Fibro-purF RNA motif
The Fibro-"purF" RNA motif is a conserved RNA structure that was discovered by bioinformatics. Fibro-"purF" motif RNAs are found in Fibrobacterota, a group of bacteria that are common in cow rumen. Additionally, the RNAs are found in metagenomic sequences of DNA isolated from cow rumen. Fibro-"purF" motif RNAs likely function as cis-regulatory elements, in view of their positions upstream of protein-coding genes. In fact, instances of the Fibro-"purF" RNA motif are often located nearby to the predicted Shine-Dalgarno sequence of the downstream gene. This arrangement is consistent with a model of "cis"-regulation where  the RNA allosterically controls access to the Shine-Dalgarno sequence, thus regulating the gene translationally. All known Fibro-"purF" RNAs are found upstream of "purF" genes, which encode amidophosphoribosyltransferase, which participates in the biosynthesis of biological purine molecules. The Fibro-"purF" motif is found only in closely related organisms. As a result, the RNAs do not exhibit a high degree of mutations when compared, and it was difficult to study the RNA's secondary structure by covariation methods. As a result, the predicted secondary structure is uncertain in some places. The Fibro-"purF" RNA motif was considered a potential candidate riboswitch, but no  ligand has been established, as of 2022. However, experiments have confirmed that the RNA motif corresponds to a regulatory element and "in vitro" experiments corroborate the predicted structure. Due to the instability of the proposed ligand, 5-phospho-D-ribosylamine, direct experiments to confirm this ligand have not yet been conducted.