A transcription factor is a protein involved in regulating DNA transcription, but not forming part of the RNA polymerase. Transcription factors (TF) can act by recognizing and binding to specific DNA sequences, binding to other factors, or directly binding to RNA polymerase.
- What are DNA Polymerase and its function in DNA Replication
- What is the Fidelity of DNA Replication in Normal?
Transcription factors are stimulated by cytoplasmic signals. When they activated to acquire the ability to regulate gene expression in the cell nucleus, by activating either repressing transcription of various genes. Certain transcription factors may undergo mutations that continuously remain active without external signals, thus becoming oncogenes, uncontrolled stimulating the synthesis of proteins involved in the regulation of the cell cycle.
This may lead to uncontrolled growth of cells and therefore a tumor. Numerous TFs whose importance has been demonstrated in cancer. Among these transcription factors that act as oncoproteins include Myc, Max, Myb, Fos, Jun, Rel, Ets, etc.
Transcription factors can be selectively activated or deactivated by other proteins, often as a final step in the chain of transmission of signals intracellular.
Transcription complexes in cells eukaryotes are much more complex than in prokaryotes due to the larger size genome eukaryotic, so the eukaryotic transcription complex needs a higher number of steps for assembly. Because no eukaryotic RNA polymerase can bind to the promoter sequences without involving another protein, the DNA sequence of the promoter region should be recognized by various transcriptional factors.
Types of Transcription Factors
Specificity of transcription factors
Transcription factors are usually class-specific genes. TF recognize and bind to specific sequences near the start site of transcription.
- The sequences to which the transcription factors gene promoters bind to class I vary tremendously from organism to organism.
- Transcription factors of the class II genes are joined to promoter sequences located upstream of the site of transcription initiation. The TATA box is the most common.
- The binding sites of transcription factors of class III genes are located within the transcribed region of the gene and internal controls are called regions. Internal control regions bind to various factors and form a stable transcription complex that binds RNA polymerase III. Transcription complexes remain stable assembled over many cycles of transcription.
- In some Pathogenic fungi transcription factor (PACC) recognizes the number of nucleotides 5′-GCCARG-3 ‘found in the promoter of several genes of importance in the pathogenesis process.
- The baseline factor transcription is necessary to initiate RNA synthesis in all promoters. RNA polymerase constitutes the basal transcription apparatus.
- The generic factors recognize promoters of genes expressed constitutionally.
- The specific factors recognize specific gene promoters of tissue. An example would be the genes T-box.
How do transcription factors work?
A typical transcription factor binds to DNA at a certain target sequence. Once it’s bound, the transcription factor makes it either harder or easier for RNA polymerase to bind to the promoter of the gene.
Some transcription factors activate transcription. For instance, they may help the general transcription factors and/or RNA polymerase bind to the promoter, as shown in the diagram below.
Diagram of an activator attached to a specific DNA sequence that is its binding site. The other end of the transcriptional activator (the one not bound to the DNA) interacts with general transcription factors, helping the general transcription factors and polymerase assemble tat the nearby promoter.
Other transcription factors repress transcription. This repression can work in a variety of ways. As one example, a repressor may get in the way of the basal transcription factors or RNA polymerase, making it so they can’t bind to the promoter or begin transcription.
Diagram of a repressor attached to a specific DNA sequence that is its binding site. When bound to this site, the repressor blocks formation of the transcription initiation complex at the promoter of a nearby gene.
c. Binding sites
The binding sites for transcription factors are often close to a gene’s promoter. However, they can also be found in other parts of the DNA, sometimes very far away from the promoter, and still affect the transcription of the gene.
The parts of an activator protein: the DNA binding domain (which attaches to the recognition site in the DNA) and the activation domain, which is the “business end” of the activator that actually promotes transcription, e.g., by facilitating the formation of the transcription initiation complex. The flexibility of DNA is what allows transcription factors at distant binding sites to do their job.
The DNA loops like cooked spaghetti bring far-off binding sites and factors of transcription close to general transcription factors or “mediator” proteins. In the cartoon above, and activating the transcription factor bound at a far-away site helps RNA polymerase bind to the promoter and start transcribing.