Transcriptional regulation by a riboswitch mechanism

(a) Transcription of thi operon genes in Bacillus subtilis occurs when TPP concentration is low. (b) At high TPP concentration, TPP binds to riboswitch sequences, resulting in transcription termination(a) Transcription of thi operon genes in Bacillus subtilis occurs when TPP concentration is low. (b) At high TPP concentration, TPP binds to riboswitch sequences, resulting in transcription termination

(a) Low TPP: Riboswitch not active (b) High TPP: Riboswitch active Riboswitch sequences Transcription continues into the thi operon; proteins are produced. Termination stem loop -—> mRNA 5' ‘=/ mRNA S'm’ ’- UUUUU 3’—>lntrinsic Antitermination termination; stem IOOP no proteins . c i are produced Low TPP concentration generates High TPP concentration initiates antitermination and thi operon the riboswitch that terminates transcription. transcription by intrinsic termination.