FIGURE 3.

Posttranscriptional control of PrfA expression. At low temperatures (<30°C), the Shine-Dalgarno site (SD, red square) is sequestered in an RNA thermosensor, preventing binding of the 30S subunit (green sphere) of the ribosome. Possibly, the binding of the 30S subunit involves a ribosomal standby site, where it primarily binds to the coding region of the prfA mRNA, where it scans the transcript. At permissive temperatures (∼37°C), the RNA thermosensor dissociates, allowing access of the 30S subunit to the SD, where it can recruit the 50S subunit (light green sphere), and translation can initiate. Under conditions in which a terminated S-adenosylmethionine riboswitch is produced (+SreA), it can bind to the RNA thermosensor and by an unknown mechanism prevent PrfA production.