Opportunities for Riboswitch Inhibition by Targeting Co-Transcriptional RNA Folding Events

Antibiotic resistance is a critical global health concern, causing millions of prolonged bacterial infections every year and straining our healthcare systems. Novel antibiotic strategies are essential to combating this health crisis and bacterial non-coding RNAs are promising targets for new antibiotics. In particular, a class of bacterial non-coding RNAs called riboswitches has attracted significant interest as antibiotic targets. Riboswitches reside in the 5'-untranslated region of an mRNA transcript and tune gene expression levels in cis by binding to a small-molecule ligand. Riboswitches often control expression of essential genes for bacterial survival, making riboswitch inhibitors an exciting prospect for new antibacterials. Synthetic ligand mimics have predominated the search for new riboswitch inhibitors, which are designed based on static structures of a riboswitch's ligand-sensing aptamer domain or identified by screening a small-molecule library. However, many small-molecule inhibitors that bind an isolated riboswitch aptamer domain with high affinity in vitro lack potency in vivo. Importantly, riboswitches fold and respond to the ligand during active transcription in vivo. This co-transcriptional folding is often not considered during inhibitor design, and may explain the discrepancy between a low K_d in vitro and poor inhibition in vivo. In this review, we cover advances in riboswitch co-transcriptional folding and illustrate how intermediate structures can be targeted by antisense oligonucleotides-an exciting new strategy for riboswitch inhibitor design.