B2 RNA binds directly to RNA polymerase II to repress transcript synthesis

SINE RNA induces severe developmental defects in Arabidopsis thaliana and interacts with HYL1 (DRB1), a key member of the DCL1 complex

The proper temporal and spatial expression of genes during plant development is governed, in part, by the regulatory activities of various types of small RNAs produced by the different RNAi pathways. Here we report that transgenic Arabidopsis plants constitutively expressing the rapeseed SB1 SINE retroposon exhibit developmental defects resembling those observed in some RNAi mutants. We show that SB1 RNA interacts with HYL1 (DRB1), a double-stranded RNA-binding protein (dsRBP) that associates with the Dicer homologue DCL1 to produce microRNAs. RNase V1 protection assays mapped the binding site of HYL1 to a SB1 region that mimics the hairpin structure of microRNA precursors. We also show that HYL1, upon binding to RNA substrates, induces conformational changes that force single-stranded RNA regions to adopt a structured helix-like conformation. Xenopus laevis ADAR1, but not Arabidopsis DRB4, binds SB1 RNA in the same region as HYL1, suggesting that SINE RNAs bind only a subset of dsRBPs. Consistently, DCL4-DRB4-dependent miRNA accumulation was unchanged in SB1 transgenic Arabidopsis, whereas DCL1-HYL1-dependent miRNA and DCL1-HYL1-DCL4-DRB4-dependent tasiRNA accumulation was decreased. We propose that SINE RNA can modulate the activity of the RNAi pathways in plants and possibly in other eukaryotes.

Short interspersed elements (SINEs) are transposable elements in eukaryotic genomes that mobilize through an RNA intermediate. Recently, mammalian SINE RNAs were shown to have roles as noncoding riboregulators in stress situations or in specific tissues. Mammalian SINE RNAs modulate the level of mRNAs and proteins by interacting with key proteins involved in gene transcription and translation. Here we show that constitutive production of a plant SINE RNA induces developmental defects in Arabidopsis thaliana and that this SINE RNA interacts with HYL1, a double-stranded RNA-binding protein required for the production of microRNA and trans-acting small interfering (tasi)RNA. We mapped the binding site of HYL1 to a SINE RNA region that mimics the hairpin structure of microRNA precursors. We also found that HYL1 induces conformational changes upon binding to RNA substrates. These data suggest that SINE RNAs modulate the activity of RNAi pathways in Arabidopsis.

Genome-wide RNA polymerase II: not genes only!

RNA polymerase (Pol) II transcriptional regulation is an essential process for guiding eukaryotic gene expression. Early in vitro studies deciphered the essential steps for transcription, including recruitment, initiation, elongation and termination. Based on these findings, the idea emerged that Pol II should essentially be located on promoters or genic regions of transcribed genes. The development of in vivo localization protocols has enabled the investigation of genome-wide Pol II occupancy. Recent studies from yeast to human show that Pol II can be poised at the transcription start site or can be located outside of gene-coding regions, sometimes dependent on the growth or differentiation stage. These recent results regarding Pol II genomic location and transcription challenge our classical views of transcriptional regulation.

Modular RNA heats up

In this issue of Molecular Cell, Mariner et al. (2008) demonstrate that Alu RNA from a human SINE represses RNA polymerase II transcription during heat shock. This noncoding RNA is the first example of a "protein-like" transcription factor with a distinct modular architecture.

Human Alu RNA is a modular transacting repressor of mRNA transcription during heat shock

Noncoding RNAs (ncRNAs) have recently been discovered to regulate mRNA transcription in trans, a role traditionally reserved for proteins. The breadth of ncRNAs as transacting transcriptional regulators and the diversity of signals to which they respond are only now becoming recognized. Here we show that human Alu RNA, transcribed from short interspersed elements (SINEs), is a transacting transcriptional repressor during the cellular heat shock response. Alu RNA blocks transcription by binding RNA polymerase II (Pol II) and entering complexes at promoters in vitro and in human cells. Transcriptional repression by Alu RNA involves two loosely structured domains that are modular, a property reminiscent of classical protein transcriptional regulators. Two other SINE RNAs, human scAlu RNA and mouse B1 RNA, also bind Pol II but do not repress transcription in vitro. These studies provide an explanation for why mouse cells harbor two major classes of SINEs, whereas human cells contain only one.

TFIIB aptamers inhibit transcription by perturbing PIC formation at distinct stages

Transcription in eukaryotes is a multistep process involving the assembly and disassembly of numerous inter- and intramolecular interactions between transcription factors and nucleic acids. The roles of each of these interactions and the regions responsible for them have been identified and studied primarily by the use of mutants, which destroy the inherent properties of the interacting surface. A less intrusive but potentially effective way to study the interactions as well as the surfaces responsible for them is the use of RNA aptamers that bind to the interacting factors. Here, we report the isolation and characterization of high-affinity RNA aptamers that bind to the yeast general transcription factor TFIIB. These aptamers fall into two classes that interfere with TFIIB's interactions with either TBP or RNA polymerase II, both of which are crucial for transcription in yeast. We demonstrate the high affinity and specificity of these reagents, their effect on transcription and preinitiation complex formation and discuss their potential use to address mechanistic questions in vitro as well as in vivo.

Studies on the expression of 6S RNA from E. coli: involvement of regulators important for stress and growth adaptation

The small bacterial 6S RNA has been recognized as a transcriptional regulator, facilitating the transition from exponential to stationary growth phase by preferentially inhibiting Eσ70RNA polymerase holoenzyme transcription. Consistent with this function, the cellular concentration of 6S RNA increases with stationary phase. We have studied the underlying mechanisms responsible for the growth phase-dependent differences in 6S RNA concentration. To this aim, we have analyzed the effects of the typical bacterial growth phase and stress regulators FIS, H-NS, LRP and StpA on 6S RNA expression. Measurements of 6S RNA accumulation in strains deficient in each one of these proteins support their contribution as potential regulators. Specific binding of the four proteins to DNA fragments containing 6S RNA promoters was demonstrated by gel retardation and DNase I footprinting. Moreover,in vitrotranscription analysis with both RNA polymerase holoenzymes, Eσ70and Eσ38, demonstrated a direct inhibition of 6S RNA transcription by H-NS, StpA and LRP, while FIS seems to act as a dual regulator.In vitrotranscription in the presence of ppGpp indicates that 6S RNA promoters are not stringently regulated. Our results underline that regulation of 6S RNA transcription depends on a complex network, involving a set of bacterial regulators with general importance in the adaptation to changing growth conditions.

Non-coding RNAs regulating the transcriptional machinery

During the past decade, numerous ncRNAs (non-coding RNAs) have been identified as regulators of transcription. This review focuses on a few examples of ncRNAs that directly interact with and regulate components of the transcription machinery. Artificial RNA aptamers have been selected against components of the transcriptional machinery. The bacterial 6S RNA and the eukaryotic B2 RNA directly target RNA polymerases. The 7SK RNA, U1 snRNA (small nuclear RNA) and SRA (steroid receptor RNA activator) RNA bind to and regulate the activity of transcription factors. Xist (X-inactive-specific transcript) and roX (RNA on the X) RNAs are involved in epigenetic regulation of transcription through the recruitment of histone-modifying enzymes.

Developmentally Regulated Activation of a SINE B2 Repeat as a Domain Boundary in Organogenesis

The temporal and spatial regulation of gene expression in mammalian development is linked to the establishment of functional chromatin domains. Here, we report that tissue-specific transcription of a retrotransposon repeat in the murine growth hormone locus is required for gene activation. This repeat serves as a boundary to block the influence of repressive chromatin modifications. The repeat element is able to generate short, overlapping Pol II-and Pol III-driven transcripts, both of which are necessary and sufficient to enable a restructuring of the regulated locus into nuclear compartments. These data suggest that transcription of interspersed repetitive sequences may represent a developmental strategy for the establishment of functionally distinct domains within the mammalian genome to control gene activation.

Regulating bacterial transcription with small RNAs

In recent years, the combinations of computational and molecular approaches have led to the identification of an increasing number of small, noncoding RNAs encoded by bacteria and their plasmids and phages. Most of the characterized small RNAs have been shown to operate at a posttranscriptional level, modulating mRNA stability or translation by base-pairing with the 5' regions of the target mRNAs. However, a subset of small RNAs has been found to regulate transcription. One example is the abundant 6S RNA that has been proposed to compete for DNA binding of RNA polymerase by mimicking the open conformation of promoter DNA. Other small RNAs affect transcription termination via base-pairing interactions with sequences in the mRNA. Here, we discuss current understanding and questions regarding the roles of small RNAs in regulating transcription.

Comparative analysis of structured RNAs in S. cerevisiae indicates a multitude of different functions

Background

Non-coding RNAs (ncRNAs) are an emerging focus for both computational analysis and experimental research, resulting in a growing number of novel, non-protein coding transcripts with often unknown functions. Whole genome screens in higher eukaryotes, for example, provided evidence for a surprisingly large number of ncRNAs. To supplement these searches, we performed a computational analysis of seven yeast species and searched for new ncRNAs and RNA motifs.

Results

A comparative analysis of the genomes of seven yeast species yielded roughly 2800 genomic loci that showed the hallmarks of evolutionary conserved RNA secondary structures. A total of 74% of these regions overlapped with annotated non-coding or coding genes in yeast. Coding sequences that carry predicted structured RNA elements belong to a limited number of groups with common functions, suggesting that these RNA elements are involved in post-transcriptional regulation and/or cellular localization. About 700 conserved RNA structures were found outside annotated coding sequences and known ncRNA genes. Many of these predicted elements overlapped with UTR regions of particular classes of protein coding genes. In addition, a number of RNA elements overlapped with previously characterized antisense transcripts. Transcription of about 120 predicted elements located in promoter regions and other, previously un-annotated, intergenic regions was supported by tiling array experiments, ESTs, or SAGE data.

Conclusion

Our computational predictions strongly suggest that yeasts harbor a substantial pool of several hundred novel ncRNAs. In addition, we describe a large number of RNA structures in coding sequences and also within antisense transcripts that were previously characterized using tiling arrays.

RNA-based affinity purification reveals 7SK RNPs with distinct composition and regulation

Recent studies have uncovered an unanticipated diversity of noncoding RNAs (ncRNAs), although these studies provide limited insight into their biological significance. Numerous general methods for identification and characterization of protein interactions have been developed, but similar approaches for characterizing cellular ncRNA interactions are lacking. Here we describe RNA Affinity in Tandem (RAT), an original, entirely RNA tag-based method for affinity purification of endogenously assembled RNP complexes. We demonstrate the general utility of RAT by isolating RNPs assembled in vivo on ncRNAs transcribed by RNA polymerase II or III. Using RAT in conjunction with protein identification by mass spectrometry and protein-RNA interaction assays, we define and characterize previously unanticipated protein subunits of endogenously assembled human 7SK RNPs. We show that 7SK RNA resides in a mixed population of RNPs with different protein compositions and responses to cellular stress. Depletion of a newly identified 7SK RNP component, hnRNP K, alters the partitioning of 7SK RNA among distinct RNPs. Our results establish the utility of a generalizable RNA-based RNP affinity purification method and provide insight into 7SK RNP dynamics.

Characterization of the structure, function, and mechanism of B2 RNA, an ncRNA repressor of RNA polymerase II transcription

We previously found that the SINE-encoded mouse B2 RNA binds RNA polymerase II and represses mRNA transcription during the cellular heat-shock response. In vitro B2 RNA assembles into preinitiation complexes on promoter DNA via its interaction with the polymerase, thus rendering the complexes inactive. With the goal of understanding which regions of B2 RNA are important for high-affinity binding to RNA polymerase II and repression of transcription, we performed a structural and deletion analysis of a 178 nucleotide (nt) B2 RNA. We describe an experimentally derived secondary structure model for B2 RNA, and show that RNA polymerase II protects a specific region from RNase digestion. Deletion studies revealed that a 51-nt region of B2 RNA is sufficient for high-affinity binding to RNA polymerase II, association with preinitiation complexes, and repression of transcription in vitro, the latter of which involves a large predominately single-stranded region within the RNA. In addition, this piece of B2 RNA blocked the polymerase from properly associating with template DNA during the assembly of elongation complexes. Further deletion revealed that a 33-nt piece of B2 RNA binds RNA polymerase II, associates with preinitiation complexes, but cannot repress transcription. These results support a model in which RNA polymerase II contains a high-affinity ncRNA docking site to which a distinct region of B2 RNA binds, thereby allowing another region of the RNA to repress transcription. Moreover, the mechanism of transcriptional repression by B2 RNA likely involves disrupting critical contacts between RNA polymerase II and the promoter DNA.

6S RNA: a small RNA regulator of transcription

Appreciation for the prevalence and diversity of noncoding, small RNAs (sRNAs) has grown enormously in the past decade. A major role for sRNAs in all organisms is to regulate gene expression, often at the level of mRNA translation or stability. However, a few sRNAs have been shown to function by regulating transcription. The bacterial 6S RNA was the first sRNA shown to inhibit transcription by binding directly to the housekeeping holoenzyme form of RNA polymerase (i.e. sigma70-RNA polymerase in E. coli). It resides within the active site of RNA polymerase, blocks access to promoter DNA and, surprisingly, is used as a template for RNA synthesis. 6S RNA regulation of transcription leads to altered cell survival, perhaps by redirecting resource utilization under nutrient-limiting conditions.

Studies on the function of the riboregulator 6S RNA from E. coli: RNA polymerase binding, inhibition of in vitro transcription and synthesis of RNA-directed de novo transcripts

Escherichia coli 6S RNA represents a non-coding RNA (ncRNA), which, based on the conserved secondary structure and previous functional studies, had been suggested to interfere with transcription. Selective inhibition of sigma-70 holoenzymes, preferentially at extended -10 promoters, but not stationary-phase-specific transcription was described, suggesting a direct role of 6S RNA in the transition from exponential to stationary phase. To elucidate the underlying mechanism, we have analysed 6S RNA interactions with different forms of RNA polymerase by gel retardation and crosslinking. Preferred binding of 6S RNA to Esigma(70) was confirmed, however weaker binding to Esigma(38) was also observed. The crosslinking analysis revealed direct contact between a central 6S RNA sequence element and the beta/beta' and sigma subunits. Promoter complex formation and in vitro transcription analysis with exponential- and stationary-phase-specific promoters and the corresponding holoenzymes demonstrated that 6S RNA interferes with transcription initiation but does not generally distinguish between exponential- and stationary-phase-specific promoters. Moreover, we show for the first time that 6S RNA acts as a template for the transcription of defined RNA molecules in the absence of DNA. In conclusion, this study reveals new aspects of 6S RNA function.

Synthetic RNA circuits

Natural and engineered RNA 'parts' can perform a variety of functions, including hybridizing to targets, binding ligands and undergoing programmed conformational changes, and catalyzing reactions. These RNA parts can in turn be assembled into synthetic genetic circuits that regulate gene expression by acting either in cis or in trans on mRNAs. As more parts are discovered and engineered, it should be increasingly possible to create synthetic RNA circuits that are able to carry out complex logical operations in cells, either superimposed on or autonomous to extant gene regulation.

Recent structural studies of RNA polymerases II and III

Here, I review three new structural studies from our laboratory. First, the crystal structure of RNA polymerase (Pol) II in complex with an RNA inhibitor revealed that this RNA blocks transcription initiation by preventing DNA loading into the active-centre cleft. Secondly, the structure of the SRI (Set2 Rpb1-interacting) domain of the histone methyltransferase Set2 revealed a novel fold for specific interaction with the doubly phosphorylated CTD (C-terminal repeat domain) of Pol II. Finally, we obtained the first structural information on Pol III, in the form of an 11-subunit model obtained by combining a homology model of the nine-subunit core enzyme with a new X-ray structure of the subcomplex C17/25.

Eukaryotic regulatory RNAs: an answer to the 'genome complexity' conundrum

A large portion of the eukaryotic genome is transcribed as noncoding RNAs (ncRNAs). While once thought of primarily as "junk," recent studies indicate that a large number of these RNAs play central roles in regulating gene expression at multiple levels. The increasing diversity of ncRNAs identified in the eukaryotic genome suggests a critical nexus between the regulatory potential of ncRNAs and the complexity of genome organization. We provide an overview of recent advances in the identification and function of eukaryotic ncRNAs and the roles played by these RNAs in chromatin organization, gene expression, and disease etiology.

What is so special about oskar wild?

The amazing world of regulatory noncoding RNA has been at the center of biologists' attention in many different fields, from structural biology to transcriptional regulation and cell signaling. The latest example comes from developmental biology. A mutation in the Drosophila gene Oskar reveals a novel developmental function for the 3' untranslated region (UTR) of the oscar mRNA. This study further suggests that, when transcribed, the noncoding parts of the genome may well carry essential regulatory functions fundamental for the coordinated gene expression and development of multicellular organisms.

Common evolutionary trends for SINE RNA structures

Short interspersed elements (SINEs) and long interspersed elements (LINEs) are transposable elements in eukaryotic genomes that mobilize through an RNA intermediate. Understanding their evolution is important because of their impact on the host genome. Most eukaryotic SINEs are ancestrally related to tRNA genes, although the typical tRNA cloverleaf structure is not apparent for most SINE consensus RNAs. Using a cladistic method where RNA structural components were coded as polarized and ordered multistate characters, we showed that related structural motifs are present in most SINE RNAs from mammals, fishes and plants, suggesting common selective constraints imposed at the SINE RNA structural level. Based on these results, we propose a general multistep model for the evolution of tRNA-related SINEs in eukaryotes.

Heat-shock proteins: new keys to the development of cytoprotective therapies

As molecular chaperones, heat-shock proteins (HSPs) function to limit protein aggregation, facilitate protein refolding and chaperone other proteins. Under conditions of cellular stress, intracellular HSP levels increase in order to provide cellular protection and maintain homeostasis. Evidence exists that the HSP family may be secreted into the circulation via lipid raft-mediated, granule-mediated or exosome-mediated exocytosis in haematopoietic and tumour cells. Extracellular HSPs exert immunomodulatory activities and play an important role in innate immune activation against pathogen infection. Membrane-bound Hsp70 in tumour cells or released chaperone-tumour associated antigen complex represent a target structure for the cytolytic attack by natural killer cells or T lymphocytes. Cellular stresses induce stress granule formation to evade detrimental cellular effects, mediating preconditioning phenotype. Therefore, induction of cellular stress tolerance by preconditioning (e.g., heat shock) might be potential therapeutic targets.

RNA polymerase III transcribes human microRNAs

Prior work demonstrates that mammalian microRNA (miRNA or miR) expression requires RNA polymerase II (Pol II). However, the transcriptional requirements of many miRNAs remain untested. Our genomic analysis of miRNAs in the human chromosome 19 miRNA cluster (C19MC) revealed that they are interspersed among Alu repeats. Because Alu transcription occurs through RNA Pol III recruitment, and we found that Alu elements upstream of C19MC miRNAs retain sequences important for Pol III activity, we tested the promoter requirements of C19MC miRNAs. Chromatin immunoprecipitation and cell-free transcription assays showed that Pol III, but not Pol II, is associated with miRNA genomic sequence and sufficient for transcription. Moreover, the mature miRNA sequences of approximately 50 additional human miRNAs lie within Alu and other known repetitive elements. These findings extend the current view of miRNA origins and the transcriptional machinery driving their expression.

Human sat III and Drosophila hsr omega transcripts: a common paradigm for regulation of nuclear RNA processing in stressed cells

Exposure of cells to stressful conditions elicits a highly conserved defense mechanism termed the heat shock response, resulting in the production of specialized proteins which protect the cells against the deleterious effects of stress. The heat shock response involves not only a widespread inhibition of the ongoing transcription and activation of heat shock genes, but also important changes in post-transcriptional processing. In particular, a blockade in splicing and other post-transcriptional processing has been described following stress in different organisms, together with an altered spatial distribution of the proteins involved in these activities. However, the specific mechanisms that regulate these activities under conditions of stress are little understood. Non-coding RNA molecules are increasingly known to be involved in the regulation of various activities in the cell, ranging from chromatin structure to splicing and RNA degradation. In this review, we consider two non-coding RNAs, the hsrω transcripts in Drosophila and the sat III transcripts in human cells, that seem to be involved in the dynamics of RNA-processing factors in normal and/or stressed cells, and thus provide new paradigms for understanding transcriptional and post-transcriptional regulations in normal and stressed cells.

Gene control by large noncoding RNAs

Large noncoding RNAs (lncRNAs) have emerged as key players in regulating various fundamental cellular processes. Recent reports identify a functional lncRNA, Evf-2, that operates during development to control the expression of specific homeodomain proteins, and they provide important insights into the mechanism of cooperation between a newly discovered nuclear receptor co-repressor protein (SLIRP) and steroid receptor activator RNA. Evf-2 is the first example of lncRNA directly involved in organogenesis in vertebrates.

Genetic regulation by non-coding RNAs

Large scale cDNA sequencing and genome tiling array studies have shown that around 50% of genomic DNA in humans is transcribed, of which 2% is translated into proteins and the remaining 98% is non-coding RNAs (ncRNAs). There is mounting evidence that these ncRNAs play critical roles in regulating DNA structure, RNA expression, protein translation and protein functions through multiple genetic mechanisms, and thus affect normal development of organisms at all levels. Today, we know very little about the regulatory mechanisms and functions of these ncRNAs, which is clearly essential knowledge for understanding the secret of life. To promote this emerging research subject of critical importance, in this paper we review (1) ncRNAs' past and present, (2) regulatory mechanisms and their functions, (3) experimental strategies for identifying novel ncRNAs, (4) experimental strategies for investigating their functions, and (5) methodologies and examples of the application of ncRNAs.

6S RNA - an ancient regulator of bacterial RNA polymerase rediscovered

The bacterial riboregulator 6S RNA was one of the first non-coding RNAs to be discovered in the late 1960s, but its cellular role remained enigmatic until the year 2000. 6S RNA, only recognized to be ubiquitous among bacteria in 2005, binds to RNA polymerase in a sigma factor-dependent manner to repress transcription from a subgroup of promoters. The common feature of a double-stranded rod with a central bulge has led to the proposal that 6S RNA may mimic an open promoter complex.

RNA regulation in mammals

Noncoding RNAs (ncRNA) are ubiquitous regulatory factors affecting gene expression in all organisms. In eukaryotes ncRNAs have been shown to operate on virtually every level of transmission of genetic information. They are implicated in processes that are crucial for the correct growth and development of multicellular organisms. Changes in their expression are often related to stress conditions or associated with diseases or developmental disorders.

SV40 large T antigen up-regulates the retrotransposition frequency of viral-like 30 elements

The regulation of non-autonomous retrotransposition is not known. A recombinant bearing a hygromycin gene and a viral-like 30 (VL30) retrotransposon tagged with an enhanced green fluorescent protein (EGFP) gene-based retrotransposition cassette was constructed and used for detection of retrotransposition events. Transfection of this recombinant produced retrotransposition events, detected both by EGFP fluorescence and PCR analysis, in hygromycin-selected clones of two established simian virus 40 (SV40)-transformed mouse NIH3T3 cell lines but not in normal NIH3T3 cells. The retrotransposition potential of this recombinant, as a provirus, was studied in stably transfected NIH3T3 clones. Transfection of these clones with either a wild-type or a mutant LE1135T SV40 large T antigen gene, not expressing small t protein, induced retrotransposition events at high frequencies as measured by fluorescence-activated cell scanning (FACS). In addition, measuring retrotransposition frequencies over a period of nine days following infection with isolated SV40 particles, revealed that the frequency of retrotransposition was time-dependent and induced as early as 24 h, increasing exponentially to high levels (>10(-2) events per cell per generation) up to nine days post-infection. Furthermore, ectopic expression of a cloned MoMLV-reverse transcriptase gene also produced retrotransposition events and suggested that the large T antigen most likely acted through induction of expression of endogenous reverse transcriptase genes. Our results show a direct correlation between SV40-cell transformation and VL30 retrotransposition and provide for the first time strong evidence that SV40 large T antigen up-regulates the retrotransposition of VL30 elements.

Non-coding-RNA regulators of RNA polymerase II transcription

Several non-coding RNAs (ncRNAs) that regulate eukaryotic mRNA transcription have recently been discovered. Their mechanisms of action and biological roles are extremely diverse, which indicates that, so far, we have only had a glimpse of this new class of regulatory factor. Many surprises are likely to be revealed as further ncRNA transcriptional regulators are identified and characterized.

Beating the heat: A translation factor and an RNA mobilize the heat shock transcription factor HSF1

Cells respond to stress via orchestrated modifications in gene expression; in a recent publication in Nature, Shamovsky et al. (2006) report that an RNA and a translation factor regulate the activity of a transcriptional activator essential to the mammalian heat shock response.

Thermodynamic and kinetic characterization of ligand binding to the purine riboswitch aptamer domain

Riboswitches are cis-acting genetic regulatory elements found commonly in bacterial mRNAs that consist of a metabolite-responsive aptamer domain coupled to a regulatory switch. Purine riboswitches respond to intracellular concentrations of either adenine or guanine/hypoxanthine to control gene expression. The aptamer domain of the purine riboswitch contains a pyrimidine residue (Y74) that forms a Watson-Crick base-pairing interaction with the bound purine nucleobase ligand that discriminates between adenine and guanine. We sought to understand the structural basis of this specificity and the mechanism of ligand recognition by the purine riboswitch. Here, we present the 2,6-diaminopurine-bound structure of a C74U mutant of the xpt-pbuX guanine riboswitch, along with a detailed thermodynamic and kinetic analysis of nucleobase recognition by both the native and mutant riboswitches. These studies demonstrate clearly that the pyrimidine at position 74 is the sole determinant of purine riboswitch specificity. In addition, the mutant riboswitch binds adenine and adenine derivatives well compared with the guanine-responsive riboswitch. Under our experimental conditions, 2,6-diaminopurine binds the RNA with DeltaH=-40.3 kcal mol(-1), DeltaS=-97.6 cal mol(-1)K(-1), and DeltaG=-10.73 kcal mol(-1). A kinetic determination of the slow rate (0.15 x 10(5)M(-1)s(-1) and 2.1 x 10(5)mM(-1)s(-1) for 2-aminopurine binding the adenine-responsive mutant riboswitch and 7-deazaguanine-binding guanine riboswitch, respectively) of association under varying experimental conditions allowed us to propose a mechanism for ligand recognition by the purine riboswitch. A conformationally dynamic unliganded state for the binding pocket is stabilized first by the Watson-Crick base pairing between the ligand and Y74, and by the subsequent ordering of the J2/3 loop, enclosing the ligand within the three-way junction.

Non-coding RNA

The term non-coding RNA (ncRNA) is commonly employed for RNA that does not encode a protein, but this does not mean that such RNAs do not contain information nor have function. Although it has been generally assumed that most genetic information is transacted by proteins, recent evidence suggests that the majority of the genomes of mammals and other complex organisms is in fact transcribed into ncRNAs, many of which are alternatively spliced and/or processed into smaller products. These ncRNAs include microRNAs and snoRNAs (many if not most of which remain to be identified), as well as likely other classes of yet-to-be-discovered small regulatory RNAs, and tens of thousands of longer transcripts (including complex patterns of interlacing and overlapping sense and antisense transcripts), most of whose functions are unknown. These RNAs (including those derived from introns) appear to comprise a hidden layer of internal signals that control various levels of gene expression in physiology and development, including chromatin architecture/epigenetic memory, transcription, RNA splicing, editing, translation and turnover. RNA regulatory networks may determine most of our complex characteristics, play a significant role in disease and constitute an unexplored world of genetic variation both within and between species.

Engineering RNA-based circuits

Nucleic acids can modulate gene function by base-pairing, via the molecular recognition of proteins and metabolites, and by catalysis. This diversity of functions can be combined with the ability to engineer nucleic acids based on Watson-Crick base-pairing rules to create a modular set of molecular "tools" for biotechnological and medical interventions in cellular metabolism. However, these individual RNA-based tools are most powerful when combined into rational logical or regulatory circuits, and the circuits can in turn be evolved for optimal function. Examples of genetic circuits that control translation and transcription are herein detailed, and more complex circuits with medical applications are anticipated.

AtCyp59 is a multidomain cyclophilin from Arabidopsis thaliana that interacts with SR proteins and the C-terminal domain of the RNA polymerase II

AtCyp59 and its orthologs from different organisms belong to a family of modular proteins consisting of a peptidyl-prolyl cis-trans isomerase (PPIase) domain, followed by an RNA recognition motif (RRM), and a C-terminal domain enriched in charged amino acids. AtCyp59 was identified in a yeast two-hybrid screen as an interacting partner of the Arabidopsis SR protein SCL33/SR33. The interaction with SCL33/SR33 and with a majority of Arabidopsis SR proteins was confirmed by in vitro pull-down assays. Consistent with these interactions, AtCyp59 localizes to the cell nucleus, but it does not significantly colocalize with SR proteins in nuclear speckles. Rather, it shows a punctuate localization pattern resembling transcription sites. Indeed, by using yeast two-hybrid, in vitro pull-down, and immunoprecipitation assays, we found that AtCyp59 interacts with the C-terminal domain (CTD) of the largest subunit of RNA polymerase II. Ectopic expression of the tagged protein in Arabidopsis cell suspension resulted in highly reduced growth that is most probably due to reduced phosphorylation of the CTD. Together our data suggest a possible function of AtCyp59 in activities connecting transcription and pre-mRNA processing. We discuss our data in the context of a dynamic interplay between transcription and pre-mRNA processing.

The Y chromosome as a target for acquired and amplified genetic material in evolution

The special properties of the Y chromosome stem form the fact that it is a non-recombining degenerate derivative of the X chromosome. The absence of homologous recombination between the X and the Y chromosome leads to gradual degeneration of various Y chromosome genes on an evolutionary timescale. The absence of recombination, however, also favors the accumulation of transposable elements on the Y chromosome during its evolution, as seen with both Drosophila and mammalian Y chromosomes. Alongside these processes, the acquisition and amplification of autosomal male benefit genes occur. This review will focus on recent studies that reveal the autosome-acquired genes on the Y chromosome of both Drosophila and humans. The evolution of the acquired and amplified genes on the Y chromosome is also discussed. Molecular and comparative analyses of Y-linked repeats in the Drosophila melanogaster genome demonstrate that there was a period of their degeneration followed by a period of their integration into RNAi silencing, which was beneficial for male fertility. Finally, the function of non-coding RNA produced by amplified Y chromosome genetic elements will be discussed.

Regulatory RNAs in mammals

Recent years have brought a dramatic change in our understanding of the role of ribonucleic acids (RNAs) within the cell. In addition to the already well-known classes of RNAs that take part in the transmission of genetic information from DNA to proteins, a new highly heterogeneous group of RNA molecules has emerged. The regulatory nonprotein-coding RNAs (npcRNAs) have been shown to be involved in modulation of gene expression on both the transcriptional and post-transcriptional level. They participate in mechanisms of chromatin modification, regulation of transcription factor activity, and influencing mRNA stability, processing, and translation. npcRNAs are key factors in genetic imprinting, dosage compensation of X-chromosome-linked genes, and many processes of differentiation and development.

Probing the secondary structure of salmon SmaI SINE RNA

SmaI is a short interspersed element (SINE) of the salmon genome, and is derived from tRNA(Lys). We probed the secondary structure of SmaI SINE RNA by enzymatic cleavage and found that the RNA structure comprises three separate domains. The 5'-terminal region (the 5' domain) forms a tRNA-like cloverleaf structure, whereas the 3'-terminal region (the 3' domain) forms an extended stem-loop. The loop region is thought to be recognized by the reverse transcriptase (RT) encoded by the long interspersed element (LINE). The two structural domains are linked by a single-stranded region (the linker domain). Our melting profile analyses indicated the presence of two structural domains having different thermal stabilities, thus supporting the domain composition described above. Based on these results, we discuss the structural generality and evolutionary advantage of the domain composition of SINE RNA.

Structure of an RNA polymerase II–RNA inhibitor complex elucidates transcription regulation by noncoding RNAs

The noncoding RNA B2 and the RNA aptamer FC bind RNA polymerase (Pol) II and inhibit messenger RNA transcription initiation, but not elongation. We report the crystal structure of FC(*), the central part of FC RNA, bound to Pol II. FC(*) RNA forms a double stem-loop structure in the Pol II active center cleft. B2 RNA may bind similarly, as it competes with FC(*) RNA for Pol II interaction. Both RNA inhibitors apparently prevent the downstream DNA duplex and the template single strand from entering the cleft after DNA melting and thus interfere with open-complex formation. Elongation is not inhibited, as nucleic acids prebound in the cleft would exclude the RNA inhibitors. The structure also indicates that A-form RNA could interact with Pol II similarly to a B-form DNA promoter, as suggested for the bacterial transcription inhibitor 6S RNA.

A new frontier for molecular medicine: noncoding RNAs

It is now becoming evident that the variety of noncoding RNA (ncRNA) molecules play important roles in many cellular processes and they are not just mere intermediates in transfer of genetic information from DNA to proteins. Recent data, from the analyses of transcriptional activity of human genome, suggest that it may contain roughly equal numbers of protein- and RNA-encoding transcription units. Many of the ncRNAs described in humans as well as in other mammals have been linked, through specific chromosomal localization or expression patterns, with certain diseases including complex congenital syndromes, neurobehavioral and developmental disorders and cancer. These findings clearly indicate that an expression of genes of which end-products are RNA molecules is crucial for development, differentiation and normal functioning of the cells. The ncRNAs expression patterns can therefore be used as molecular markers for specific diagnostic methods.

An abundance of RNA regulators

The importance of small, noncoding RNAs that act as regulators of transcription, of RNA modification or stability, and of mRNA translation is becoming increasingly apparent. Here we discuss current knowledge of regulatory RNA function and review how the RNAs have been identified in a variety of organisms. Many of the regulatory RNAs act through base-pairing interactions with target RNAs. The base-pairing RNAs can be grouped into two general classes: those that are encoded on the opposite strand of their target RNAs such that they contain perfect complementarity with their targets, and those that are encoded at separate locations on the chromosome and have imperfect base-pairing potential with their targets. Other regulatory RNAs act by modifying protein activity, in some cases by mimicking the structures of other RNA or DNA molecules.

6S RNA is a widespread regulator of eubacterial RNA polymerase that resembles an open promoter

6S RNA is an abundant noncoding RNA in Escherichia coli that binds to sigma70 RNA polymerase holoenzyme to globally regulate gene expression in response to the shift from exponential growth to stationary phase. We have computationally identified >100 new 6S RNA homologs in diverse eubacterial lineages. Two abundant Bacillus subtilis RNAs of unknown function (BsrA and BsrB) and cyanobacterial 6Sa RNAs are now recognized as 6S homologs. Structural probing of E. coli 6S RNA and a B. subtilis homolog supports a common secondary structure derived from comparative sequence analysis. The conserved features of 6S RNA suggest that it binds RNA polymerase by mimicking the structure of DNA template in an open promoter complex. Interestingly, the two B. subtilis 6S RNAs are discoordinately expressed during growth, and many proteobacterial 6S RNAs could be cotranscribed with downstream homologs of the E. coli ygfA gene encoding a putative methenyltetrahydrofolate synthetase. The prevalence and robust expression of 6S RNAs emphasize their critical role in bacterial adaptation.

L1-mediated retrotransposition of murine B1 and B2 SINEs recapitulated in cultured cells

SINEs are short interspersed nucleotide elements with transpositional activity, present at a high copy number (up to a million) in mammalian genomes. They are 80-400 bp long, non-coding sequences which derive either from the 7SL RNA (e.g. human Alus, murine B1s) or tRNA (e.g. murine B2s) polymerase III-driven genes. We have previously demonstrated that Alus very efficiently divert the enzymatic machinery of the autonomous L1 LINE (long interspersed nucleotide element) retrotransposons to transpose at a high rate. Here we show, using an ex vivo assay for transposition, that both B1 and B2 SINEs can be mobilized by murine LINEs, with the hallmarks of a bona fide retrotransposition process, including target site duplications of varying lengths and integrations into A-rich sequences. Despite different phylogenetic origins, transposition of the tRNA-derived B2 sequences is as efficient as that of the human Alus, whereas that of B1s is 20-100-fold lower despite a similar high copy number of these elements in the mouse genome. We provide evidence, via an appropriate nucleotide substitution within the B1 sequence in a domain essential for its intracellular targeting, that the current B1 SINEs are not optimal for transposition, a feature most probably selected for the host sake in the course of evolution.

A highly conserved 6S RNA structure is required for regulation of transcription

6S RNA, a highly abundant noncoding RNA, regulates transcription through interaction with RNA polymerase in Escherichia coli. Computer searches identified 6S RNAs widely among gamma-proteobacteria. Biochemical approaches were required to identify more divergent 6S RNAs. Two Bacillus subtilis RNAs were found to interact with the housekeeping form of RNA polymerase, thereby establishing them as 6S RNAs. A third B. subtilis RNA was discovered with distinct RNA polymerase-binding activity. Phylogenetic comparison and analysis of mutant RNAs revealed that a conserved secondary structure containing a single-stranded central bulge within a highly double-stranded molecule was essential for 6S RNA function in vivo and in vitro. Reconstitution experiments established the marked specificity of 6S RNA interactions for sigma(70)-RNA polymerase, as well as the ability of 6S RNA to directly inhibit transcription. These data highlight the critical importance of structural characteristics for 6S RNA activity.

Chromatin marks and machines, the missing nucleosome is a theme: gene regulation up and downstream

A remarkable insight to emerge from chromatin immunoprecipitation studies is that the steps leading to chromatin remodeling and preinitiation complex (PIC) assembly differ significantly, depending upon the gene and its biological context (Cosma, 2002). However, when multiple systems are compared, the differences illuminate checkpoints and generalities that provide insights into the most salient features of mechanism. This concept dominated presentations at the 2004 Chromatin and Transcription by RNA Polymerase II meeting held at the Lake Tahoe Granlibakken Conference Center.

The profile of repeat-associated histone lysine methylation states in the mouse epigenome

Histone lysine methylation has been shown to index silenced chromatin regions at, for example, pericentric heterochromatin or of the inactive X chromosome. Here, we examined the distribution of repressive histone lysine methylation states over the entire family of DNA repeats in the mouse genome. Using chromatin immunoprecipitation in a cluster analysis representing repetitive elements, our data demonstrate the selective enrichment of distinct H3-K9, H3-K27 and H4-K20 methylation marks across tandem repeats (e.g. major and minor satellites), DNA transposons, retrotransposons, long interspersed nucleotide elements and short interspersed nucleotide elements. Tandem repeats, but not the other repetitive elements, give rise to double-stranded (ds) RNAs that are further elevated in embryonic stem (ES) cells lacking the H3-K9-specific Suv39h histone methyltransferases. Importantly, although H3-K9 tri- and H4-K20 trimethylation appear stable at the satellite repeats, many of the other repeat-associated repressive marks vary in chromatin of differentiated ES cells or of embryonic trophoblasts and fibroblasts. Our data define a profile of repressive histone lysine methylation states for the repetitive complement of four distinct mouse epigenomes and suggest tandem repeats and dsRNA as primary triggers for more stable chromatin imprints.