transcription

Examples of transcription in the following topics:

• The Promoter and the Transcription Machinery

  • When transcription factors bind to the promoter region, RNA polymerase is placed in an orientation that allows transcription to begin.
  • The purpose of the promoter is to bind transcription factors that control the initiation of transcription.
  • RNA polymerase binds to the transcription initiation complex, allowing transcription to occur.
  • To initiate transcription, a transcription factor (TFIID) is the first to bind to the TATA box.
  • In addition to the general transcription factors, other transcription factors can bind to the promoter to regulate gene transcription.

• Cancer and Transcriptional Control

  • Alterations in cells that give rise to cancer can affect the transcriptional control of gene expression.
  • Mutations that activate transcription factors, such as increased phosphorylation, can increase the binding of a transcription factor to its binding site in a promoter.
  • This could lead to increased transcriptional activation of that gene that results in modified cell growth.
  • This can lead to increased phosphorylation of key transcription factors that increase transcription.
  • Transcription factors, especially some that are proto-oncogenes or tumor suppressors, help regulate the cell cycle; however, when regulation gives rise to cancer cells, then transcriptional control of gene expression is affected.

• Transcriptional Enhancers and Repressors

  • Enhancers increase the rate of transcription of genes, while repressors decrease the rate of transcription.
  • Enhancer regions are binding sequences, or sites, for transcription factors.
  • Transcriptional repressors can bind to promoter or enhancer regions and block transcription.
  • Like the transcriptional activators, repressors respond to external stimuli to prevent the binding of activating transcription factors.
  • An enhancer is a DNA sequence that promotes transcription.

• Prokaryotic versus Eukaryotic Gene Expression

  • When the resulting protein is no longer needed, transcription stops.
  • When more protein is required, more transcription occurs.
  • Therefore, in prokaryotic cells, the control of gene expression is mostly at the transcriptional level.
  • The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus within the cytoplasm.
  • Prokaryotic transcription and translation occur simultaneously in the cytoplasm, and regulation occurs at the transcriptional level.

• Initiation of Transcription in Eukaryotes

  • Initiation is the first step of eukaryotic transcription and requires RNAP and several transcription factors to proceed.
  • The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription pre-initiation complex (PIC).
  • The TATA box, as a core promoter element, is the binding site for a transcription factor known as TATA-binding protein (TBP), which is itself a subunit of another transcription factor: Transcription Factor II D (TFIID).
  • Activator proteins increase the transcription rate, and repressor proteins decrease the transcription rate.
  • Transcription factors recognize the promoter, RNA polymerase II then binds and forms the transcription initiation complex.

• Attenuation

  • In bacteria, transcription and translation are capable of proceeding simultaneously.
  • Transcription-attenuation is characterized by the presence of 5'-cis acting regulatory regions that fold into alternative RNA structures which can terminate transcription.
  • These RNA structures dictate whether transcription will proceed successfully or be terminated early, specifically, by causing transcription-attenuation.
  • This characterizes the mechanisms of transcription-attenuation.
  • This schematic represents transcriptional-attenuation as the formation of mRNA stem-loops prevents the continuance of transcription based on the levels of tryptophan in the metabolic environment.

• Regulation of Sigma Factor Activity

  • Sigma factors are proteins that function in transcription initiation .
  • Sigma factor synthesis is controlled at the levels of both transcription and translation.
  • If transcription of genes involved in growth is necessary, the sigma factors will be translated to allow for transcription initiation to occur.
  • However, if transcription of genes is not required, sigma factors will not be active.
  • The anti-sigma factors are responsible for regulating inhibition of transcriptional activity in organisms that require sigma factor for proper transcription initiation.

• Cancer and Post-Transcriptional Control

  • Modifications, such as the overexpression of miRNAs, in the post-transcriptional control of a gene can result in cancer.
  • Post-transcriptional regulation is the control of gene expression at the RNA level; therefore, between the transcription and the translation of the gene.
  • After being produced, the stability and distribution of the different transcripts is regulated (post-transcriptional regulation) by means of RNA-binding proteins (RBP) that control the various steps and rates of the transcripts: events such as alternative splicing, nuclear degradation (exosome), processing, nuclear export (three alternative pathways), sequestration in DCP2-bodies for storage or degradation, and, ultimately, translation.
  • Changes in the post-transcriptional control of a gene can result in cancer.

• Prokaryotic Transcription and Translation Are Coupled

  • Prokaryotic transcription is the process in which messenger RNA transcripts of genetic material in prokaryotes are produced, to be translated for the production of proteins.
  • Prokaryotic transcription occurs in the cytoplasm alongside translation .
  • Prokaryotic transcription and translation can occur simultaneously.
  • Transcription is controlled by a variety of regulators in prokaryotes.
  • Additional transcription regulation comes from transcription factors that can affect the stability of the holoenzyme structure at initiation.

• Initiation of Transcription in Prokaryotes

  • These subunits assemble each time a gene is transcribed; they disassemble once transcription is complete.
  • The fifth subunit, σ, is involved only in transcription initiation.
  • A promoter is a DNA sequence onto which the transcription machinery binds and initiates transcription .
  • The transcription initiation phase ends with the production of abortive transcripts, which are polymers of approximately 10 nucleotides that are made and released.
  • The σ subunit dissociates from the polymerase after transcription has been initiated.