Novel benzene ring biosynthesis from C3 and C4 primary metabolites by two enzymes

The shikimate pathway, including seven enzymatic steps for production of chorismate via shikimate from phosphoenolpyruvate and erythrose-4-phosphate, is common in various organisms for the biosynthesis of not only aromatic amino acids but also most biogenic benzene derivatives. 3-Amino-4-hydroxybenzoic acid (3,4-AHBA) is a benzene derivative serving as a precursor for several secondary metabolites produced by Streptomyces, including grixazone produced by Streptomyces griseus. Our study on the biosynthesis pathway of grixazone led to identification of the biosynthesis pathway of 3,4-AHBA from two primary metabolites. Two genes, griI and griH, within the grixazone biosynthesis gene cluster were found to be responsible for the biosynthesis of 3,4-AHBA; the two genes conferred the in vivo production of 3,4-AHBA even on Escherichia coli. In vitro analysis showed that GriI catalyzed aldol condensation between two primary metabolites, L-aspartate-4-semialdehyde and dihydroxyacetone phosphate, to form a 7-carbon product, 2-amino4,5-dihydroxy-6-one-heptanoic acid-7-phosphate, which was subsequently converted to 3,4-AHBA by GriH. The latter reaction required Mn2+ ion but not any cofactors involved in reduction or oxidation. This pathway is independent of the shikimate pathway, representing a novel, simple enzyme system responsible for the synthesis of a benzene ring from the C-3 and C-4 primary metabolites.