Biochemical and genetic characterization of an early step in a novel pathway for the biosynthesis of aromatic amino acids and p-aminobenzoic acid in the archaeon Methanococcus maripaludis

Methanococcus maripaludis is a strictly anaerobic, methane-producing archaeon and facultative autotroph capable of biosynthesizing all the amino acids and vitamins required for growth. In this work, the novel 6-deoxy-5-ketofructose-1-phosphate (DKFP) pathway for the biosynthesis of aromatic amino acids (AroAAs) and p-aminobenzoic acid (PABA) was demonstrated in M. maripaludis. Moreover, PABA was shown to be derived from an early intermediate in AroAA biosynthesis and not from chorismate. Following metabolic labelling with [U-¹³C]-acetate, the expected enrichments for phenylalanine and arylamine derived from PABA were observed. DKFP pathway activity was reduced following growth with aryl acids, an alternative source of the AroAAs. Lastly, a deletion mutant of aroA′, which encodes the first step in the DKFP pathway, required AroAAs and PABA for growth. Complementation of the mutants by an aroA′ expression vector restored the wild-type phenotype. In contrast, a deletion of aroB′, which encodes the second step in the DKFP pathway, did not require AroAAs or PABA for growth. Presumably, methanococci contain an alternative activity for this step. These results identify the initial reactions of a new pathway for the biosynthesis of PABA in methanococci.