Antisense-acting riboswitches: A poorly characterized yet important model of transcriptional regulation in prokaryotic organisms.

Riboswitches are RNA elements involved in regulating genes that participate in the biosynthesis or transport of essential metabolites. They are characterized by their ability to recognize their target molecules with high affinity and specificity. Riboswitches are commonly cotranscribed with their target genes and are located at the 5' end of their transcriptional units. To date, only two exceptional cases of riboswitches being situated at the 3' end and transcribing in the antisense direction of their regulated genes have been described. The first case involves a SAM riboswitch located at the 3' end of the ubiG-mccB-mccA operon in Clostridium acetobutylicum involved in converting methionine to cysteine. The second case concerns a Cobalamin riboswitch in Listeria monocytogenes that regulates the transcription factor PocR related to this organism's pathogenic process. In almost a decade since the first descriptions of antisense-acting riboswitches, no new examples have been described. In this work, we performed a computational analysis to identify new examples of antisense-acting riboswitches. We found 292 cases in which, according to the available information, we infer that the expected regulation of the riboswitch is consistent with the signaling molecule it senses and the metabolic function of the regulated gene. The metabolic implications of this novel type of regulation are thoroughly discussed.