RIBOSE BIOSYNTHESIS AND EVIDENCE FOR AN ALTERNATIVE FIRST STEP IN THECOMMON AROMATIC AMINO-ACID PATHWAY IN METHANOCOCCUS-MARIPALUDIS

An acetate-requiring mutant of Methanococcus maripaludis allowed effic ient labeling of riboses following growth in minimal medium supplement ed with [2-C-13]acetate. Nuclear magnetic resonance and mass spectrosc opic analysis of purified cytidine and uridine demonstrated that the C -1' of the ribuse was about 67% enriched for UC. This value was incons istent with the formation of erythrose 4-phosphate (E4P) exclusively b y the carboxylation of a triose. Instead, these results suggest that e ither (i) E4P is formed by both the nonoxidative pentose phosphate and triose carboxylation pathways or (ii) E4P is formed exclusively by th e nonoxidative pentose phosphate pathway and is not a precursor of aro matic amino acids.