9.8
RNA Polymerase II Accessory Proteins
Proteins that regulate transcription can do so either via direct contact with RNA Polymerase or through indirect interactions facilitated by adaptors,…
In the eukaryotic cells, the initiation of transcription is a complex process that needs meticulous coordination between certain DNA elements and protein components.
Besides RNA polymerase II and transcription factors, other regulatory factors are also involved in gene expression regulation during transcription initiation.
Enhancer sequences are one such transcriptional control element that can exert their effects independent of their distance from the promoter.
They are regulatory DNA segments of about 50 to 200 base pairs that contain multiple binding sites for gene regulatory proteins known as transcriptional activators.
Transcriptional activators are modular proteins consisting of two domains, a DNA binding domain and an activation domain.
The DNA binding domain anchors the protein to a specific enhancer sequence, while the activation domain interacts with different elements of the transcriptional machinery to stimulate transcription.
However, this interaction between the activator and the transcriptional machinery requires the presence of a multi-subunit complex called the mediator.
The structurally flexible network of proteins in the mediator relays signals from the activator to the RNA Polymerase II and transcriptional factors, which results in the upregulation of the transcription of a linked gene that would otherwise be only transcribed at a basal level.
In contrast to the activators, repressors regulate gene expression by inhibiting transcription.
Active repressors contain functional domains that interact with general transcription factors to interfere with their binding to the DNA.
Another repressor type binds near the transcription start site, blocking the interaction of RNA polymerase or general transcription factors with the promoter.
The third type of repressor competes with activators for binding to enhancers.
Overall, the combined actions of multiple different transcriptional regulatory proteins are responsible for gene expression control during embryonic development and cell differentiation.
View the full transcript and gain access to JoVE Core videos
Q1: What role do accessory proteins play in RNA polymerase II function?
RNA polymerase II accessory proteins regulate transcription by facilitating enzyme recruitment, stabilizing protein-DNA interactions, and modulating polymerase activity. These proteins work alongside general transcription factors to enable efficient initiation and elongation of mRNA synthesis. They also coordinate with chromatin remodeling complexes to access DNA templates within condensed chromatin structures.
Q2: How do accessory proteins influence transcription elongation?
Accessory proteins regulate transcription elongation by controlling RNA polymerase II pausing and resumption. Transcription elongation factors work with the polymerase to overcome pause sites and maintain productive transcription. These proteins also facilitate the transition from initiation to elongation phases, ensuring continuous mRNA synthesis.
Q3: What is the relationship between RNA polymerase II accessory proteins and pre-mRNA processing?
RNA polymerase II accessory proteins coordinate with pre-mRNA processing machinery during transcription. These proteins help couple transcription with capping, splicing, and polyadenylation events. By recruiting processing factors to the elongating polymerase, accessory proteins ensure that pre-mRNA modification occurs co-transcriptionally, enhancing efficiency and accuracy of mRNA maturation.
Q4: How do accessory proteins help RNA polymerase II navigate chromatin?
Accessory proteins facilitate RNA polymerase II transcription through chromatin by recruiting histone modifiers and chromatin remodeling complexes. These proteins help establish transcription-permissive chromatin states and maintain polymerase progression through nucleosome-rich regions. Chromatin structure regulates pre-mRNA processing efficiency, making accessory protein function critical for gene expression in eukaryotes.
Q5: What distinguishes RNA polymerase II accessory proteins from general transcription factors?
General transcription factors are required for basal transcription initiation at all protein-coding genes, while accessory proteins provide regulatory functions beyond basal levels. Accessory proteins enable gene-specific regulation, respond to cellular signals, and modulate transcription rates. They work cooperatively with general transcription factors to achieve regulated and efficient mRNA synthesis.
Q6: How do accessory proteins contribute to alternative RNA splicing regulation?
RNA polymerase II accessory proteins influence alternative RNA splicing by recruiting specific splicing factors and controlling polymerase elongation rates. Slow polymerase progression allows weak splice sites to be recognized, while rapid elongation favors strong sites. This coupling of transcription and splicing through accessory proteins enables regulated splicing exons and introns selection.
Q7: Why are accessory proteins essential for productive transcription in eukaryotes?
Accessory proteins are essential because they enable RNA polymerase II to overcome multiple regulatory checkpoints and physical barriers during transcription. They facilitate promoter recognition, stabilize polymerase-DNA complexes, promote elongation through chromatin, and couple transcription with RNA processing. Without accessory proteins, eukaryotic transcription would be inefficient and poorly regulated.