The hisL gene encodes a leader peptide that mediates transcriptional attenuation in response to histidine availability, thereby regulating the expression of the full his operon. When sufficient charged histidyl-tRNAs are present, translation of the hisL-encoded leader peptide occurs rapidly, promoting the formation of an intrinsic, rho-independent terminator and halting transcription of the operon [203]. An artificial open reading frame, hisL', has been designed to enhance the expression of the hisG gene [291].