L-arginine influences the structure and function of arginase mRNA in Aspergillus nidulans

Discovering riboswitches: the past and the future

Riboswitches are structured noncoding RNA domains used by many bacteria to monitor the concentrations of target ligands and regulate gene expression accordingly. In the past 20 years over 55 distinct classes of natural riboswitches have been discovered that selectively sense small molecules or elemental ions, and thousands more are predicted to exist. Evidence suggests that some riboswitches might be direct descendants of the RNA-based sensors and switches that were likely present in ancient organisms before the evolutionary emergence of proteins. We provide an overview of the current state of riboswitch research, focusing primarily on the discovery of riboswitches, and speculate on the major challenges facing researchers in the field.

Former orphan riboswitches reveal unexplored areas of bacterial metabolism, signaling, and gene control processes

Comparative sequence analyses have been used to discover numerous classes of structured noncoding RNAs, some of which are riboswitches that specifically recognize small-molecule or elemental ion ligands and influence expression of adjacent downstream genes. Determining the correct identity of the ligand for a riboswitch candidate typically is aided by an understanding of the genes under its regulatory control. Riboswitches whose ligands were straightforward to identify have largely been associated with well-characterized metabolic pathways, such as coenzyme or amino acid biosynthesis. Riboswitch candidates whose ligands resist identification, collectively known as orphan riboswitches, are often associated with genes coding for proteins of unknown function, or genes for various proteins with no established link to one another. The cognate ligands for 16 former orphan riboswitch motifs have been identified to date. The successful pursuit of the ligands for these classes has provided insight into areas of biology that are not yet fully explored, such as ion homeostasis, signaling networks, and other previously underappreciated biochemical or physiological processes. Herein we discuss the strategies and methods used to match ligands with orphan riboswitch classes, and overview the lessons learned to inform and motivate ongoing efforts to identify ligands for the many remaining candidates.

Nitric oxide homeostasis is required for light-dependent regulation of conidiation in Aspergillus

Nitric oxide (NO) can be biologically synthesized from nitrite or from arginine. Although NO is involved as a signal in many biological processes in bacteria, plants, and mammals, still little is known about the role of NO in fungi. Here we show that NO levels are regulated by light as an environmental signal in Aspergillus nidulans. The flavohaemoglobin-encoding fhbB gene involved in NO oxidation to nitrate, and the arginine-regulated arginase encoded by agaA, which controls the intracellular concentration of arginine, are both up-regulated by light. The phytochrome fphA is required for the light-dependent induction of fhbB and agaA, while the white-collar gene lreA acts as a repressor when arginine is present in the media. The intracellular arginine pools increase upon induction of both developmental programs (conidiation and sexual development), and the increase is higher under conditions promoting sexual development. The presence of low concentrations of arginine does not affect the light-dependent regulation of conidiation, but high concentrations of arginine overrun the light signal. Deletion of fhbB results in the partial loss of the light regulation of conidiation on arginine and on nitrate media, while deletion of fhbA only affects the light regulation of conidiation on nitrate media. Our working model considers a cross-talk between environmental cues and intracellular signals to regulate fungal reproduction.

Cellular conditions of weakly chelated magnesium ions strongly promote RNA stability and catalysis

Most RNA folding studies have been performed under non-physiological conditions of high concentrations (≥10 mM) of Mg²⁺_free, while actual cellular concentrations of Mg²⁺_free are only ~1 mM in a background of greater than 50 mM Mg²⁺_total. To uncover cellular behavior of RNA, we devised cytoplasm mimic systems that include biological concentrations of amino acids, which weakly chelate Mg²⁺. Amino acid-chelated Mg²⁺ (aaCM) of ~15 mM dramatically increases RNA folding and prevents RNA degradation. Furthermore, aaCM enhance self-cleavage of several different ribozymes, up to 100,000-fold at Mg²⁺_free of just 0.5 mM, indirectly through RNA compaction. Other metabolites that weakly chelate magnesium offer similar beneficial effects, which implies chelated magnesium may enhance RNA function in the cell in the same way. Overall, these results indicate that the states of Mg²⁺ should not be limited to free and bound only, as weakly bound Mg²⁺ strongly promotes RNA function under cellular conditions.

Challenges of ligand identification for the second wave of orphan riboswitch candidates

Orphan riboswitch candidates are noncoding RNA motifs whose representatives are believed to function as genetic regulatory elements, but whose target ligands have yet to be identified. The study of certain orphans, particularly classes that have resisted experimental validation for many years, has led to the discovery of important biological pathways and processes once their ligands were identified. Previously, we highlighted details for four of the most common and intriguing orphan riboswitch candidates. This facilitated the validation of riboswitches for the signaling molecules c-di-AMP, ZTP, and ppGpp, the metal ion Mn2+, and the metabolites guanidine and PRPP. Such studies also yield useful linkages between the ligands sensed by the riboswitches and numerous biochemical pathways. In the current report, we describe the known characteristics of 30 distinct classes of orphan riboswitch candidates - some of which have remained unsolved for over a decade. We also discuss the prospects for uncovering novel biological insights via focused studies on these RNAs. Lastly, we make recommendations for experimental objectives along the path to finding ligands for these mysterious RNAs.

Riboswitch diversity and distribution

Riboswitches are commonly used by bacteria to detect a variety of metabolites and ions to regulate gene expression. To date, nearly 40 different classes of riboswitches have been discovered, experimentally validated, and modeled at atomic resolution in complex with their cognate ligands. The research findings produced since the first riboswitch validation reports in 2002 reveal that these noncoding RNA domains exploit many different structural features to create binding pockets that are extremely selective for their target ligands. Some riboswitch classes are very common and are present in bacteria from nearly all lineages, whereas others are exceedingly rare and appear in only a few species whose DNA has been sequenced. Presented herein are the consensus sequences, structural models, and phylogenetic distributions for all validated riboswitch classes. Based on our findings, we predict that there are potentially many thousands of distinct bacterial riboswitch classes remaining to be discovered, but that the rarity of individual undiscovered classes will make it increasingly difficult to find additional examples of this RNA-based sensory and gene control mechanism.

Transcriptome analysis of the filamentous fungus Aspergillus nidulans directed to the global identification of promoters

BACKGROUND

The filamentous fungus Aspergillus nidulans has been a tractable model organism for cell biology and genetics for over 60 years. It is among a large number of Aspergilli whose genomes have been sequenced since 2005, including medically and industrially important species. In order to advance our knowledge of its biology and increase its utility as a genetic model by improving gene annotation we sequenced the transcriptome of A. nidulans with a focus on 5' end analysis.

RESULTS

Strand-specific whole transcriptome sequencing showed that 80-95% of annotated genes appear to be expressed across the conditions tested. We estimate that the total gene number should be increased by approximately 1000, to 11,800. With respect to splicing 8.3% of genes had multiple alternative transcripts, but alternative splicing by exon-skipping was very rare. 75% of annotated genes showed some level of antisense transcription and for one gene, meaB, we demonstrated the antisense transcript has a regulatory role. Specific sequencing of the 5' ends of transcripts was used for genome wide mapping of transcription start sites, allowing us to interrogate over 7000 promoters and 5' untranslated regions.

CONCLUSIONS

Our data has revealed the complexity of the A. nidulans transcriptome and contributed to improved genome annotation. The data can be viewed on the AspGD genome browser.

RrmA regulates the stability of specific transcripts in response to both nitrogen source and oxidative stress

Differential regulation of transcript stability is an effective means by which an organism can modulate gene expression. A well-characterized example is glutamine signalled degradation of specific transcripts in Aspergillus nidulans. In the case of areA, which encodes a wide-domain transcription factor mediating nitrogen metabolite repression, the signal is mediated through a highly conserved region of the 3′ UTR. Utilizing this RNA sequence we isolated RrmA, an RNA recognition motif protein. Disruption of the respective gene led to loss of both glutamine signalled transcript degradation as well as nitrate signalled stabilization of niaD mRNA. However, nitrogen starvation was shown to act independently of RrmA in stabilizing certain transcripts. RrmA was also implicated in the regulation of arginine catabolism gene expression and the oxidative stress responses at the level of mRNA stability. ΔrrmA mutants are hypersensitive to oxidative stress. This phenotype correlates with destabilization of eifE and dhsA mRNA. eifE encodes eIF5A, a translation factor within which a conserved lysine is post-translationally modified to hypusine, a process requiring DhsA. Intriguingly, for specific transcripts RrmA mediates both stabilization and destabilization and the specificity of the signals transduced is transcript dependent, suggesting it acts in consort with other factors which differ between transcripts.

Dynamic energy landscapes of riboswitches help interpret conformational rearrangements and function

Riboswitches are RNAs that modulate gene expression by ligand-induced conformational changes. However, the way in which sequence dictates alternative folding pathways of gene regulation remains unclear. In this study, we compute energy landscapes, which describe the accessible secondary structures for a range of sequence lengths, to analyze the transcriptional process as a given sequence elongates to full length. In line with experimental evidence, we find that most riboswitch landscapes can be characterized by three broad classes as a function of sequence length in terms of the distribution and barrier type of the conformational clusters: low-barrier landscape with an ensemble of different conformations in equilibrium before encountering a substrate; barrier-free landscape in which a direct, dominant “downhill” pathway to the minimum free energy structure is apparent; and a barrier-dominated landscape with two isolated conformational states, each associated with a different biological function. Sharing concepts with the “new view” of protein folding energy landscapes, we term the three sequence ranges above as the sensing, downhill folding, and functional windows, respectively. We find that these energy landscape patterns are conserved in various riboswitch classes, though the order of the windows may vary. In fact, the order of the three windows suggests either kinetic or thermodynamic control of ligand binding. These findings help understand riboswitch structure/function relationships and open new avenues to riboswitch design.

Riboswitches are RNAs that modulate gene expression by ligand-induced conformational changes. However, the way that sequence dictates alternative folding pathways of gene regulation remains unclear. In this study, we mimic transcription by computing energy landscapes which describe accessible secondary structures for a range of sequence lengths. Consistent with experimental evidence, we find that most riboswitch landscapes can be characterized by three broad classes as a function of sequence length in terms of the distribution and barrier type of the conformational clusters: Low-barrier landscape with an ensemble of conformations in equilibrium before encountering a substrate; barrier-free landscape with a dominant “downhill” pathway to the minimum free energy structure; and barrier-dominated landscape with two isolated conformational states with different functions. Sharing concepts with the “new view” of protein folding energy landscapes, we term the three sequence ranges above as the sensing, downhill folding, and functional windows, respectively. We find that these energy landscape patterns are conserved between riboswitch classes, though the order of the windows may vary. In fact, the order of the three windows suggests either kinetic or thermodynamic control of ligand binding. These findings help understand riboswitch structure/function relationships and open new avenues to riboswitch design.

Riboswitches and the RNA world

Riboswitches are structured noncoding RNA domains that selectively bind metabolites and control gene expression (Mandal and Breaker 2004a; Coppins et al. 2007; Roth and Breaker 2009). Nearly all examples of the known riboswitches reside in noncoding regions of messenger RNAs where they control transcription or translation. Newfound classes of riboswitches are being reported at a rate of about three per year (Ames and Breaker 2009), and these have been shown to selectively respond to fundamental metabolites including coenzymes, nucleobases or their derivatives, amino acids, and other small molecule ligands. The characteristics of some riboswitches suggest they could be modern descendents of an ancient sensory and regulatory system that likely functioned before the emergence of enzymes and genetic factors made of protein (Nahvi et al. 2002; Vitreschak et al. 2004; Breaker 2006). If true, then some of the riboswitch structures and functions that serve modern cells so well may accurately reflect the capabilities of RNA sensors and switches that existed in the RNA World. This article will address some of the characteristics of modern riboswitches that may be relevant to ancient versions of these metabolite-sensing RNAs.

Amino acid recognition and gene regulation by riboswitches

Riboswitches specifically control expression of genes predominantly involved in biosynthesis, catabolism and transport of various cellular metabolites in organisms from all three kingdoms of life. Among many classes of identified riboswitches, two riboswitches respond to amino acids lysine and glycine to date. Though these riboswitches recognize small compounds, they both belong to the largest riboswitches and have unique structural and functional characteristics. In this review, we attempt to characterize molecular recognition principles employed by amino acid-responsive riboswitches to selectively bind their cognate ligands and to effectively perform a gene regulation function. We summarize up-to-date biochemical and genetic data available for the lysine and glycine riboswitches and correlate these results with recent high-resolution structural information obtained for the lysine riboswitch. We also discuss the contribution of lysine riboswitches to antibiotic resistance and outline potential applications of riboswitches in biotechnology and medicine.

The structural and functional diversity of metabolite-binding riboswitches

The cellular concentrations of certain metabolites are assiduously monitored to achieve appropriate levels of gene expression. Although proteins have long been known to act as sensors in this capacity, metabolite-binding RNAs, or riboswitches, also play an important role. More than 20 distinct classes of riboswitches have been identified to date, and insights to the molecular recognition strategies of a significant subset of these have been provided by detailed structural studies. This diverse set of metabolite-sensing RNAs is found to exploit a variety of distinct mechanisms to regulate genes that are fundamental to metabolism.

Expanding roles for metabolite-sensing regulatory RNAs

Metabolite-sensing regulatory RNAs, oft referred to as riboswitches, are widely used among eubacteria for control of diverse biochemical pathways and transport mechanisms. Great strides have been made in understanding the general structure and biochemistry of individual riboswitch classes. However, along with these advancements, it has become clear that metabolite-sensing riboswitches respond to an increasingly structurally diverse range of metabolite and metal ligands. Moreover, the recent accruement of new riboswitches has uncovered individual examples and classes that utilize unique regulatory strategies or employ a regulatory logic other than simple feedback inhibition.

AcpA, a member of the GPR1/FUN34/YaaH membrane protein family, is essential for acetate permease activity in the hyphal fungus Aspergillus nidulans

In a previous study, alcS, a gene of the Aspergillus nidulans alc cluster, was shown to encode a protein that belongs to the GPR1/FUN34/YaaH membrane protein family. BLAST screening of the A. nidulans genome data identified additional genes encoding hypothetical proteins that could belong to this family. In this study we report the functional characterization of one of them, AN5226. Its expression is induced by ethanol and ethyl acetate (two inducers of the alc genes) and is mediated by the specific transcriptional activator of genes of the acetate-utilization pathway FacB. Growth of a null mutant (DeltaAN5226) is notably affected when acetate is used as sole carbon source at low concentration and in a high pH medium, i.e. when protonated acetate, the form that can enter the cell by passive diffusion, is present in low amounts. Consistently, expression of AN5226 is also induced by acetate, but only when the latter is present at low concentrations. (14)C-labelled acetate uptake experiments using germinating conidia demonstrate an essential role for AN5226 in mediated acetate transport. To our knowledge this report is the first to provide evidence for the identification of an acetate transporter in filamentous fungi. We have designated AN5226 as acpA (for acetate permease A).

Purine sensing by riboswitches

Structured mRNA elements called riboswitches control gene expression by binding to small metabolites. Over a dozen riboswitch classes have been characterized that target a broad range of molecules and vary widely in size and secondary structure. Four of the known riboswitch classes recognize purines or modified purines. Three of these classes are closely related in conserved sequence and secondary structure, but members of these classes selectively recognize guanine, adenine or 2'-deoxyguanosine. Members of the fourth riboswitch class adopt a distinct structure to form a selective binding pocket for the guanine analogue preQ(1) (7-aminomethyl-7-deazaguanine). All four classes of purine-sensing riboswitches are most likely to recognize their respective metabolites by utilizing a riboswitch residue to make a canonical Watson-Crick base-pair with the ligand. This review will provide a summary of the purine-sensing riboswitches, as well as discuss the complex functions and applications of these RNAs.

Ribozymes, riboswitches and beyond: regulation of gene expression without proteins

Although various functions of RNA are carried out in conjunction with proteins, some catalytic RNAs, or ribozymes, which contribute to a range of cellular processes, require little or no assistance from proteins. Furthermore, the discovery of metabolite-sensing riboswitches and other types of RNA sensors has revealed RNA-based mechanisms that cells use to regulate gene expression in response to internal and external changes. Structural studies have shown how these RNAs can carry out a range of functions. In addition, the contribution of ribozymes and riboswitches to gene expression is being revealed as far more widespread than was previously appreciated. These findings have implications for understanding how cellular functions might have evolved from RNA-based origins.

Tetracycline aptamer-controlled regulation of pre-mRNA splicing in yeast

Splicing of pre-mRNA is a critical step in mRNA maturation and disturbances cause several genetic disorders. We apply the synthetic tetracycline (tc)-binding riboswitch to establish a gene expression system for conditional tc-dependent control of pre-mRNA splicing in yeast. Efficient regulation is obtained when the aptamer is inserted close to the 5′splice site (SS) with the consensus sequence of the SS located within the aptamer stem. Structural probing indicates limited spontaneous cleavage within this stem in the absence of the ligand. Addition of tc leads to tightening of the stem and the whole aptamer structure which probably prevents recognition of the 5′SS. Combination of more then one aptamer-regulated intron increases the extent of regulation leading to highly efficient conditional gene expression systems. Our findings highlight the potential of direct RNA–ligand interaction for regulation of gene expression.

Control of alternative RNA splicing and gene expression by eukaryotic riboswitches

Bacteria make extensive use of riboswitches to sense metabolites and control gene expression, and typically do so by modulating premature transcription termination or translation initiation. The most widespread riboswitch class known in bacteria responds to the coenzyme thiamine pyrophosphate (TPP), which is a derivative of vitamin B1. Representatives of this class have also been identified in fungi and plants, where they are predicted to control messenger RNA splicing or processing. We examined three TPP riboswitches in the filamentous fungus Neurospora crassa, and found that one activates and two repress gene expression by controlling mRNA splicing. A detailed mechanism involving riboswitch-mediated base-pairing changes and alternative splicing control was elucidated for precursor NMT1 mRNAs, which code for a protein involved in TPP metabolism. These results demonstrate that eukaryotic cells employ metabolite-binding RNAs to regulate RNA splicing events that are important for the control of key biochemical processes.