Transcription factors are protins that bind specific sites or elements in regulatory regoions of DNA, known as promoters or enhancers, where they control the transcription or expression of target genes. They are encoded by a unique calss of genes amounting in the animal genome to only a few percent of the total number of protein- coding genes. The recognition of DNA sequence by TFs occurs by chemical interactions of the amino acid side chains of the TF protein with base pair residues of DNA functioning as regulatory sequence. The TFs thus read the genomic sequence and the most fundamental fact about this mechanism is that it is the sequence recognition function ion which informational aspects of all regulatory transactions controlling gene expression depend. Only a relatively small, finite number of protein structural motifs have evolved which have the capacity to recognize and bind to specific DNA sequence. With few exceptions, all animals utilise the same families of TFs, representatives of which were evidently present in their common evolutionary ancestor. Transcription factors consist of DNa- binding domains by which these families are defined and effector domains that mediate interactions with other proteins necessary for transcription.
There are three functional classes of transcription factors: general transcription factors, which are ubiquitous and represent the core machinery of transcription; the most common are abbreviated as TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH; (2) constitutively expressed factors that in each cell type constitutively stimulate or repress transcription; (3) and inducible transcription factors, which are similar...
