transcription factor
/ tran-SKRIP-shun FAK-tor /
Imagine a stage manager standing by a long line of light switches, deciding which lamps come on for tonight's scene — turning some up, dimming others, leaving most off. Transcription factors are the cell's stage managers for its genes. They are proteins that bind to specific spots on DNA and decide whether nearby genes get copied, and how vigorously.
Precisely, a transcription factor is a protein that recognizes a particular short DNA sequence and binds it, thereby influencing transcription of one or more genes. Activators turn genes up — often by helping recruit or stabilize the copying machinery — while repressors turn genes down, by blocking it or by recruiting other dampening factors. Each transcription factor reads its own preferred sequence, so where it can act is written into the genome itself. A separate group, the general (basal) transcription factors, are needed at nearly every gene just to get RNA polymerase started; the regulatory ones described here add the gene-specific control on top.
Transcription factors matter because they are the master controllers of gene expression and therefore of cell identity. Which set of them is active in a cell largely decides whether it behaves as a neuron, a muscle fiber, or an immune cell. They are central to development, to how cells respond to signals, and to disease — many cancers involve a transcription factor stuck on or off. A single transcription factor often controls dozens of genes at once, which is why a small change in one can ripple widely.
The transcription factor MyoD can, on its own, push many ordinary connective-tissue cells to switch on a whole muscle program and become muscle-like — one master switch reaching across dozens of genes.
One transcription factor, a whole cell-type program.
Whether a transcription factor counts as an 'activator' or 'repressor' is not fixed: the same protein can activate one gene and repress another, depending on the context and what other factors are present.