GAL4 activates the transcription of structural genes in yeas

GAL4 activates the transcription of structural genes in yeast. GAL80 inhibits GAL4\'s ability to activate transcription unless galactose is present. It was assumed that it mediated this effect by interacting directly with GAL80, changing its conformation and thereby changing the way it interacts with GAL4 (i.e. analogous to the conformational change induced in lac repressor when it binds inducer). However, there is strong evidence arguing against this possibility. A new regulatory gene has been identified, called gal3. The following question concerns the role of GAL3 in the regulation of the yeast gal genes. The phenotypes associated with mutations in gal3 are shown in the table below. (Note that gal7 is one of the structural genes regulated by GAL4. Assume that the alleles described in this table are equivalent to deletion of the corresponding gene.) Draw out a regulatory scheme that is consistent with these results and describe the regulatory interactions between GAL3, GAL4, and GAL80. Your scheme should be diagrammed using the symbols for activation and repression that clearly distinguishable (e.g. A rightarrow B ---| C rightarrow structural genes.) Your scheme should end with GAL4 rightarrow structural genes. Now, consider two more pieces of data: A missense mutation in gal80 (gene) is discovered that causes GAL80 to inhibit GAL4 even in the presence of galactose. The gal structural genes are not induced by galactose in this strain. A second-site suppressor of this mutation is discovered that is within gal3; this second mutation restores the galactose-inducibility of the gal structural genes. GAL3 (gene product) is closely related to the enzyme galactokinase (phosphorylates galactose), which uses galactose as a substrate. However, it lacks galactokinase activity. Propose a hypothesis for the molecular roles of GAL80, GAL3, and GAL4 in the regulation of the gal genes. Describe how your hypothesis accounts for each of the findings outlined above.

Solution

(a)

In the presence of galactose

Gal3 -------> Gal80 ------->/ Gal4 --------> Structural genes

In the absence of galactose

Gal3 ------->/ Gal80 -------> Gal4 -------->/ Structural genes

(b) Hypothesis:

In the presence of galactose, the Gal3 binds to the Gal80 and inhibit its binding to Gal4. The latter is thus free to bind to the structural genes.

Findings:

1. The missense mutation in gal80 prevents the binding of its product, Gal 80 with Gal 3. It implies that the Gal 80 is not modified and is free to inhibit Gal4.

The second site suppressor mutation restores the binding of Gal 3 with Gal 80 and as a result, the Gal 4 activates the structural genes.

2. Gal3 is closely related to galactokinase in galactose binding site but lack the kinase activity. Therefore, the galactose binds to Gal3 and activate it to bind to the Gal80.