CLUES FROM A HALOPHILIC METHANOGEN ABOUT AROMATIC AMINO-ACID BIOSYNTHESIS IN ARCHAEBACTERIA

Extensive diversity in features of aromatic amino acid biosynthesis an d regulation has become recognized in eubacteria, but almost nothing i s known about the extent to which such diversity exists within the arc haebacteria. Methanohalophilus mahii, a methylotrophic halophilic meth anogen, was found to synthesize L-phenylalanine and L-tyrosine via phe nylpyruvate and 4-hydroxyphenylpyruvate, respectively. Enzymes capable of using L-arogenate as substrate were not found. Prephenate dehydrog enase was highly sensitive to feedback inhibition by L-tyrosine and co uld utilize either NADP+ (preferred) or NAD+ as cosubstrate. Tyrosine- pathway dehydrogenases having the combination of narrow specificity fo r a cyclohexadienyl substrate but broad specificity for pyridine nucle otide cofactor have not been described before. The chorismate mutase e nzyme found is a member of a class which is insensitive to allosteric control. The most noteworthy character state was prephenate dehydratas e which proved to be subject to multimetabolite control by feedback in hibitor (L-phenylalanine) and allosteric activators (L-tyrosine, L-try ptophan, L-leucine, L-methionine and L-isoleucine). This interlock typ e of prephenate dehydratase, also known to be broadly distributed amon g the gram-positive lineage of the eubacteria, was previously shown to exist in the extreme halophile, Halobacterium vallismortis. The resul ts are consistent with the conclusion based upon 16S rRNA analyses tha t Methanomicrobiales and the extreme halophiles cluster together.