The transcription activator protein GAL4 has three functionally distinct regions
ID: 93925 • Letter: T
Question
The transcription activator protein GAL4 has three functionally distinct regions. Explain what happens to the following three yeast strains with respect to regulation and the galactose gene regulation and the strain's ability to grow on galactose. a) Strain one has a mutation that inactivates the GAL80 binding region of GAL4 so that it cannot bind the GAL80 protein. b) Strain two has a mutation that inactivates the DNA binding region of the GAL4 protein. c) Strain three has mutations that delete the Gal 1 and Gal 3 genes.Explanation / Answer
a) GAL80 is an inhibitor of GAL4. Its binds to GAL4 thereby hindering its binding to it promoter region (UAS = upstream activating sequence). GAL80 holds inactivated upon induction by galactose. If the GAL80 binding region is mutated hence GAL80 no longer can hold back GAL4 from initiating the transcription of the gene (involved in galactose metabolism) which lies downstream to UAS eg. Gal1. Hence, as the regulation is lost, this gene would now be constitutively expressed in the strain1 yeast irrespective of the fact that galactose is present or not. It will grow on galactose anyway.
b) Mutation in the DNA binding domain would not allow GAL4 from sitting on its specific UAS, irrespective of the fact that whether GAL80 is present or not. Hence the gene lying downstream of this UAS is permanently shut off and will never be expressed in strain2. This means that genes for galactose metabolism are not transcribed and so this strain will not be able to grow on galactose.
c) Gal1 is the genes involved in galactose metabolism which are activated/trancribed upon the binding of GAL4 to UAS while Gal3 is an activator of GAL4. If these gene are deleted,the whole GAL80-GAL4-UAS regulation system holds futile i.e. no galactose metabolism would occur . Hence, strain3 would not be able to grow on galactose.
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