Question

The yeast Saccharomyces cerevisiae can grow as haploid or diploid cells. You have two haploid yeast strains that each carry recessive mutations that affect regulation of the genes required for galactose metabolism. One strain has a deletion of the region of the genome on chromosome II that lies between the GAL1 and GAL10 genes (deltaUAS). The other strain carries a mutant allele of the GAL80 gene on chromosome XIII that produces no functional GAL80 protein. Which of the following correctly describe GAL80? (a) GAL80 is a transcriptional activator. (b) GAL80 is a trans-acting factor in the regulation of the GAL structural proteins. (c) GAL80 is allosterically regulated by galactose. (d) GAL80 masks the activation domain of GAL4. (e) GAL80 is an enzyme involved in the catabolism of galactose. Which of the following correctly describe UAS? (a) UAS is a cis-acting regulatory DNA sequence. (b) UAS is bound by the DNA-binding domain of GAL4. (c) UAS is a trans-acting factor that silences GAL gene expression. (d) UAS is allosterically regulated by binding galactose. (e) UAS is used in the enhancer of multiple GAL genes to result in coordinated induction. You cross these two strains together to get diploid progeny. Write down the diploid genotype. Expression of the GAL1 and GAL10 genes in the progeny will be (a) Permanently repressed (b) Induced in the precense of galactose (c) Constitutive (permanently expressed) You then induce meiosis to produce haploid progeny. Write down the haploid progeny genotypes. What proportion of the haploid progeny (expressed as a decimal) will have permanently repressed expression of GAL1 and GAL10? ____________ What proportion of the haploid progeny (expressed as a decimal) will have inducible expression of GAL1 and GAL10 in the presence of galactose? ____________ What proportion of the haploid progeny (expressed as a decimal) will have contstitutive expression of GAL1 and GAL10? ____________

Answer 1

Gal80 is a negative regulator for galactose genes. It mainly go and bind to the Gal4 thus inhibiting its expression when no inducing sugar(Galactose) is present. Thus it is option d.

UAS is mainly bound by the Gal4 protein. UAS is a cis regualtory site where the Gal4 binds and transcribe the target gene. Thus it is more preferably option b.

As gal80 is mutated, it can no longer mask gal4 and thus gal 4 can be produced but since uas is mutated gal 4 cannot go and bind to the sequence and the genes will not get expressed. Thus it is repressed permanently.