The histidine operon leader is an RNA element that was found upstream of some histidine biosynthesis his operons in Escherichia coli and related bacteria and that is involved in the amino acid dependent attenuation of their transcription [6186194]. The leader sequence can assume two different secondary structures known as the terminator and the anti-terminator structure. The leader also codes for very short peptide sequence that is rich in Histidine. The terminator structure is recognized as a termination signal for RNA polymerase and the operon is not transcribed. This structure forms when the cell has an excess of histidine and ribosome movement over the leader transcript is not impeded. more

When there is a deficiency of the charged histidyl tRNA the ribosome translating the leader peptide stalls and the antiterminator structure can form. This allows RNA polymerase to transcribe the operon. Computational searches for similar RNA structures in the leader regions of various bacteria have identified the conserved Trp attenuators in various Gamma- and Alphaproteobacteria [15135544].