FIGURE 1.

Sequence and secondary structure of the B. subtilis pbuE adenine riboswitch. A, proposed secondary structure of the adenine-bound state, in which the switching sequence (outlined sequence, nucleotides 79–94) is incorporated into the aptamer. The adenine nucleobase interacts with nucleotide 89 via Watson-Crick base pairing. Within the expression platform, only P4b likely forms. B, proposed structure of the terminated state, in which P1 and P3 are disrupted by pairing of the switching sequence with a downstream sequence (outlined) to form P4a, which is part of the intrinsic terminator element.