Sherlock et al. studied a tandem riboswitch, made of a guanine riboswitch (blue) and a PRPP riboswitch (purple), that regulates genes involved in purine biosynthesis. These genes are transcribed when PRPP binds to its riboswitch (first and third rows in table) or when neither guanine nor PRPP are available (second row). However, a transcription terminator structure (red) forms and blocks transcription when PRPP is absent and guanine binds to its riboswitch (last row). This is the only known example of an IMPLY logic gate in nature. The PRPP riboswitch belongs to the larger family of ykkC riboswitches, which is subdivided in three main groups based on their tertiary structures. The riboswitches in this family bind to guanidine and participate in the guanidine detoxification process (Battaglia et al., 2017; Huang et al., 2017a; Huang et al., 2017b; Nelson et al., 2017; Reiss and Strobel, 2017; Reiss et al., 2017; Sherlock and Breaker, 2017; Sherlock et al., 2017).