Explain how the following mutations would affect transcription of the yeast GAL1 gene in the presence of galactose.
A point mutation in the GAL1 core promoter that alters the sequence of the TATA box.

Verified step by step guidance

1. Understand the role of the TATA box in transcription: The TATA box is a core promoter element that helps position RNA polymerase II and general transcription factors to initiate transcription at the correct site.
2. Recognize that a point mutation in the TATA box sequence can disrupt the binding affinity of the transcription machinery, potentially reducing or abolishing the recruitment of RNA polymerase II.
3. Consider that in the presence of galactose, the GAL1 gene is normally activated by specific transcription factors, but if the core promoter is defective, these activators cannot effectively initiate transcription.
4. Predict that the mutation would likely lead to decreased or no transcription of the GAL1 gene despite the presence of galactose, because the basal transcriptional machinery cannot properly assemble at the promoter.
5. Summarize that the mutation in the TATA box impairs the fundamental step of transcription initiation, overriding the positive regulatory signals from galactose-induced activators.

Key Concepts

Role of the TATA Box in Transcription Initiation

The TATA box is a core promoter element that helps position RNA polymerase II for accurate transcription initiation. It is recognized by the TATA-binding protein (TBP), which is part of the transcription factor complex. Mutations in the TATA box can disrupt TBP binding, leading to reduced or abolished transcription initiation.

GAL1 Gene Regulation in Yeast

The GAL1 gene in yeast is induced in the presence of galactose and repressed when glucose is available. Its expression depends on the activation of transcription factors that bind upstream regulatory sequences, but proper transcription also requires an intact core promoter, including the TATA box, to recruit the transcription machinery.

Effects of Point Mutations on Promoter Function

A point mutation in the promoter region can alter critical DNA sequences necessary for transcription factor binding. In the TATA box, even a single nucleotide change can impair the assembly of the pre-initiation complex, reducing transcription efficiency or completely preventing gene expression despite the presence of activating signals like galactose.