Figure 7.

Riboswitches: RNA sensors and RNA thermometers. (A) Ligand (magenta sphere) induced formation of transcriptional terminator or anti-terminator RNA structures in the mRNA (green colored) that result in premature transcription termination of the RNAP (gray colored) or allow downstream transcription elongation, respectively. (B) Translational attenuation mechanisms whereby ligand binding traps the RBS (magenta box) in a RNA secondary structure and is not accessible for ribosome (yellow colored) binding or, inversely, stabilizes an alternative structure and frees the RBS, thus allowing initiation of translation. (C) RNA thermometer. Here the RBS is trapped in a secondary structure and not available for ribosome binding. Melting of this structure in a zipper-like manner at increasing temperatures liberates the RBS, thus allowing translation initiation. (D) Reversible and temperature-dependent formation of alternative secondary RNA structures that may affect premature transcription termination or translation initiation.