The SUC2 Gene in Yeast Encodes an Enzyme to Convert

Short Answer

The SUC2 gene in yeast encodes an enzyme to convert the sugar, sucrose, into glucose and fructose, which is necessary for yeast to use sucrose as a source of food. In the presence of glucose, SUC2 expression is switched off. But in the absence of glucose, SUC2 expression increases 100-fold. The expression of SUC2 in wild-type yeast and two mutant yeast strains is shown below. The mutations occur in two genes other than SUC2: SNF1 and SSN6. $$\begin{array} { | l | c c } \hline \text { Genotype } & { \text { SUC2 Expression } } \\\hline & + \text { glucose } & - \text { glucose } \\\hline \text { Wild type } & 1 & 100 \\\hline \text { snf1 } & < 1 & < 1 \\\hline \text { ssn } 6 & 100 & 100 \\\hline\end{array}$$ a. Is the SNF1 gene normally important for activation or repression of SUC2 expression?
b. Is the SSN6 gene normally important for activation or repression of SUC2 expression?
c. If SNF1 and SSN6 work in the same pathway to regulate SUC2 expression, the order in which the genes acts can be one of two possibilities (where $\rightarrow$ indicates activation and $\perp$ indicates inhibition): (1) SNF1 $\quad$ SSN6 $\quad$ SUC2
SSN6 acts to repress SUC2 expression, and SNF1 activates SUC2 by inhibiting the inhibitor, SSN6.
$\quad\quad\quad-\quad\quad\quad-$
(2) SSN6 $\quad$ SNF1 $\rightarrow$ SUC2
SNF1 activates $S U C 2$ expression, and SSN6 represses $S U C 2$ by inhibiting the action of SNF1. Design an experiment to distinguish between these two possible orders. What would you expect the outcome of your experiment to be in each of the two possible cases?