Mutagenesis in Yeast

Mutations of numerous types can be induced in yeast. The basic principle is to bring the yeast in contact with the mutagen (UV light, X-rays, EMS, MMS, nitrous acid, nitrosoguanidine [NNG], ICR-170, nitrogen mustard, and so on), for long enough to bring about 50–95% killing, after which the mutagen is removed. The yeast strain may be grown in nutrient medium for a round of cell division to fix the mutation in the cells (a “recovery” step), and the cells are plated out on complete medium. Finally, the cells are replica plated to minimal medium, if auxotrophic mutants are desired, or on various other media designed to detect mutations such as resistance to a number of compounds (canavanine, cycloheximide, heavy metals, and similar toxic compounds). Recessive mutations are generally induced in haploid strains, but dominant mutations, including many of the mutations to resistance, can be induced in cells of higher ploidy. Mutations in the mitochondrial (mt) DNA form a special class of mutations in Saccharomyces cerevisiae , the classic petite colonie mutation, which manifests itself as a small colony on agar, and which lacks the ability to grow on nonfermentable substrates. The mtDNA in this mutation is nonfunctional or absent, giving rise to the respiratory deficiency, and the mutation occurs spontaneously, or is induced by manganese, and by many carcinogenic drugs, such as acriflavin, ethidium bromide, adriamycin, daunomycin, and benzidine, to name a few. Mutations conferring resistance to such antibiotics as chloramphenicol, erythromycin, oligomycin, and paromomycin, affecting the mitochondrial system, also occur in mtDNA.