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Transcription factor II B
Transcription Factor II B (TFIIB) is a general transcription factor that is involved in the formation of the RNA polymerase II preinitiation complex (PIC) and aids in stimulating transcription initiation. TFIIB is localised to the nucleus and provides a platform for PIC formation by binding and stabilising the DNA-TBP (TATA-binding protein) complex and by recruiting RNA polymerase II and other transcription factors. It is encoded by the gene.
==Structure==
TFIIB is a single 33kDa polypeptide consisting of 316 amino acids. It was originally thought to be essential at all promoters in order to recruit RNA polymerase II and initiate transcription, however recent research has shown that a depletion in TFIIB is not lethal to cells and transcription levels are not significantly affected. This is because over 90% of mammalian promoters do not contain a BRE (B recognition element) or TATA box sequence which are required for TFIIB to bind. In addition to this, TFIIB levels have been shown to fluctuate in different types of cell, and at different points in the cell cycle, supporting the evidence that it is not required for all RNA polymerase II transcription.
TFIIB is made up of four functional domains: The C terminal core domain; the B linker; the B reader and the amino terminal zinc ribbon. The protease resistant C terminal core〔 stabilises the DNA-TBP complex by interacting with nonspecific sequences either side of the TATA box called the upstream and downstream B recognition elements (BREu and BREd), as well as interacting with the Initiator element (INR). The core domain consists of two alpha helical structures that form nearly identical domains connected by a short linker region and rotated by 90 degrees between each other. Each of the domains has 5 alpha helices with a hydrophobic core. These two domains show a high sequence and structural similarity to cyclin A and are held together by intramolecular hydrophobic forces. The C terminus consists of another short alpha helix and a random coil. The B reader is formed of an alpha helix and mobile loop that is thought to play a role in the identification of the transcription start site.
Amino terminal zinc ribbon takes part in the recruitment of RNA polymerase II. The zinc ion is coordinated by cysteine and histidine residues arranged in beta sheets.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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