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Carlson, M.B., C. Osmond, L. Neigeborn, & D. Botstein (1984) A suppressor of snfl mutations causes constitutive high-level invertase synthesis in yeast. Genetics 107: 19-32.

1. The authors isolated 30 revertants of sucrose nonfermenting mutant strains DBY934, DBY1052, and DBY1053, each of which carries the snfl-28 allele.

(a) Describe the selection method used to isolate the revertants.

(b) 'To test for dominance, each revertant was crossed to a snfl SSN strain.' Diagram one example of such a cross showing only the snfl and ssn genes. Give the genotype and phenotype of the parental strains and the diploid.

(c) If the revertant had been a true revertant to wild-type SNF1, what would have been the phenotype of the diploid from the cross in part (b) above?

(d) Were any of these revertants true revertants or were they all suppressors of snfl, that is a suppressor mutation in an ssn gene? How do the mapping results shown in Table 4 support the conclusion that these revertants were intergenic suppressors?

2. Diagram the cross used to demonstrate that:

(a) ssn6-3 and ssn6-2 are in the same complementation group.

(b) ssn6-2 and ssn5-l are in different complementation groups.

For both of these crosses give the SSN and SNF1 genotype and the phenotype of both parental strains and the resulting diploid.

3. Answer the following questions based on the results reported in Table 3.

(a) Which ssn mutations are epistatic to snfll

(b) Which ssn mutations cause constitutive expression of invertase?

(c) What do the authors feel is unique about the ssn6 mutations?

4. Based on the results reported in Table 4, are strains of the ssn SNF1 genotype sucrose fermenters or nonfermenters and how did you determine this?

5. Answer the following questions based on the results reported in Table 5.

(a) Which result(s) indicate that ssn6-l is recessive to the wild-type allele?

(b) Which result(s) suggest that cyc8-l might be partially dominant to the wild-type allele?

6. Present evidence to support the following statements.

(a) Ssn6 protein has multiple functions in the cell.

(b) Ssn6 protein acts as a repressor of SUC2 expression.

7. Mutations in SSN6 have been identified in several laboratories from different mutant hunts. List the four other names for SSN6 and the mutant phenotype used to identify these mutant alleles.

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