TPA Accession No. BK000964). Analysis of the intergenic spacer (IGS) showed a high density of simple sequence repeats and transposable elements. The IGS contains two long sequence blocks, which are repeated tandemly. Some of the sequences in these blocks are also present in other parts of the IGS. A difference in the mutation rate along the mouse IGS was observed. The significance of sequence motifs in the IGS for transcription enhancement, transcription termination, origin of replication, and nucleolar organization is discussed.

Functional dissection of RNA polymerase III termination using a peptide nucleic acid as a transcriptional roadblock

We have shown previously that a T(10) peptide nucleic acid (PNA) bound to the transcriptional terminator of a Saccharomyces cerevisiae tDNA(Ile)(TAT) gene arrests elongating yeast RNA polymerase (pol) III at a position that precedes by 20 bp the upstream end of the PNA roadblock (Dieci, G., Corradini, R., Sforza, S., Marchelli, R., and Ottonello, S. (2001) J. Biol. Chem. 276, 5720-5725). Here, a PNA-binding cassette was placed at various distances downstream of a functional tDNA(Ile) transcriptional terminator (T(6)) that is not bound by the T(10) PNA, and the effect of the PNA roadblock on RNA 3'-end formation, transcript release, and transcription reinitiation was examined. With a PNA roadblock placed as close as 5 bp downstream of the T(6) terminator, pol III could still reach the termination site and complete pre-tRNA synthesis, implying that the catalytic site-to-front edge (C-F) distance of the polymerase can shorten by >10 bp upon recognition of the terminator element. In addition, transcripts synthesized by a PNA-roadblocked terminating pol III were found to be released from transcription complexes. Interestingly, however, the same roadblock dramatically reduced the rate of transcription reinitiation. Also, when placed 5 bp downstream of a mutationally inactivated terminator element (T(3)GT(2)), the PNA roadblock restored transcription termination, thus indicating that the inactivated terminator is compromised in its ability to cause pol III pausing, but can still induce C-F distance shortening and transcript release. The latter two activities were found to be further impaired in variants of the inactivated terminator bearing fewer than three consecutive T residues (T(2)G(2)T(2) and TG(2)TGT). The data indicate that RNA polymerase pausing, C-F distance shortening, and transcript release are functionally distinguishable features of the termination process and point to the RNA release propensity of pol III as a major determinant of its remarkably high termination efficiency.

Isolation of human NURF: a regulator of Engrailed gene expression

The modification of chromatin structure is an important regulatory mechanism for developmental gene expression. Differential expression of the mammalian ISWI genes, SNF2H and SNF2L, has suggested that they possess distinct developmental roles. Here we describe the purification and characterization of the first human SNF2L-containing complex. The subunit composition suggests that it represents the human ortholog of the Drosophila nucleosome-remodeling factor (NURF) complex. Human NURF (hNURF) is enriched in brain, and we demonstrate that it regulates human Engrailed, a homeodomain protein that regulates neuronal development in the mid-hindbrain. Furthermore, we show that hNURF potentiates neurite outgrowth in cell culture. Taken together, our data suggess a role for an ISWI complex in neuronal growth.

Repression of RNA polymerase I transcription by nucleolin is independent of the RNA sequence that is transcribed

Nucleolin is one of the most abundant non-ribosomal proteins of the nucleolus. Several studies in vitro have shown that nucleolin is involved in several steps of ribosome biogenesis, including the regulation of rDNA transcription, rRNA processing, and ribosome assembly. However, the different steps of ribosome biogenesis are highly coordinated, and therefore it is not clear to what extent nucleolin is involved in each of these steps. It has been proposed that the interaction of nucleolin with the rDNA sequence and with nascent pre-rRNA leads to the blocking of RNA polymerase I (RNA pol I) transcription. To test this model and to get molecular insights into the role of nucleolin in RNA pol I transcription, we studied the function of nucleolin in Xenopus oocytes. We show that injection of a 2-4-fold excess of Xenopus or hamster nucleolin in stage VI Xenopus oocytes reduces the accumulation of 40 S pre-rRNA 3-fold, whereas transcription by RNA polymerase II and III is not affected. Direct analysis of rDNA transcription units by electron microscopy reveals that the number of polymerase complexes/rDNA unit is drastically reduced in the presence of increased amounts of nucleolin and corresponds to the level of reduction of 40 S pre-rRNA. Transcription from DNA templates containing various combinations of RNA polymerase I or II promoters in fusion with rDNA or CAT sequences was analyzed in the presence of elevated amounts of nucleolin. It was shown that nucleolin leads to transcription repression from a minimal polymerase I promoter, independently of the nature of the RNA sequence that is transcribed. Therefore, we propose that nucleolin affects RNA pol I transcription by acting directly on the transcription machinery or on the rDNA promoter sequences and not, as previously thought, through interaction with the nascent pre-rRNA.

The transcript release factor PTRF augments ribosomal gene transcription by facilitating reinitiation of RNA polymerase I

Termination of murine rDNA transcription by RNA polymerase I (Pol I) requires pausing of Pol I by terminator-bound TTF-I (transcription termination factor for Pol I), followed by dissociation of the ternary complex by PTRF (Pol I and transcript release factor). To examine the functional correlation between transcription termination and initiation, we have compared transcription on terminator-containing and terminator-less rDNA templates. We demonstrate that terminated RNA molecules are more efficiently synthesized than run-off transcripts, indicating that termination facilitates reinitiation. Transcriptional enhancement is observed in multiple- but not single-round transcription assays measuring either promoter-dependent or promoter-independent Pol I transcription. Increased synthesis of terminated transcripts is observed in crude extracts but not in a PTRF-free reconstituted transcription system, indicating that PTRF-mediated release of pre-rRNA is responsible for transcriptional enhancement. Consistent with PTRF serving an important role in modulating the efficiency of rRNA synthesis, PTRF exhibits pronounced charge heterogeneity, is phosphorylated at multiple sites and fractionates into transcriptionally active and inactive forms. The results suggest that regulation of PTRF activity may be an as yet unrecognized means to control the efficiency of ribosomal RNA synthesis.

Survey and summary: transcription by RNA polymerases I and III

The task of transcribing nuclear genes is shared between three RNA polymerases in eukaryotes: RNA polymerase (pol) I synthesizes the large rRNA, pol II synthesizes mRNA and pol III synthesizes tRNA and 5S rRNA. Although pol II has received most attention, pol I and pol III are together responsible for the bulk of transcriptional activity. This survey will summarise what is known about the process of transcription by pol I and pol III, how it happens and the proteins involved. Attention will be drawn to the similarities between the three nuclear RNA polymerase systems and also to their differences.

Vaccinia virus gene A18R DNA helicase is a transcript release factor

Prior phenotypic analysis of a vaccinia virus gene A18R mutant, Cts23, showed the synthesis of longer than wild type (Wt) length viral transcripts during the intermediate stage of infection, indicating that the A18R protein may act as a negative transcription elongation factor. The purpose of the work described here was to determine a biochemical activity for the A18R protein. Pulse-labeled transcription complexes established from intermediate virus promoters on bead-bound DNA templates were assayed for transcript release during an elongation step that contained nucleotides and various proteins. Pulse-labeled transcription complexes elongated in the presence of only nucleotides were unable to release nascent RNA. The addition of Wt extract during the elongation phase resulted in release of the nascent transcript, indicating that additional factors present in the Wt extract are capable of inducing transcript release. Extract from Cts23 or mock-infected cells was unable to induce release. The lack of release upon addition of Cts23 extract suggests that A18R is involved in release of nascent RNA. By itself, purified polyhistidine-tagged A18R protein (His-A18R) was unable to induce release; however, release did occur in the presence of purified His-A18R protein plus extract from either Cts23 or mock-infected cells. These data taken together indicate that A18R is necessary but not sufficient for release of nascent transcripts. We have also demonstrated that the combination of A18R protein and mock extract induces transcript release in an ATP-dependent manner, consistent with the fact that the A18R protein is an ATP-dependent helicase. Further analysis revealed that the release activity is not restricted to a vaccinia intermediate promoter but is observed using pulse-labeled transcription complexes initiated from all three viral gene class promoters. Therefore, we conclude that A18R and an as yet unidentified cellular factor(s) are required for the in vitro release of nascent RNA from a vaccinia virus transcription elongation complex.

Identification of a mammalian RNA polymerase I holoenzyme containing components of the DNA repair/replication system

Traditional models for transcription initiation by RNA polymerase I include a stepwise assembly of basic transcription factors/regulatory proteins on the core promoter to form a preinitiation complex. In contrast, we have identified a preassembled RNA polymerase I (RPI) complex that contains all the factors necessary and sufficient to initiate transcription from the rDNA promoter in vitro. The purified RPI holoenzyme contains the RPI homolog of TFIID, SL-1 and the rDNA transcription terminator factor (TTF-1), but lacks UBF, an activator of rDNA transcription. Certain components of the DNA repair/replication system, including Ku70/80, DNA topoisomerase I and PCNA, are also associated with the RPI complex. We have found that the holo-enzyme supported specific transcription and that specific transcription was stimulated by the RPI transcription activator UBF. These results support the hypothesis that a fraction of the RPI exists as a preassembled, transcriptionally competent complex that is readily recruited to the rDNA promoter, i.e. as a holoenzyme, and provide important new insights into the mechanisms governing initiation by RPI.

The interferon-inducible nucleolar p204 protein binds the ribosomal RNA-specific UBF1 transcription factor and inhibits ribosomal RNA transcription

p204, a member of the interferon-inducible p200 family of murine proteins, is primarily nucleolar. We generated cell lines in which p204 was inducible by muristerone. This induction resulted in retardation of cell proliferation and inhibition of rRNA transcription in vivo. Interferon treatment, resulting in p204 induction and retardation of proliferation, also caused inhibition of rRNA transcription in vivo. p204 also inhibited rRNA transcription in vitro. This inhibition was overcome by addition of UBF1, the rRNA-specific transcription factor. A direct interaction between p204 and UBF1 was revealed in vitro in pull-down assays, and in vivo by co-immunoprecipitation from cell extracts. UBF1 bound strongly to at least two regions of p204: the N-terminal segment linked to the conserved 200 amino acid a segment, and the conserved 200 amino acid b segment. Cleavage of the a or b segments into two segments (encoded by single exons) resulted in a strong decrease or loss of binding. The inhibition of rRNA transcription by p204 may be due to the inhibition by p204 of the specific DNA binding of UBF1. This was revealed in electrophoretic mobility shift, magnetic bead and footprinting assays. Thus, p204 serves as a mediator of the inhibition of rRNA transcription by interferon.

Regulation of mammalian ribosomal gene transcription by RNA polymerase I

All cells, from prokaryotes to vertebrates, synthesize vast amounts of ribosomal RNA to produce the several million new ribosomes per generation that are required to maintain the protein synthetic capacity of the daughter cells. Ribosomal gene (rDNA) transcription is governed by RNA polymerase I (Pol I) assisted by a dedicated set of transcription factors that mediate the specificity of transcription and are the targets of the pleiotrophic pathways the cell uses to adapt rRNA synthesis to cell growth. In the past few years we have begun to understand the specific functions of individual factors involved in rDNA transcription and to elucidate on a molecular level how transcriptional regulation is achieved. This article reviews our present knowledge of the molecular mechanism of rDNA transcriptional regulation.

A human RNA polymerase II transcription termination factor is a SWI2/SNF2 family member

We obtained protein sequence information from Drosophila factor 2, an ATP-dependent RNA polymerase II transcription termination factor, and discovered that it was identical to a SWI2/SNF2 family member called lodestar. Portions of putative human and Caenorhabditis elegans homologues were found in the sequence data bases and a complete cDNA for the human factor was generated using polymerase chain reaction techniques. Recombinant human factor 2 was produced in a baculovirus expression system, purified, and characterized. Similar to the authentic Drosophila factor, the human factor displayed a strong double-stranded DNA-dependent ATPase activity that was inhibited by single-stranded DNA and exhibited RNA polymerase II termination activity. Both factors were able to work on elongation complexes from either species. We discuss the mechanism of termination by factor 2 and the implications for the role of factor 2 in cellular activities.

Cloning and functional characterization of PTRF, a novel protein which induces dissociation of paused ternary transcription complexes

Termination of transcription by RNA polymerase I (Pol I) is a two-step process which involves pausing of elongating transcription complexes and release of both pre-rRNA and Pol I from the template. In mouse, pausing of elongation complexes is mediated by the transcription termination factor TTF-I bound to the 'Sal box' terminator downstream of the rDNA transcription unit. Dissociation of paused ternary complexes requires a cellular factor, termed PTRF for Pol I and transcript release factor. Here we describe the molecular cloning of a cDNA corresponding to murine PTRF. Recombinant PTRF is capable of dissociating ternary Pol I transcription complexes in vitro as revealed by release of both Pol I and nascent transcripts from the template. Consistent with its function in transcription termination, PTRF interacts with both TTF-I and Pol I. Moreover, we demonstrate specific binding of PTRF to transcripts containing the 3' end of pre-rRNA. Substitution of 3'-terminal uridylates by guanine residues abolishes PTRF binding and impairs release activity. The results reveal a network of protein-protein and protein-nucleic acid interactions that governs termination of Pol I transcription.

Terminating transcription in eukaryotes: lessons learned from RNA polymerase I

Within the past few years, the genes encoding transcription terminator proteins for RNA polymerase I (pol I) have been cloned from organisms as diverse as yeast and mammals. The availability of terminator proteins has allowed construction of in vitro transcription systems that terminate pol I at the same sites as used in vivo and thus allows study of termination mechanisms. This has resulted in a burst of information concerning pol I termination mechanisms, which this review will attempt to summarize.

RNA polymerase I transcription termination: similar mechanisms are employed by yeast and mammals

Termination of RNA polymerase I (Pol I) transcription requires the interaction of a specific DNA binding factor with terminator elements downstream of the pre-rRNA coding region. Both the terminator elements and the respective termination factors are distinct in yeast and mammals, and differences in the mechanism of transcription termination have been postulated. We have compared in vitro transcription termination of yeast and mouse Pol I using both the murine factor TTF-I, and the yeast homolog Reb1p. We show that, similar to TTF-I, Reb1p was sufficient for pausing of Pol I from either species, but was unable to cause release of the nascent transcripts from the paused ternary complex. The deficiency of Reb1p to mediate transcript release from Pol I of either species was complemented by the recently characterized murine release factor. Thus, both yeast and mouse Pol I termination requires a trans-acting factor that, in conjunction with the T-rich flanking sequence, releases the transcripts and Pol I from the template. The observation that the murine factor causes dissociation of ternary transcription complexes arrested by Reb1p suggests that the mechanism of Pol I termination is highly conserved from yeast to mammals.