Natural Variability in S -Adenosylmethionine (SAM)-Dependent Riboswitches: S-Box Elements in Bacillus subtilis Exhibit Differential Sensitivity to SAM In Vivo and In Vitro

Riboswitches are regulatory systems in which changes in structural elements in the 5' region of the nascent RNA transcript (the "leader region") control expression of the downstream coding sequence in response to a regulatory signal in the absence of a trans-acting protein factor. The S-box riboswitch, found primarily in low-G+C gram-positive bacteria, is the paradigm for riboswitches that sense S-adenosylmethionine (SAM). Genes in the S-box family are involved in methionine metabolism, and their expression is induced in response to starvation for methionine. S-box genes exhibit conserved primary sequence and secondary structural elements in their leader regions. We previously demonstrated that SAM binds directly to S-box leader RNA, causing a structural rearrangement that results in premature termination of transcription at S-box leader region terminators. S-box genes have a variety of physiological roles, and natural variability in S-box structure and regulatory response could provide additional insight into the role of conserved S-box leader elements in SAM-directed transcription termination. In the current study, in vivo and in vitro assays were employed to analyze the differential regulation of S-box genes in response to SAM. A wide range of responses to SAM were observed for the 11 S-box-regulated transcriptional units in Bacillus subtilis, demonstrating that S-box riboswitches can be calibrated to different physiological requirements.