Transcriptional and Translational S-box Riboswitches Differ in Ligand-Binding Properties

There are a number of riboswitches that utilize the same ligand-binding domain to regulate either transcription or translation. S-box (SAM-I) riboswitches, including the riboswitch present in the Bacillus subtilis metI gene, which encodes cystathionine gamma-synthase, regulate the expression of genes involved in methionine metabolism in response to SAM, primarily at the level of transcriptional attenuation. A rarer class of S-box riboswitches is predicted to regulate translation initiation. Here, we identified and characterized a translational S-box riboswitch in the metI gene from Desulfurispirillum indicum The regulatory mechanisms of riboswitches are influenced by the kinetics of ligand interaction. The half-life of the translational D. indicum metI RNA-SAM complex is significantly shorter than that of the transcriptional B. subtilis metI RNA. This finding suggests that unlike the transcriptional RNA, the translational metI riboswitch can make multiple reversible regulatory decisions. Comparison of both RNAs revealed that the second internal loop of helix P3 in the transcriptional RNA usually contains an A residue, whereas the translational RNA contains a C residue that is conserved in other S-box RNAs that are predicted to regulate translation. Mutational analysis indicated that the presence of an A or C residue correlates with RNA-SAM complex stability. These analyses indicate that the internal loop sequence critically determines the stability of the RNA-SAM complex by influencing the flexibility of residues involved in SAM binding and thereby affects the molecular mechanism of riboswitch function.