Association of Methionine Requirement with Methyl Mercury Resistant Mutants of Yeast

IT has been known for several years that strains resistant to mercury can be obtained in several bacterial species1–4. Soon after the correlation between resistance to antibiotics and to mercury was recognised2,5, it was established that genetic elements conferring resistance to antibiotics, mercury and other heavy metals in Escherichia coli and Salmonella typhimurium6and Staphylococcus aureus7,8reside on extrachromosomal resistance transfer factors or plasmids. Among fungi, mercury resistant strains of Botrytis cinerea9, Penicillium notatum, Sclerotinia fructicola, Stemphylium sarcinaeforme10, and Saccharomyces cerevisiae11,12have been reported. In most cases1,3,9–12, this was accomplished by ‘training’ the normal strains for growth on media supplemented with successively increasing concentrations of mercury compounds, and in some cases10the resistance was lost when subcultured on mercury-free media. It is noteworthy that in none of the mercury-adapted strains of fungi has the genetic basis of resistance been determined. In this report we describe a method of isolation and characterisation of methyl mercury resistant mutants of S. cerevisiae. This study was undertaken with the view that the examination of physiological changes associated with genetically defined resistant mutants will be useful in studying the mechanisms of cellular detoxification of organic mercurials.