Questions for Thought

1) Hartwell identified ste4 mutations but no alleles of STE18 and SCG1. As we discussed previously, Hartwell’s screen was not saturating for a variety of reasons. Why would he have missed these genes? Think of as many reasons as possible for each gene. (Don’t worry about the logic of Whiteway et alia’s screen and why they recovered their alleles; if we have time at the end we’ll come back to it.)

2) How do they clone these genes? What is the logic? How do they show they have cloned the correct genes?

3) Table 2: Double mutant analyses between scg1 and ste4 or ste18 show that ste4 and ste18 are epistatic to scg1. Work through the table sufficiently to convince yourself that the markers for the double mutants (LEU⁺, URA⁺) show the ste18 or ste4 phenotype and not the scg1 phenotype. Rather than use the logic of Whiteway et al. to interpret these results, apply the logic of Hereford and Hartwell (Table 2) to analyze these interactions (i.e., what are the four ways these genes could be placed into a pathway?). What do you conclude? How does your conclusion compare to Whiteway et al.?

4) Now apply the Whiteway et al. logic to Hereford and Hartwell’s double mutant analysis. Why do the analyses give different interpretations? Who is correct? Why?

These analyses may be confusing but try to think through the logic as much as you can.

5) The authors suggest four hypotheses to explain their data. How well do the data fit these hypotheses? How would you distinguish among them?