FIGURE 1 The tyrS riboswitch senses the cellular level of uncharged tRNATyr. Both charged and uncharged tRNATyr interact with the leader region of the mRNA at the detector codon, but only uncharged tRNATyr is capable of a second interaction between its amino acid arm and the antiterminator. This second interaction stabilizes the antiterminator and promotes continued transcription of the tyrosyl-