Transcription factor

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In molecular biology, a transcription factor is a protein that regulates transcription. In particular, transcription factors regulate the binding of RNA polymerase and the initiation of transcription. A transcription factor binds upstream or downstream to either enhance or repress transcription of a gene by assisting or blocking RNA polymerase binding.

Transcription factors are usually modular proteins, have several effector domains (DNA binding & activator domain), and affect regulation of gene expression.

Transcription factors frequently bind to other bound transcription factors but may also bind directly to the DNA promoter region. A cluster of transcription factors is the preinitiation complex (PIC) that recruits and activates RNA polymerase. Conversely, repressor transcription factors inhibit transcription by blocking the attachment of activator proteins.

1 Regulation
2 Classes
3 DNA-Binding motifs found in transcription factors
4 Examples of transcription factors
- 4.1 STAT
  - 4.1.1 Function of STAT proteins
  - 4.1.2 Activation of STAT proteins
- 4.2 Core binding factors
5 See also
6 External links
- 6.1 Entries of transcription factors at genetic databases

[edit] Regulation

The regulation of transcription factors is a highly complex process as it is dependent upon a number of events, most notable of which are the presence of other DNA binding proteins (including other transcription factors) as well as local chromatin structure. Initial models, based on in vitro experiments suggested that there is a definite assembly sequence of transcription factors dictated by the DNA sequence.

It is, however, becoming increasingly obvious that the events leading to activation of transcription are dependent on a large number of factors and are highly intertwined. Furthermore epigenetic information present on DNA appears to play an important role in transcriptional activation.

[edit] Classes

There are three classes of transcription factors:

General transcription factors are involved in the formation of a preinitiation complex. The most common are abbreviated as TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH. They are ubiquitous and interact with the core promoter region surrounding the transcription start site(s) of all class II genes.
Upstream transcription factors are proteins that bind somewhere upstream of the initiation site to stimulate or repress transcription.
Inducible transcription factors are similar to upstream transcription factors but require activation or inhibition.

General Definition- Protein involved in recognition by RNA polymerases of specific regulatory sequences in eukaryotic genes.

[edit] DNA-Binding motifs found in transcription factors

Helix-turn-helix (HTH) bind the major groove of the DNA.
Zinc fingers function as structural platforms for DNA binding.
Leucine zippers function in associating the transcription factors with each other.
Basic-helix-loop-helix (bHLH) bind DNA with two alpha helices containing basic amino acid residues which are linked by a loop and are typically dimeric.
G-quadruplex Motifs are a focus for current studies in their role as a TF binding site.
The MADS-box encodes a DNA-binding domain found in e.g. floral homeotic genes.

[edit] Examples of transcription factors

[edit] STAT

The Signal Transducers and Activator of Transcription (STAT) proteins regulate many aspects of cell growth, survival and differentiation. The transcription factors of this family are activated by the Janus Kinase JAK and dysregulation of this pathway is frequently observed in primary tumors and leads to increased angiogenesis and enhanced survival of tumors. Knockout studies have provided evidence that STAT proteins are involved in the development and function of the immune system and play a role in maintaining immune tolerance and tumor surveillance.

[edit] Function of STAT proteins

STAT proteins were originally described as latent cytoplasmic transcription factors that require phosphorylation for nuclear retention. The unphosphorylated STAT proteins shuttle between the cytosol and the nucleus waiting for its activation signal. Once the activated transcription factors reach the nucleus, they bind to a consensus DNA-recognition motif called gamma activated sites (GAS) in the promoter region of cytokine-inducible genes and activate transcription of these genes.....

[edit] Activation of STAT proteins

Extracellular binding of Cytokines induces activation of the intracellular Janus kinase that phosphorylates a specific tyrosine residue in the STAT protein which promotes the dimerization of STAT monomers via their SH2 domain. The phosphorylated dimer is then actively transported in the nucleus via importin a/b and RanGDP complex. Once inside the nucleus the active STAT dimer binds to cytokine inducible promoter regions of genes containing gamma activated site (GAS) motif and activate transcription of this proteins. The STAT protein can be dephosphorylated by nuclear phosphatases which leads to inactivation of STAT and the transcription factor becomes transported out of the nucleus by exportin crm1/RanGTP.