The ilvIH operon in E. coli contains two genes that encode the two subunits of the acetolactate synthase isozyme 3 protein. This protein is used in the biosynthesis of L-isoleucine in the bacteria. The operon is activated by the binding of the leucine-responsive regulatory protein (Lrp) to either of two regions upstream of the promoter. Binding of Lrp to the first upstream region (between -260 and -190) appears to be required for operon transcription, while binding to the downstream region (between -150 and -40) enhances it without being required. The operon can also be negatively regulated by the binding of the DNA-binding protein H-NS, which binds to DNA, altering its structure and preventing RNA polymerase from binding and transcribing the operon. The operon is also negatively regulated by the presence of leucine in the cell, which binds to Lrp, changing its conformation and preventing it from binding to its target sites in the operon. This means that if leucine is present in the cell in high enough concentrations, the operon will be inactivated. The genes in the operon, ilvI and ilvH, encode the acetolactate synthase isozyme 3 large and small subunits respectively. Two large and two small subunits combine to form the final enzyme, which is responsible for the conversion of pyruvate in the cell to (2S)-2-acetolactate, which is then used in future steps to form L-isoleucine.

Operon: Acetolactate Synthase III Activation References