N-(5′-Phosphoribosyl)anthranilate isomerase–indol-3-ylglycerol phosphate synthetase of tryptophan biosynthesis. Relationship between the two activities of the enzyme from Escherichia coli

Abstract

Further evidence is presented to confirm the previous conclusion that the enzyme from Escherichia coli catalysing the two sequential reactions in tryptophan biosynthesis, N-(5′-phosphoribosyl)anthranilic acid (PRA) → 1-(o-carboxyphenyl-amino)-1-deoxyribulose 5-phosphate (CdRP) → indol-3-ylglycerol phosphate (InGP)+CO₂+H₂O, consists of a single polypeptide chain. The kinetic properties of the enzyme demonstrate that intermediate CdRP formed from PRA must dissociate from the enzyme before it can be converted into InGP. It is concluded that there are two distinct and non-overlapping catalytic sites on the enzyme for the two reactions. The expected complementation between a mutationally altered form of the enzyme lacking the first reaction and a mutationally altered form lacking the second reaction has been demonstrated in vitro by InGP formation from PRA. This system thus exhibits intracistronic complementation with a non-oligomeric protein gene product.