Ligand Modulates Cross-Coupling between Riboswitch Folding and Transcriptional Pausing

Abstract

HighlightsO_LIsmFRET, biochemistry and simulations probe co-transcriptional riboswitch folding\nC_LIO_LINascent RNA folding is disfavored by the DNA template and aided by RNAP\nC_LIO_LITranscriptional pausing is stabilized by RNA pseudoknot and destabilized by ligand\nC_LI\n\nSUMMARYNumerous classes of riboswitches have been found to regulate bacterial gene expression in response to physiological cues, offering new paths to anti-bacterial drugs. As common studies of isolated riboswitches lack the functional context of the transcription machinery, we here combine single-molecule, biochemical and simulation approaches to investigate the coupling between co-transcriptional folding of the pseudoknot-structured preQ1 riboswitch and RNA polymerase (RNAP) pausing. We show that pausing at a site immediately downstream of the riboswitch requires a ligand-free pseudoknot in the nascent RNA, a precisely spaced sequence resembling the pause consensus, and electrostatic and steric interactions with the RNAP exit channel. While interactions with RNAP stabilize the native fold of the riboswitch, binding of the ligand signals RNAP release from the pause. Our results demonstrate that the nascent riboswitch and its ligand actively modulate the function of RNAP and vice versa, a paradigm likely to apply to other cellular RNA transcripts.