The 5' untranslated region (5' UTR) of eukaryotic mRNA plays an important role in translation. Here we report the function of the 5' UTR mRNA of  S -adenosylmethionine synthetase ( sam1 ) in translational modulation in the presence of SAM in fission yeast  Schizosaccharomyces pombe  Reporter assays, binding and chemical probing experiments, and mutational analysis show that the 5' UTR mRNA of  sam1  binds to SAM to effect translation. Translational modulation is dependent on a tertiary structure transition in the RNA upon SAM binding. The characterization of such an RNA that is directly associated with an essential metabolic process in eukaryotes provides additional evidence that ligand binding by RNAs plays an important role in eukaryotic gene regulation.