Cyclic di-GMP is a circular RNA dinucleotide that functions as a second messenger in diverse species of bacteria to trigger wide-ranging physiological changes, including cell differentiation, conversion between motile and biofilm lifestyles, and virulence gene expression. However, the mechanisms used by cyclic di-GMP to regulate gene expression have remained a mystery. We demonstrate that cyclic di-GMP in many bacterial species is sensed by a riboswitch class in mRNA that controls the expression of genes involved in numerous fundamental cellular processes. A variety of cyclic di-GMP regulons are revealed, including some riboswitches associated with virulence gene expression, pili formation, and flagellar organelle biosynthesis. In addition, sequences matching the consensus for cyclic di-GMP riboswitches are present in the genome of a bacteriophage.

Messenger RNAs are typically thought of as passive carriers of genetic information that are acted upon by protein- or small RNA-regulatory factors and by ribosomes during the process of translation. We report that the 5'-untranslated sequence of the Escherichia coli btuB mRNA assumes a more proactive role in metabolic monitoring and genetic control. The mRNA serves as a metabolite-sensing genetic switch by selectively binding coenzyme B(12) without the need for proteins. This binding event establishes a distinct RNA structure that is likely to be responsible for inhibition of ribosome binding and consequent reduction in synthesis of the cobalamin transport protein BtuB. This finding, along with related observations, supports the hypothesis that metabolic monitoring through RNA-metabolite interactions is a widespread mechanism of genetic control.

The expression of certain genes involved in fundamental metabolism is regulated by metabolite-binding ''riboswitch'' elements embedded within their corresponding mRNAs. We have identified at least six additional elements within the Bacillus subtilis genome that exhibit characteristics of riboswitch function (glmS, gcvT, ydaO͞yuaA, ykkC͞yxkD, ykoK, and yybP͞ykoY). These motifs exhibit extensive sequence and secondary-structure conservation among many bacterial species and occur upstream of related genes. The element located upstream of the glmS gene in Grampositive organisms functions as a metabolite-dependent ribozyme that responds to glucosamine-6-phosphate. Other motifs form complex folded structures when transcribed as RNA molecules and carry intrinsic terminator structures. These findings indicate that riboswitches serve as a major genetic regulatory mechanism for the control of metabolic genes in many microbial species. R iboswitches are highly structured domains within mRNAs that precisely sense metabolites and control gene expression (1). These RNA elements are capable of binding to a variety of target compounds and subsequently modulating transcription and translation with performance characteristics that are similar to those of protein genetic factors. Typically, each riboswitch is composed of a conserved metabolite-binding domain (aptamer) located upstream of a variable sequence region (expression platform) that dictates the level of gene expression. Allosteric changes brought about by metabolite binding to the aptamer are harnessed by the expression platform to modulate the expression of the adjacent gene or operon. Riboswitches are versatile genetic control elements. In some instances, both transcription and translation control are used by the same aptamer class in the same prokaryotic organism (e.g., see ref.2). Evidence also shows that riboswitches can use mRNA-processing events to modulate gene expression (3, 4).The various metabolites that are detected by known riboswitches are of fundamental importance to living systems (5). On this basis, we have speculated that modern riboswitches might be the remaining representatives of an ancient metabolitemonitoring system that was present in the RNA World (5-9). The wide distribution of some riboswitch classes among microbes (e.g., see refs. 5 and 9-14) and the presence of metabolitebinding RNA domains in eukaryotes (4) support this hypothesis. Each of the seven classes of riboswitches reported (1, 5) was examined for metabolite-binding function because published genetic evidence showed that these elements were important for genetic control. Because the regulation of many metabolism genes has not been characterized in detail, it is possible that numerous other metabolite-binding RNA motifs exist in nature.The riboswitches known to be present in prokaryotes are typically located in noncoding or intergenic regions (IGRs). Therefore, the examination of unusually long IGRs for indications of conserved sequence and secondary-structure elements should yield new...

Most riboswitches are metabolite-binding RNA structures located in bacterial messenger RNAs where they control gene expression. We have discovered a riboswitch class in many bacterial and archaeal species whose members are selectively triggered by fluoride but reject other small anions, including chloride. These fluoride riboswitches activate expression of genes that encode putative fluoride transporters, enzymes that are known to be inhibited by fluoride, and additional proteins of unknown function. Our findings indicate that most organisms are naturally exposed to toxic levels of fluoride and that many species use fluoride-sensing RNAs to control the expression of proteins that alleviate the deleterious effects of this anion.

Group I self-splicing ribozymes commonly function as components of selfish mobile genetic elements. We identified an allosteric group I ribozyme, wherein self-splicing is regulated by a distinct riboswitch class that senses the bacterial second messenger c-di-GMP. The tandem RNA sensory system resides in the 5′ untranslated region of the messenger RNA for a putative virulence gene in the pathogenic bacterium Clostridium difficile. c-di-GMP binding by the riboswitch induces folding changes at atypical splice site junctions to modulate alternative RNA processing. Our findings indicate that some self-splicing ribozymes are not selfish elements, but are harnessed by cells as metabolite sensors and genetic regulators.

Riboswitches are metabolite-binding RNA structures that serve as genetic control elements for certain messenger RNAs. These RNA switches have been identified in all three kingdoms of life and are typically responsible for the control of genes whose protein products are involved in the biosynthesis, transport or utilization of the target metabolite. Herein, we report that a highly conserved RNA domain found in bacteria serves as a riboswitch that responds to the coenzyme S-adenosylmethionine (SAM) with remarkably high affinity and specificity. SAM riboswitches undergo structural reorganization upon introduction of SAM, and these allosteric changes regulate the expression of 26 genes in Bacillus subtilis. This and related findings indicate that direct interaction between small metabolites and allosteric mRNAs is an important and widespread form of genetic regulation in bacteria.

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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.