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

Abstract

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 met 15 locus, and the remaining 16% occur at the met 2 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 met 2, met 15, and to a lesser extent for met 6, but not for any other methionine mutants. This loss of methionine requirement is produced with methyl mercury, phenyl mercury, and mercuric chloride although met 2 and met 15 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 met 2 and met 15 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 met 2 and met 15 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.