Cotranscriptional RNA strand displacement underlies the regulatory function of the E. coli thiB TPP translational riboswitch.

Riboswitches are cis-regulatory RNA elements that regulate gene expression in response to ligand through the coordinated action of a ligand-binding aptamer domain (AD) and a downstream expression platform (EP). Previous studies of transcriptional riboswitches have uncovered diverse examples that utilize cotranscriptional strand displacement to mediate the switching mechanism. The coupling of transcription and translation in bacteria motivates the intriguing question as to whether translational riboswitches can utilize the same mechanistic features. Here we investigate this question by studying the Escherichia coli thiB thiamine pyrophosphate (TPP) riboswitch. Using cellular gene expression assays, we first confirmed that the riboswitch acts at the level of translational regulation. Deletion mutagenesis showed the importance of the AD-EP linker sequence for riboswitch function, which based on sequence complementarity with the AD P1 stem suggested the possibility of an intermediate structure reminiscent of transcriptional riboswitches that exploit strand displacement. Point mutation analysis of this intermediate structure, followed by designed changes to P1, supported a strand displacement mechanism for E. coli thiB. This work provides an important new example of diverse riboswitch AD-EP combinations that exploit this switching mechanism.