This chapter deals only with the yeast S. cerevisiae, and related interbreeding species. The fission yeast Schizosaccharomyces pombe, which is only distantly related to S. cerevisiae, has equally important features, but is not as well characterized. The general principles of the numerous classical and modern approaches for investigating S. cerevisiae are described, and the explanation of terms and nomenclature used in current yeast studies are emphasized . This article should be particularly useful to the uninitiated who are exposed for the first time to experimental studies of yeast. Detailed protocols are described in the primary literature and in a number of reviews in the books listed in the Bibliography. The original citations for the material covered in this chapter also can be found in these comprehensive reviews.

Although yeasts have greater genetic complexity than bacteria, containing 3.5 times more DNA than Escherichia coli cells, they share many of the technical advantages that permitted rapid progress in the molecular genetics of prokaryotes and their viruses. Some of the properties that make yeast particularly suitable for biological studies include rapid growth, dispersed cells, the ease of replica plating and mutant isolation, a well-defined genetic system, and most important, a highly versatile DNA transformation system. Unlike many other microorganisms, S. cerevisiae is viable with numerous markers. Being nonpathogenic, yeast can be handled with little precautions. Large quantities of normal bakers’ yeast are commercially available and can provide a cheap source for biochemical studies.

Unlike most other microorganisms, strains of S. cerevisiae have both a stable haploid and diploid state. Thus, recessive mutations can be conveniently isolated and manifested in haploid strains, and complementation tests can be carried out in diploid strains. The development of DNA transformation has made yeast particularly accessible to gene cloning and genetic engineering techniques. Structural genes corresponding to virtually any genetic trait can be identified by complementation from plasmid libraries. Plasmids can be introduced into yeast cells either as replicating molecules or by integration into the genome. In contrast to most other organisms, integrative recombination of transforming DNA in yeast proceeds exclusively via homologous recombination. Exogenous DNA with at least partial homologous segments can therefore be directed at will to specific locations in the genome. Also, homologous recombination, coupled with yeasts’ high levels of gene conversion, has led to the development of techniques for the direct replacement of genetically engineered DNA sequences into their normal chromosome locations. Thus, normal wild-type genes, even those having no previously known mutations, can be conveniently replaced with altered and disrupted alleles. The phenotypes arising after disruption of yeast genes has contributed significantly toward understanding of the function of certain proteins in vivo. Many investigators have been shocked to find viable mutants with little of no detrimental phenotypes after disrupting genes that were previously assumed to be essential. Also unique to yeast, transformation can be carried out directly with synthetic oligonucleotides, permitting the convenient productions of numerous altered forms of proteins. These techniques have been extensively exploited in the analysis of gene regulation, structure-function relationships of proteins, chromosome structure, and other general questions in cell biology. The overriding virtues of yeast are illustrated by the fact that mammalian genes are being introduced into yeast for systematic analyses of the functions of the corresponding gene products.

In addition, yeast has proved to be valuable for studies of other organisms, including the use of the two-hybrid screening system for the general detection of protein-protein interactions, the use of YACs for cloning large fragments of DNA, and expression systems for the laboratory and commercial preparation of heterologous proteins. Many of these techniques are described herein.