A role for a pyridoxne derivative in the multivalent repression of the isoleucine and valine biosynthetic enzymes

Starvation of a pdx mutant of Escherichia coli strain B in the presence of repressing levels of isoleucine, valine and leucine leads to a derepression of the normally repressible ilv genes. The derepression of the ilvA gene under these conditions results in the accumulation of apothreonine deaminase. Addition of pyridoxine leads to a sudden increase in threonine deaminase activity, and to restoration of repression. The pyridoxine component needed for the repression signal is probably not threonine deaminase but, more likely, some transient (“immature”) form of the enzyme.