(A) Comparison of the consensus sequence and secondary structure models for the guanidine-I riboswitch aptamer (subtype 1, left) and the aptamers for ppGpp (subtype 2a, middle; ME Sherlock, N Sudarsan, RR Breaker, manuscript submitted), and PRPP (subtype 2b, right). For sequence alignments of the 257 unique PRPP riboswitch aptamers in Stockholm format, see Supplementary file 2. (B) Consensus sequence and secondary structure model for the 127 unique examples of adjacent guanidine-PRPP aptamers that form the corresponding tandem riboswitch architecture. For sequence alignments of the tandem guanine- PRPP riboswitch aptamers in Stockholm format, see Supplementary file 3. (C) Only genes in the purine biosynthetic pathway that are necessary for the production of adenosine nucleotides are commonly associated with PRPP riboswitches.