酵母细胞利用半乳糖的几种酶基因GAL7-GAL10-GAL1,它们的转录是如何受调控因子GAL4和GAL80调节的?中科院01年的考题

Question

酵母细胞利用半乳糖的几种酶基因GAL7-GAL10-GAL1,它们的转录是如何受调控因子GAL4和GAL80调节的?
中科院01年的考题

Answer 1

GAL7,GAL10,and GAL1 are three related enzymes that enzymes used to convert galactose to glucose.They are clustered close together on chromosome 2.These genes are induced by galactose and are controlled by the positive regulator gene gal4 and the negative regulator gene gal80.
A galactose-specific upstream activating sequence (UASG) that serves as a binding site for a galactose-specific transcription enhancing factor (the GAL4 gene product) is associated with the promoter for each of these genes.The binding domain is a 17 bp palindrome,which is repeated four times in the UAS.
Transcription of all three of these genes (and also of the GAL2 gene,which is on a separate chromosome) requires the binding of GAL4 to the UAS to be activated above a very low basal level.The GAL4 protein has the expected properties of a transcription activating factor.These include a DNA-binding domain that is specific for UASG,and two separate transcriptional activation domains that must both be intact for full activity.However,the GAL4 protein does not interact directly with galactose.Instead,it has a binding site for the GAL80 protein,and is maintained in an inactive condition when galactose is not present by being bound to that protein.
The GAL80 gene product is the only protein involved in induction of the lactose utilization genes that is capable of direct interaction with galactose.In a sense,the GAL80 protein functions as a repressor,but it does not do so by binding an operator site.Insterad,in the absence of galactose,the GAL80 protein binds to the GAL4 transcriptional activator protein and blocks its ability to interact with the UASG and enhance transcription of the lactose utilization genes.When galactose enters the cell,it binds to the GAL80 protein and causes it to undergo an allosteric change,which in turn causes dissociation of the GAL 80 protein from the GAL4 protein and allows the GAL4 protein to function as a positive-acting transcription factor for the galactose utilization genes.The increase in transcription is presumed to be accomplished by an interaction between the activation domains of GAL 4 and the RNA polymerase II initiation complex,as depicted in Figure 19.13 for classical enhancers.
In the absence of galactose,GAL80 binds to GAL4 and prevents it from acting as a transcription-enhancing factor for GAL1,GAL7,and GAL10.When galactose is present,it binds to GAL80,causing it to dissociate from GAL4.GAL4 then binds to the UASG associated with each of GAL1,GAL7,and GAL10 and stimulates transcription of all three of these genes.
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