You have isolated five yeast mutants (a - e) unable to metabolize galactose. All of the mutants fail to transcribe the GAL2 gene, which encodes the enzyme for the first step in metabolizing galactose. You want to determine whether each mutant is
- a cis-acting mutation of a regulatory sequence for GAL2itself;
- a trans-acting mutation in GAL4, which activates transcription ofGAL2 and other enzymatic genes in the galactose metabolic pathway; or
- a trans-acting mutation in GAL3, which interacts with GAL80 to de-repress GAL4 when galactose is present.
In analyzing each mutant, you make the following observations. NOTE: Yeast are normally haploid, but the diploid cells resulting from mating can persist as diploid colonies until induced to undergo meiosis.
Mutant a: When this mutant is crossed to a known GAL4 recessive mutant strain, the resulting diploid yeast colonies are all unable to metabolize galactose (i.e. failure to complement).
Mutant b: You do a cross and find this mutation maps to the vicinity of GAL3, and not to GAL4 or GAL2.
Mutant c: A double mutant haploid with both this mutation and a GAL80 mutation restores galactose metabolism, and GAL2 is transcribed even when galactose is absent.
Mutant d: All other enzyme-encoding genes (other than GAL2) in the galactose metabolism pathway are transcribed normally in this mutant.
Mutant e: The DNA sequence region containing the UAS sites upstream of GAL2 is deleted in this mutant.
Which mutant or mutants show evidence of a cis-regulatory mutation of GAL2?
Which mutant or mutants show evidence of a mutation in GAL4?
Which mutant or mutants show evidence of a mutation in GAL3?