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The MET15 gene of the yeast Saccharomyces cerevisiae is becoming a popular marker for molecular biological and genetical applications. The purpose of this page is to disseminate information concering the gene and its uses. All vectors and strains described here are available from the American Type Culture Collection. Here's how it works: yeast auxotrophic for the MET15 gene (also known as MET17 and MET25) overproduce hydrosulfide ions, which leak out of the cell. If the culture medium contains divalent lead ions (Pb2+), a dark brown to black precipitate is formed on the surface of the cell, rendering met15 strains easily differentiable from their otherwise congenic counterparts. As shown below, when the cells contain a CEN/ARS plasmid covering a chromosomal mutation, random loss of the plasmid can be observed as distinct colored sectors within the colony. Colony color can be distinguished in a matter of hours. Even better, the MET15 gene is counter-selectable (you can select for met15) with methylmercury. The MET15 gene thus combines the advantages of the widely used ADE2 and URA3 genes. Various cloning vectors and strains containing complete deletions of the chromosomal locus are available (see below).
Currently, lead dropout medium does not work. Phosphates and sulfates in YNB precipitate the lead when added. Synthetic complete (SC) medium (without lead) should also lack cysteine, as cysteine can be converted by S. cerevisiae into methionine via a MET15 independent pathway.
Methyl mercury links:
Despite all of the above, methylmercury(II) chloride has been used safely and successfully to isolate met15 mutants. (Cost and Boeke unpubl. data, and Dolinski et al.-see below)