Prephenate dehydratase from the aphid endosymbiont (Buchnera) displays changes in the regulatory domain that suggest its desensitization to inhibition by phenylalanine

Buchnera aphidicola, the prokaryotic endosymbiont of aphids, complements di etary deficiencies with the synthesis and provision of several essential am ino acids. We have cloned and sequenced a region of the genome of B. aphidi cola isolated from Acyrthosiphon pisum which includes the two-domain aroQ/p heA gene. This gene encodes the bifunctional chorismate mutase-prephenate d ehydratase protein, which plays a central role in L-phenylalanine biosynthe sis. Two changes involved in the overproduction of this amino acid have bee n detected. First, the absence of an attenuator region suggests a constitut ive expression of this gene. Second, the regulatory domain of the Buchnera prephenate dehydratase shows changes in the ESRP sequence, which is involve d in the allosteric binding of phenylalanine and is strongly conserved in p rephenate dehydratase proteins from practically all known organisms. These changes suggest the desensitization of the enzyme to inhibition by phenylal anine and would permit the bacterial endosymbiont to overproduce phenylalan ine.