Definition
Transcription factors are proteins that bind to regulatory stretches of DNA and thereby switch nearby genes on or off, or tune how strongly they are transcribed. They are the cell's hands on the genome's controls.
Sapolsky gives transcription factors a starring role because they answer a crucial question: how does an event in the outside world end up changing what a gene does? The chain runs from environment, to signal, to transcription factor, to a gene that is now read differently. They are the bridge from experience to DNA.
Why it matters
How it works
A transcription factor carries a region shaped to recognize a specific DNA sequence in a gene's promoter or enhancer. When it binds, it either recruits RNA polymerase and the transcription machinery — turning the gene up — or obstructs them, turning it down. Environmental signals such as a stress hormone, a nutrient, or a social experience change which transcription factors are present, active, or able to reach the nucleus, converting the outside event into a change in gene activity.