In E. coli CmoA is involved in conversion of ho5U to cmo5U (addition of carboxymethyl group) in tRNA. It transforms S-AdoMet and prephenate into carboxy-S-adenosyl-L-methionine (Cx-SAM) and phenyl pyruvate. Cx-SAM is the carboxymethyl group donor in the next reaction catalyzed by CmoB. Null mutation of CmoA results in the accumulation of tRNA containing mo5U34 and ho5U34. CmoA (YecO) is paralogous to AdoMet-dependent methyltransferases YgdE and FtsJ. A metabolic link exists between the biosynthesis of cmo5U and the biosynthesis of aromatic amino acids Tyr and Phe via the chorismic acid. Of note, no homolog of CmoA exists in B. subtilis, attesting that the enzymatic formation of mo5U34 in tRNA of this organism probably follows another mechanism than in E. coli.