Chapter 15, Problem 19P

A mutant haploid strain of Saccharomyces cerevisiae (yeast) called cox2-1 was found that was unable to grow on media containing glycerol as the sole source of carbon and energy. (Glycerol is a nonfermentable substrate for yeast.) This strain could, however, grow on the fermentable substrate glucose. Researchers discovered that cox2-1 cells lack a mitochondrial protein called cytochrome c oxidase.

a.	Explain why cox2-1 cells can grow on medium containing glucose but not on glycerol medium.
b.	When cox2-1 was crossed with a wild-type yeast strain and the resultant diploid cells were allowed to grow mitotically, it was found that about half the diploid clones were able to grow on glycerol, while the other half could not. The diploid clones that could grow on glycerol were induced to sporulate, and they yielded tetrads with four spores that were all able to grow on glycerol medium. In all of these tetrads, two of the haploid progeny were of mating type a and two of mating type α. The diploids that could not grow on glycerol could not sporulate. What do the results of the mating say about the location of the cox2-1 mutation?
c.	A different mutant strain of yeast called pet111-1 is also unable to grow on glycerol medium but still can grow on glucose medium. These mutant cells similarly lacked the cytochrome c oxidase. When pet111-1 was crossed with a wild-type haploid strain of the opposite mating type, the resultant diploids were able to grow on glycerol and yielded asci that all showed a 2:2 segregation of haploid cells that could or could not grow on glycerol. Explain these results in light of your answer to part (b)