A protein that binds to specific DNA sequences to regulate gene expression by turning genes on or off.A transcription factor is a protein that controls which genes a cell reads and when. It works by recognizing a specific short sequence of DNA — often just a handful of base pairs — and binding to it. Once bound, it can either recruit the molecular machinery that copies DNA into RNA, turning the gene on, or block that machinery, turning the gene off. Because a single transcription factor can regulate hundreds of different genes, and a single gene can be controlled by many transcription factors, these proteins form the wiring of the cell's decision-making. They are what allow a liver cell and a neuron to carry the same genome but behave completely differently: the difference lies not in the DNA sequence but in which transcription factors are active and which genes they switch on. Misregulation of transcription factors is behind many cancers, developmental disorders, and immune diseases, which is why they are among the most intensively studied classes of proteins in biology.
The concept emerged in the 1960s from work on gene regulation in bacteria, notably the operon model of François Jacob and Jacques Monod, which described proteins that switch genes on and off. The first eukaryotic transcription factors were identified in the 1980s, and the discovery of zinc finger and homeodomain motifs revealed how proteins could read DNA sequences with precision. Since then, hundreds of families have been catalogued.
Most transcription factors have at least two functional parts: a DNA-binding domain that recognizes a specific sequence, and an effector domain that interacts with other proteins. The DNA-binding domain often uses structural motifs such as zinc fingers, helix-turn-helix, or leucine zippers to slot into the major groove of the double helix and read the pattern of bases. When the factor binds near a gene's promoter or enhancer region, its effector domain recruits coactivators or corepressors, which in turn modify the local chromatin or directly contact RNA polymerase. Whether a given gene is on or off at a given moment depends on the combination of transcription factors present, their modifications, and the state of the chromatin around the gene.
Transcription factors are targets for drug development, especially in cancer, where drugs aim to block overactive factors like MYC or restore the function of tumor suppressors. They are also central tools in stem cell research, where introducing a few transcription factors can reprogram adult cells into induced pluripotent stem cells.
The human genome encodes roughly 1,600 transcription factors, about 6 percent of all protein-coding genes, making them one of the largest functional classes of proteins.