FIG. 1.

Genetic selection for negative regulators of α2 proteolysis. (A) Selection scheme. The screen exploits the portability of the well-characterized Deg1 degradation signal of α2 (16). Yeast cells, whose only functional copy of URA3 is the Deg1-URA3 construct, require uracil for growth, since the Deg1-Ura3 protein is rapidly degraded. Overexpressed cDNAs that stabilize Deg1-Ura3 and Deg1-β-gal were isolated by their ability to allow cell growth in the absence of uracil and by an increase in β-gal activity. (B). Overexpression of TUP1 confers uracil prototrophy. Yeast cells expressing the Deg1-URA3 construct (JY104) were transformed with the empty GAL1 expression vector or with TUP1 expressed from this vector and grown on minimal complete media or minimal media lacking uracil.