Pleiotropic Mutations at the TUP1 Locus That Affect the Expression of Mating-Type-Dependent Functions in SACCHAROMYCES CEREVISIAE

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The umr7–1 mutation, previously identified in a set of mutants that had been selected for defective UV-induced mutagenesis at CAN1, affects other cellular functions, including many of those regulated by the mating-type locus (MAT) in heterothallic Saccharomyces cerevisiae. The recessive umr7–1 allele, mapping approximately 20 cM distal to thr4 on chromosome III, causes clumpy growth in both a and α cells and has no apparent effect on a mating functions. However, α umr7 meiotic segregants fail to express several α-specific functions (e.g., high-frequency conjugation with a strains, secretion of the hormone α-factor and response to the hormone a-factor). In addition, α umr7 cells exhibit some a-specific characteristics, such as the barrier phenotype (Bar+) that prevents diffusion of α-factor and an increased mating frequency with α strains. The most striking property of α umr7 strains is their altered morphology, in which mitotic cells develop an asymmetric pear shape, like that of normal a cells induced to form "shmoos" by interaction with α-factor. Some a/α-specific diploid functions are also affected by umr7; instead of polar budding patterns, a/α umr7/umr7 diploids have medial budding like a/a, α/α and haploid strains. Moreover, a/α umr7/umr7 diploids have lost the ability to sporulate and are Bar+ like a or a/a strains. Revertant studies indicate that umr7–1 is a single point mutation. The umr7 mutant fails to complement mutants of both tup1 (selected for deoxythymidine monophosphate utilization) and cyc9 (selected for high iso-2-cytochrome c levels), and all three isolates have similar genetic and phenotypic properties. It is suggested that the product of this gene plays some common central role in the complex regulation of the expression of both MAT-dependent and MAT-independent functions.

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