New function of vitamin B12: cobamide-dependent reduction of epoxyqueuosine to queuosine in tRNAs of Escherichia coli and Salmonella typhimurium.

Queuosine (Q), 7-[(4,5-cis-dihydroxy-2-cyclopentene-1-yl)-amino)methyl)-7- deazaguanosine, and Q derivatives usually replace guanosine in the anticodon of tRNAs(GUN) of eubacteria and of cytoplasmic and mitochondrial tRNAs of lower and higher eucaryotes except yeasts. Q appears to be synthesized de novo exclusively in eubacteria, and the free-base queuine serves as a nutrient factor for eucaryotes. Recently, a Q derivative, oQ, containing a 2,3-epoxy-4,5-dihydroxycyclopentane ring, has been identified in Escherichia coli tRNA(Tyr). Here we show that oQ is formed when E. coli or Salmonella typhimurium is grown in glucose-salt medium. The formation of oQ was independent of molecular oxygen, and oQ-tRNAs were converted to Q-tRNAs by adding cobalamin to the growth medium. Under strictly anaerobic conditions, considerable amounts of Q were present in E. coli and S. typhimurium tRNAs when the bacteria were grown in the presence of cobalt ions with glycerol as the carbon source and fumarate as the electron acceptor. Under these conditions, the biosynthesis of cobalamin was induced. The results suggest that oQ is derived from ribose and that oQ is finally reduced to Q by a cobamide-dependent enzyme.