The yeast GAL4 gene encodes a transcriptional regulator that can bind DNA upstre

Question

The yeast GAL4 gene encodes a transcriptional regulator that can bind DNA upstream of genes required for the metabolism of the sugar galactose and turn them on. Gal4 has a DNAbinding domain and an activation domain. The DNA-binding domain allows it to bind to the appropriate sites in the promoters of the galactose metabolism genes. The activation domain attracts histone-modifying enzymes and also binds to a component of the RNA polymerase II enzyme complex, attracting it to the promoter so that the regulated genes can be turned on when Gal4 is also bound to the DNA. When Gal4 is expressed normally, the genes can be maximally activated. You decide to try to produce more of the galactose metabolism genes by overexpressing the Gal4 protein at levels fiftyfold greater than normal. You conduct experiments to show that you are overexpressing the Gal4 protein and that it is properly localized in the nucleus of the yeast cells. To your surprise, you find that too much Gal4 causes the galactose genes to be transcribed only at a low level. What is the most likely explanation for your findings?

Explanation / Answer

This an example of positive control or positive repression. In this excess of GAL4 protein might be altering to alter the shape so that it doesn’t bind to the DNA and transcription occurs or activating the regulator protein that binds to the operator which prevents RNA polymerase from transcribing.

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