Figure 1.

Formation of a compact structure by the U65C mutant of the A-riboswitch aptamer. (a) Alternative base pairing of the B. subtilis pbuE A-riboswitch associated with the off and on states. The off state is characterized by the presence of a transcription terminator stem-loop, whereas the on state is characterized by the stabilization of the ligand-bound aptamer, which prevents formation of the terminator stem-loop and allows transcription to proceed. (b) Sequence and secondary structure of the U65C mutant A-riboswitch aptamer illustrating the proposed interaction between G39 and C65. (c) Native-gel electrophoresis of the wild-type A-riboswitch aptamer, the U65C mutant and a control loop mutant (P2 comp; see text) in the presence of 1 µM MgCl₂.