[formula: see text] The enzyme S-adenosylmethionine:tRNA ribosyltransferase-isomerase (QueA) catalyzes the penultimate step in the biosynthesis of the tRNA nucleoside queuosine, a unique ribosyl transfer from the cofactor S-adenosylmethionine (AdoMet) to a modified-tRNA precursor. The use of AdoMet in this way is fundamentally new to the chemistry of this important biological cofactor. We report here the first mechanistic studies of this remarkable enzyme, and we propose a chemical mechanism for the reaction consistent with our experimental observations.