The Mating-Type Switch in Yeasts
In homothallic (self-compatible) strains of the yeasts Saccharomyces cerevisiae and Schizosac-charomyces pombe, individual cells switch their mating type (MT) in a programmed manner. In contrast, the cells of heterothallic (self-incompatible) strains have
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CONTENTS I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Basic Observations. . . . . . . . . . . . . . . . . . . . . . . . A. Homothallism and Heterothallism in Saccharomyces cerevisiae . . . . . . . . . . . . . . . . . . . B. Homothalism and Heterothallism in Schizosaccharomyces pombe . . . . . . . . . . . . . . . . C. Switching Rules. . . . . . . . . . . . . . . . . . . . . . . . . . . III. The Subtleties of Mating-Type Switching. . . . . A. Repression of Silent Mating-Type Loci. . . . . . . B. Directionality of Cassette Pairing. . . . . . . . . . . . C. The Mating-Type Switch . . . . . . . . . . . . . . . . . . . 1. Creation of the Double-Strand Break ..... 2. Switching Is Related to Gene Conversion. . D. Switching Genes in S. pombe ............... E. Cross-Connections with Repair of Radiation Damage and Recombination. . . . . . . . . . . . . . . . IV. Conclusions.............................. References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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I. Introduction
In homothallic (self-compatible) strains of the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, individual cells switch their mating type (MT) in a programmed manner. In contrast, the cells of heterothallic (selfincompatible) strains have a stable MT. These are called a and a in S. cerevisiae and h + and h - in S. pombe. Only cells of opposite MT can copulate. This chapter will deal with the intriguing process of MT switching, also termed MT interconversion. Yeasts are unicellular fungi; Figs. 1 and 2 provide an overview of their life cycles. S. cerevisiae has an extended diploid phase, while in S. pombe the diploid phase is normally restricted to the zygote. With regard to yeasts, the concepts "homothallism" and "heterothallism" do not relate to individual cells but to cell clones. A 1 Institut fUr Genetik, Technische Universitiit, Spielmann str. 7, D-38106 Braunschweig, Germany
strain is called homothallic when single haploid ascospores or cells yield cultures in which, under appropriate physiological conditions, copulation takes place. In single spore or cell clones of heterothallic strains, copulation does not occur. Haploid strains with opposite MTs have to be mixed in order to obtain zygotes. In the early days of Saccharomyces genetics, Winge (1935) found homothallic strains, whereas Lindegren and Lindegren (1943) isolated heterothallic strains. In his seminal paper on S. pombe, Leupold (1950) also encountered homothallic as well as heterothallic strains. Several reviews on MT switching in S. cerevisiae and S. pombe have been published by Egel (1989), Haber (1983), Herskowitz et al. (1992), Klar (1989, 1992b), Nasmyth (1982) and Strathern (1988). In these reviews, comprehensive bibliographies can be found. For recent short overviews, see Gutz and Schmidt (1990), Haber (1992) and Klar (1992a). Due to space limitations, an exhaustive synopsis of the literature cannot be presented, but some fundamental aspects of MT switching
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