Feedback regulation of Escherichia coli thrS. In the absence of bound ThrRS, ribosomes can bind to the split ribosome binding site (domains 1 and 3) and initiate translation of thrS. Dimeric ThrRS can bind to two tRNAThr molecules for aminoacylation. Alternatively, ThrRS can bind to two sites in thrS (domains 2 and 4), which inhibits ribosome binding and represses translation. The ThrRS binding sites in thrS, as well as thrS Shine-Dalgarno (SD) sequence and initiation codon, are shown. The anticodon loop of tRNAThr is shown for comparison (44). The critical anticodon (tRNAThr) and anticodon-like (thrS) residues are shown.