This is easily observed since the existence of this product turns the colony blue, where as those lacking this product remain white. In addition to the HIS3 linked to FUS1 promoter, there is another histadine gene located at a different locus which has a deletion that makes it defective. As will be discussed later, this second gene is important for identifying spontaneous mutants in the pathway after mutagenesis. A diagram of the signal transduction pathway in a- cells. The exact function of STE 5 is unknown.
Mutagenesis is the first step in a mutant hunt.
Mutants are made quickly and on a large scale by subjecting yeast to UV radiation. A level of radiation is determined that will increase the chance of producing mutants with a single mutation to avoid complicating the analysis. Multiple mutations would greatly add to the difficulty determining the nature of the mutations. Note, that growth can not be due to expression of the second histadine gene in the strain since it is nonfunctional. However, there could be other reasons for this growth, such as a mutation in the promoter region of the FUS1 leading to its constitutive expression.