Characteristics and Relationships of Mercury-resistant Mutants and Methionine Auxotrophs of Yeast

Approximately one-half of the mutants of Saccharomyces cerevisiae that are selected as resistant to methyl mercury are also found to require methionine. Eighty-four percent of these met mutations occur at the met15 locus, and the remaining 16% occur at the met2 locus. Surprisingly, the methionine-requiring mutants are recovered at a much higher frequency on methionineless media than on media supplemented with methionine. Growth patterns of the met mutants on media having a continuous concentration gradient of methionine and mercury compounds indicate that, at a critical concentration of the mercury compounds, the methionine requirement of certain met mutants is partially or completely alleviated. This was found for met2, met15, and to a lesser extent for met6, but not for any other methionine mutants. This loss of methionine requirement is produced with methyl mercury, phenyl mercury, and mercuric chloride although met2 and met15 strains can be shown to be resistant only to methyl mercury. Other methionine auxotrophs are not resistant to any of the three mercury compounds. The met2 and met15 mutants, but not the other methionine auxotrophs, develop a sheen of an unidentified product when grown on media with mercuric chloride but not with methyl mercury or phenyl mercury. It is suggested that met2 and met15 mutants produce a simple diffusible substance, which detoxifies methyl mercury, which reacts with mercuric chloride to produce a sheen, and which is the cause of the methionine requirement.