Neigeborn, L. & M. Carlson (1984) Genes affecting the regulation of SUC2 gene expression by glucose repression in Saccharomyces cerevisiae. Genetics 108: 845-858.
1. (a) Describe the method used to identify sucrose nonfermenting mutants.
Would you consider this a selection or a screen and why?
(b) Chemical mutagenesis with ethylmethane sulfonate was used to enrich for the desired class of mutant. Was this necessary or do you think that the authors could have worked with spontaneous mutations and why?
(c) Three 'isogenic' strains were used. Are they genetically identical, and if not, how do they differ? What is the purpose for including the his4 and lys2 nonsense mutations? [You may have to read about nonsense mutations and nonsense suppressors (sometimes called information suppressors) to answer this question],
2. In your own words, explain why complementation analysis can not be done with dominant mutations.
3. The authors crossed each mutant strain to an otherwise isogenic parental strain (of course of opposite mating type).
(a) What conclusion can be drawn if only PD tetrads are obtained?
(b) If TT or NPD tetrads are obtained, what does this indicate?
4. Diagram the cross between two sucrose nonfermenting mutants isolated from strains DBY782 (MATa ade2 SUC2) and the otherwise isogenic strain DBY916 (MATa his4-86oc lys2-802oc) in which the mutations in the two strains are in the same complementation group (i.e. gene). Call the gene FER1 and show the genotype and phenotype of the mutant parents, the diploid, and the resulting tetrads.
5. With regard to complementation Group 1:
(a) Why do the authors place the Group 1A, IB, and 1C mutations in a single complementation group and not three different complementation groups?
(b) How does the tetrad analysis data support this conclusion?
6. What is the evidence that complementation Group 1 (the SUC2 gene) encodes invertase?
7. What reasons are given to support the hypothesis that complementation Group 2 (the SNF1 gene) encodes a regulator of SUC gene expression?
8. What is the difference between the SUC genotype of the parental strains used in Articles 2 and 3? Why did Neigeborn & Carlson make this change for the second study?
9. (a) Define the term pleiotropic.
(b) SNF1, SNF2, SNF4, and SNF5 are pleiotropic. What phenotypes were scored?
(c) What does this pleiotrophy suggest about the functions of these genes?
(d) This suggestion is consistent with the finding that internal invertase is made in these mutant strains but not secreted invertase. Why?
10. (a) What is unique about the snf3 mutants compared with strains carrying mutations in the other SNF genes? (b) Comment on the authors' hypothesis regarding the function of Snf3 protein.
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