transcription factor
/ TF /
Imagine a vast library where every book is a gene, and most are kept shut. A transcription factor is like a knowledgeable librarian who walks straight to one particular shelf, recognizes a specific book by a code printed on its spine, and either props it open for reading or slams it shut. It does not read the book itself — it decides whether the reading machinery is allowed to get to work there.
More precisely, a transcription factor is a protein that binds to a specific short stretch of DNA and, by doing so, turns nearby gene transcription up or down. A typical transcription factor is built from two functional parts: a DNA-binding domain, the shaped surface that reads a particular DNA sequence (often six to ten letters long) and grips it, and an activation domain (or, for repressors, a repression domain), the part that talks to the rest of the transcription machinery and says 'come here and start' or 'stay away'. The DNA-binding domain is what gives a factor its address; the activation domain is what gives it its verb. Common DNA-binding shapes have names you will meet again — helix-turn-helix, zinc finger, leucine zipper — each a different way of slotting protein into the DNA grooves.
Transcription factors are the master switches of the cell. There are well over a thousand of them encoded in the human genome, and which ones are present and active in a given cell, in what combination, is largely what makes that cell a liver cell rather than a neuron. They are also where signals from outside the cell ultimately land: a hormone or growth signal often ends its journey by switching a transcription factor on, which then changes which genes are read. A caution worth carrying: 'transcription factor' is used both broadly (any protein that affects transcription) and narrowly (the sequence-specific regulators described here); in everyday usage it usually means the sequence-specific kind.
The protein p53 is a transcription factor: when DNA is damaged, it accumulates, binds its target DNA sequence near dozens of genes, and switches on those that pause the cell cycle or trigger cell death. Lose p53's function — as happens in over half of human cancers — and that protective program never fires.
Two parts, one job: a DNA-binding domain finds the gene, an activation domain throws the switch.
Do not confuse sequence-specific transcription factors (the regulators here) with the general transcription factors (TFIID and friends) that the basic machinery needs at every promoter. Both are called 'transcription factors', but they do very different jobs.