Stevenson, B. J., Rhodes, N., Errede, B. and Sprague, G. F., jr. 1992. Constitutive mutants of the protein kinase STE11 activate the yeast pheromone response pathway in the absence of the G protein. Genes Devel. 6: 1293-1304.

Questions for Thought

1) What was known at the start of this work? Summarize the known pathway. What is the evidence that STE5, STE11, STE7, FUS3, KSS1 and STE12 act downstream of SCG1/STE4/STE18? What other ways could you test these relationships?

2) What is the goal of these experiments? How do they approach the problem? What controls do they perform to eliminate unwanted mutants?

3) Table 1: How do they characterize their mutants? Why do they perform these tests?

4) The authors use a two-step protocol to identify the genes mutated in their screen. What is the logic of their approach? THINK THROUGH THIS SECTION CAREFULLY. We will emphasize these studies in our discussion. What is their result? How do they interpret it? How do they confirm it?

5) Why do the authors recreate the STE11-1 and STE11-4 alleles in a defined background and reanalyze all their phenotypes? What is their result? What do they conclude about the function of STE5 in the pathway?

7) Summarize the evidence that Ste11p acts as a kinase upstream of Ste7p, Fus3p, Kss1p, and Ste12p. If STE7, KSS1 and FUS3 encode kinases also, why didn’t the authors obtain suppressor mutations in these genes during their screen?