Question

Growth in nutrient‑rich medium represses expression of the yeast serine biosynthesis gene SER3. Martens, Laprade, and Winston repressed SER3 expression and found a highly transcribed region of DNA upstream of the SER3 gene. This upstream region contains a non‑protein‑coding gene called SRG1.

An RNA polymerase binds the SRG1 promoter and transcribes the SRG1 gene through the adjacent SER3 promoter, which leads to the repression of SER3. Mutations in the SRG1 promoter remove the repression of SER3.
Which of the statements explains how SRG1 transcription represses SER3 transcription?
(a) The SRG1 transcription process occurs at such a high rate that it titrates basal transcription factors away from other initiation sites.
(b) Transcription machinery on the SRG1 gene prevents binding of transcription factors on the SER3 promoter, blocking SER3 transcription.
(c) SRG1 RNA modifies the histones in the region of the SER3 promoter, creating a heterochromatic region around the SER3 gene.
(d) The protein product of the SRG1 gene binds to the SER3 promoter and prevents its transcription.

Answer 1

The transcription of the SRG1 gene represses SER3 transcription by hindering the binding of transcription factors to the SER3 promoter, blocking its initiation.

Step-by-step explanation:

The transcription of the SRG1 gene represses the transcription of the adjacent SER3 gene in yeast. When the SRG1 gene is transcribed, its transcriptional machinery overlies the SER3 promoter region. Given the options provided, the most likely explanation for how SRG1 transcription represses SER3 transcription is that the transcription machinery moving along the SRG1 gene interferes with the simultaneous binding of transcription factors at the SER3 promoter, thereby blocking the initiation of SER3 transcription. This is supported by the fact that mutations in the SRG1 promoter, which would likely prevent or reduce the transcription of SRG1, alleviate the repression on SER3. This indicates that the act of transcription through the SER3 promoter region, rather than a protein product of SRG1, is responsible for the repression.