GriC and GriD constitute a carboxylic acid reductase involved in grixazone biosynthesis in Streptomyces griseus.

In grixazone biosynthesis by Streptomyces griseus, a key intermediate 3-amino-4-hydroxybenzoic acid (3,4-AHBA) is converted to another key intermediate 3-amino-4-hydroxybenzaldehyde (3,4-AHBAL). Two genes griC and griD in the grixazone biosynthesis gene cluster were found to be responsible for this conversion, because disruption of each gene resulted in the extracellular accumulation of 3-acetylamino-4-hydroxybenzoic acid, a shunt product from 3,4-AHBA. Significant sequence similarity of GriC to AMP-binding proteins and of GriD to NAD(P)-dependent aldehyde dehydrogenases suggested that GriC and GriD constituted an ATP- and NAD(P)-dependent carboxylic acid reductase (CAR) catalyzing reduction of 3,4-AHBA to produce 3,4-AHBAL through acyl-AMP formation, as is found for the reactions catalyzed by some CARs. griG encoding a benzoate transporter homologue in the grixazone biosynthesis gene cluster was nonessential for grixazone biosynthesis but probably enhanced the membrane permeability for 3,4-AHBA. Simultaneous overexpression of griC, griD, and griG in S. griseus mutant cells deficient in an acetyltransferase responsible for N-acetylation of 3,4-AHBA led to efficient bioconversion of exogenously added 3,4-AHBA to 3,4-AHBAL. This system also turned out to be useful for reduction of some aryl carboxylates to the corresponding aryl aldehydes.