since they possess two different methionine synthetase enzymes: the B12-dependent MetH and MetE, which does not require B12 as a cofactor. The MetH enzyme is used preferentially when B12 is available and MetE is induced in response to accumulated homocysteine in metH mutants (Wu et al. 1992). Epoxyqueuosine reductase

     Epoxyqueuosine reductase performs the final step in the formation of the

hypermodified tRNA base, queuosine, found in the first anticodon position of tRNAasn tRNAasp, tRNAhis, and tRNAtyr. This modified base has been demonstrated to be nonessential for bacterial growth under laboratory conditions (Noguchi et al. 1982) The final reaction in its synthesis has been reported to require B12 (Frey et al. 1988). However, in E. coli, which lacks the genes for de novo B12 synthesis and which was grown in medium lacking suitable corrinoid precursors for B12 production, epoxyqueuosine could still be made. Hence, it has been suggested that the reaction does not require B12 but may be stimulated by it indirectly (Roth et al. 1996). Other B12-Dependent Reactions

Acetyl CoA synthesis

      A methyl-corrinoid/iron sulfur protein serves to transfer methyl groups in many anaerobic bacteria that utilize the Wood/Ljungdahl pathway to synthesize acetyl-CoA. This corrinoid/iron sulfur protein transfers methyl groups from methyltetrahydrofolate to CO-dehydrogenase/acetyl-CoA synthase, a bifunctional nickel containing enzyme, which subsequently catalyzes the formation of acetyl-CoA using this methyl group, carbon monoxide, and coenzyme A (Ferry 1995)