FIG 7.

Control of riboflavin-biosynthetic operon ribDEAHT expression by an FMN-binding riboswitch in B. subtilis. (A) In the absence of FMN binding to the ribD leader transcript, the RNA forms a secondary structure that includes an antiterminator stem (orange lines). Because the downstream segment of this stem is shared with a potential attenuator-encoded terminator hairpin located further downstream, formation of the antiterminator stem permits readthrough transcription into the structural genes. The dashed black line immediately downstream of the stem-loop labeled P6 represents a long segment of unstructured leader RNA. (B) When FMN binds its aptamer, the leader transcript assumes a secondary structure that includes the terminator hairpin, resulting in transcription termination at the attenuator. (C) Crystal structure of the aptamer domain of an FMN riboswitch bound to its ligand (140).