Textbook Question
How do we know that the orientation of promoters relative to the transcription start site is important while enhancers are orientation independent?
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10. Transcription
Transcription in Eukaryotes
2:05 minutesProblem 4c
Textbook Question
The diagram below shows a DNA duplex. The template strand is identified, as is the location of the nucleotide.
If this region is a eukaryotic gene transcribed by RNA polymerase III, where are the promoter consensus sequences located?
Verified step by step guidance1
Understand that RNA polymerase III transcribes specific types of genes in eukaryotes, such as tRNA genes, 5S rRNA genes, and some small nuclear RNA (snRNA) genes. Its promoter regions are distinct from those of RNA polymerase II.
Recall that RNA polymerase III promoters are often located within the transcribed region of the gene itself, unlike RNA polymerase II promoters, which are typically upstream of the transcription start site.
For tRNA genes transcribed by RNA polymerase III, the promoter consensus sequences are internal and include two conserved regions: the A-box and the B-box, which are located downstream of the transcription start site.
For 5S rRNA genes, the promoter includes an internal control region (ICR) that contains three conserved elements: Box A, an intermediate element, and Box C, all of which are also downstream of the transcription start site.
To determine the exact location of the promoter consensus sequences in the given DNA duplex, identify whether the gene in question is a tRNA gene or a 5S rRNA gene, and locate the A-box and B-box (for tRNA) or the ICR elements (for 5S rRNA) within the transcribed region.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Eukaryotic Gene Structure
Eukaryotic genes typically consist of coding regions called exons and non-coding regions known as introns. The transcription of these genes is initiated at specific sites called promoters, which are located upstream of the transcription start site. Understanding the structure of eukaryotic genes is essential for identifying where transcription factors and RNA polymerase bind to initiate transcription.
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RNA Polymerase III
RNA polymerase III is one of the three main types of RNA polymerases in eukaryotic cells, primarily responsible for transcribing small RNA molecules, including tRNA and 5S rRNA. It recognizes specific promoter sequences that are distinct from those used by RNA polymerase II, which transcribes mRNA. Knowing the role of RNA polymerase III helps in understanding the transcription process of non-coding genes.
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Promoter Consensus Sequences
Promoter consensus sequences are short, conserved sequences of nucleotides that are recognized by transcription factors and RNA polymerase to initiate transcription. In eukaryotes, these sequences often include elements like the TATA box and other regulatory motifs. Identifying these sequences is crucial for understanding how genes are regulated and transcribed in response to cellular signals.
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