Question

Answer all questions and explain in detail.

1) GAL80 loss-of-function mutants express GAL1, 2, 7, 10, genes constitutively (in the absence of galactose) because

a	transcriptional activation by GAL4 is not inhibited
b	GAL80 is not present to block GAL4 DNA-binding
c	GAL80 cannot respond to GAL3 signaling
d	A and B
e	B and C

2) Gal80-; GAL4- double mutants express the GAL1, 7, 10 genes

a	only In the presence of galactose
b	only in the absence of galactose
c	never, whether galactose is present or not
d	always, whether galactose is present or not

3) A yeast strain is defective in inducing GAL7 expression in response to galactose. The induction of GAL1, GAL2, and GAL10 was normal. This may be caused by a mutation in:

a	GAL4 DNA binding domain
b	GAL4 activation domain
c	The UAS motif upstream of GAL7
d	Any of the above
e	None of the above

Answer 1

Answer 1: a

Explanation:

GAL4 is a transcriptional activator, which has two domains; a DNA binding domain, and an activation domain. In the absence of galactose, GAL80 mast the activation domain of GAL4 and inactivates it. GAL80 loss-of-function mutants produce non-functional GAL80, whcih can not mask the GAL4's activation domain, thus the related genes are constitutively expressed..

Answer 2: c

Explanation:

GAL4 and GAL80 double mutants cannot initiate transcription of GAL genes, as the GAL4 itself is inactivated and thus cannot activate transcription.

Answer 3: c

Explanation:

UAS (upstream activation sequence) is a cis-acting element, present upstream of the promoter of any gene and enhances the expression of  that gene. If the UAS motif is defective, the related gene will not express. Thus, in the case of defective UAS of GAL7, GAL7 is not expressed but the other genes; GAL1, GAL2,and GAL10 are expressed.