In molecular biology, a riboswitch is a regulatory segment of a messenger RNA molecule that binds a small molecule, resulting in a change in production of the proteins encoded by the mRNA. Thus, an mRNA that contains a riboswitch is directly involved in regulating its own activity, in response to the concentrations of its effector molecule. The discovery that modern organisms use RNA to bind small molecules, and discriminate against closely related analogs, expanded the known natural capabilities of RNA beyond its ability to code for proteins, catalyze reactions, or to bind other RNA or protein macromolecules.
Catabolite activator protein is a trans-acting transcriptional activator that exists as a homodimer in solution. Each subunit of CAP is composed of a ligand-binding domain at the N-terminus and a DNA-binding domain at the C-terminus. Two cAMP molecules bind dimeric CAP with negative cooperativity. Cyclic AMP functions as an allosteric effector by increasing CAP's affinity for DNA. CAP binds a DNA region upstream from the DNA binding site of RNA Polymerase. CAP activates transcription through protein-protein interactions with the α-subunit of RNA Polymerase. This protein-protein interaction is responsible for (i) catalyzing the formation of the RNAP-promoter closed complex; and (ii) isomerization of the RNAP-promoter complex to the open conformation. CAP's interaction with RNA polymerase causes bending of the DNA near the transcription start site, thus effectively catalyzing the transcription initiation process. CAP's name is derived from its ability to affect transcription of genes involved in many catabolic pathways. For example, when the amount of glucose transported into the cell is low, a cascade of events results in the increase of cytosolic cAMP levels. This increase in cAMP levels is sensed by CAP, which goes on to activate the transcription of many other catabolic genes.
The YdaO/YuaA leader is a conserved RNA structure found upstream of the ydaO and yuaA genes in Bacillus subtilis and related genes in other bacteria. Its secondary structure and gene associations were predicted by bioinformatics.
The DUF1646 RNA motif is a conserved RNA structure that was discovered by bioinformatics. One of the two DUF1646 RNAs in Enterococcus faecalis was independently detected by term-seq. Data from both discoveries suggest that DUF1646 RNAs are cis-regulatory RNAss, and that at least some DUF1646 RNAs use Rho-independent transcription terminators as their mechanism to regulate gene expression.
The DUF2800 RNA motif is a conserved RNA structure that was discovered by bioinformatics. DUF2800 motif RNAs are found in Bacillota. DUF2800 RNAs are also predicted in the phyla Actinomycetota and Synergistota, although these RNAs are likely the result of recent horizontal gene transfer or conceivably sequence contamination.
The DUF2815 RNA motif is a conserved RNA structure that was discovered by bioinformatics. As of 2018, the DUF2815 motif has not been identified in any classified organism, but is known through metagenomic sequences isolated from environmental sources.
The DUF3577 RNA motif is a conserved RNA structure that was discovered by bioinformatics. DUF3577 motifs are found in the organism Cardiobacterium valvarum and metagenomic sequences from unknown organisms.
The DUF805 RNA motif is a conserved RNA structure that was discovered by bioinformatics. The motif is subdivided into the DUF805 motif and the DUF805b motif, which have similar, but distinct secondary structures. Together, these motifs are found in Bacteroidota, Chlorobiota, and Pseudomonadota.
The freshwater-2 RNA motif is a conserved RNA structure that was discovered by bioinformatics. Freshwater-2 motif RNAs are found in metagenomic sequences that are isolated from aquatic and especially freshwater environments. As of 2018, no freshwater-2 RNA has been identified in a classified organism.
The GA-cis RNA motif is a conserved RNA structure that was discovered by bioinformatics. GA-cis motif RNAs are found in one species classified within the phylum Bacillota: specifically, there are 9 predicted copies in Coprocuccus eutactus ATCC 27759.
The HTH-XRE RNA motif is a conserved RNA structure that was discovered by bioinformatics. HTH-XRE motifs are found in Clostridiales.
The Latescibacteria, OD1, OP11, TM7 RNA motif is a conserved RNA structure that was discovered by bioinformatics. LOOT motif RNAs are found in multiple bacterial phyla that have only recently been discovered, and are currently not well understood: Latescibacteria, OD1/Parcubacteria, OP11 AND TM7. In some cases, no specific organism has been isolated in the relevant phylum, but the existence of the bacterial phylum is known only through analysis of metagenomic sequences. Curiously, the LOOT motif is not known in any phylum that has been studied for a long time.
lysM RNA motifs are conserved RNA structures that were discovered by bioinformatics. Such bacterial motifs are defined by consistently being upstream of 'lysM' genes, which encode lysin protein domains, a conserved domain that participates in cell wall degradation. lysM motif RNAs likely function as cis-regulatory elements, in view of their positions upstream of protein-coding genes, although this hypothesis is not certain.
The MDR-NUDIX RNA motif is a conserved RNA structure that was discovered by bioinformatics. The MDR-NUDIX motif is found in the poorly studied phylum TM7.
The narK RNA motif is a conserved RNA structure that was discovered by bioinformatics. narK motif RNAs are found in Beta- and Gammaproteobacteria.
The nhaA-I RNA motif is a conserved RNA structure that was discovered by bioinformatics. nhaA-I motif RNAs are found in Acidobacteriota, alpha-, beta- and Gammaproteobacteria, Verrucomicrobiota and the tentative phylum NC10.
The raiA RNA motif is a conserved RNA structure that was discovered by bioinformatics. raiA motif RNAs are found in Actinomycetota and Bacillota, and have many conserved features—including conserved nucleotide positions, conserved secondary structures and associated protein-coding genes—in both of these phyla. Some conserved features of the raiA RNA motif suggest that they function as cis-regulatory elements, but other aspects of the motif suggest otherwise.
The sul1 RNA motif is a conserved RNA structure that was discovered by bioinformatics. Energetically stable tetraloops often occur in this motif. sul1 motif RNAs are found in Alphaproteobacteria.
The terC RNA motif is a conserved RNA structure that was discovered by bioinformatics. terC motif RNAs are found in Pseudomonadota, within the sub-lineages Alphaproteobacteria and Pseudomonadales.
The uup RNA motif is a conserved RNA structure that was discovered by bioinformatics. uup motif RNAs are found in Bacillota and Gammaproteobacteria.
