Riboswitch regulation mechanisms: RNA, metabolites and regulatory proteins.

Riboswitches are RNA sensors that have been shown to modulate the expression of downstream genes by altering their structure upon metabolite binding. Riboswitches are unique among cellular regulators in that metabolite detection is strictly performed using RNA interactions with the sensed metabolite and in which no regulatory protein is needed to mediate the interaction. However, recent studies have shed light on riboswitch control mechanisms relying on protein regulators to harness metabolite binding for the mediation of gene expression, thereby increasing the range of cellular factors involved in riboswitch regulation. The interaction between riboswitches and proteins adds another level of evolutionary pressure as riboswitches must maintain key residues for metabolite detection, structural switching and protein binding sites. Here, we review regulatory mechanisms involving Escherichia coli riboswitches that have recently been shown to rely on regulatory proteins. We also discuss the implication of such protein-based riboswitch regulatory mechanisms for genetic regulation.