Gene expression of the histidine operon.

Histidine biosynthesis in Salmonella typhi-murium and Escherichia coli is regulated through repression and feedback inhibition. Repression is triggered by the intracellular level of histidyl-tRNA whereas feedback inhibition is brought about by histidine itself, an allosteric inhibitor of the first biosynthetic enzyme. Several lines of evidence indicate that these two processes are interconnected and the first biosynthetic enzyme is itself a regulatory molecule for the his operon. The regulation seems to be exerted in a negative fashion, although a positive control cannot be excluded.