sigma factor
/ SIG-muh; sigma /
Picture a delivery driver who knows how to drive but has no idea which addresses to visit — for that he needs a route card. Hand him one route card and he visits the bakeries; swap it for another and he visits the pharmacies. The core of bacterial RNA polymerase is like that driver: it can transcribe, but on its own it cannot find the right genes to start at. The sigma factor is the route card that tells it where to begin.
A sigma factor is a protein subunit that joins the core RNA polymerase to form the complete enzyme (the holoenzyme) and gives it the ability to recognize promoters — the specific DNA sequences marking where genes start. Crucially, a cell carries several different sigma factors, and each one steers the polymerase to a different set of promoters. The everyday housekeeping sigma directs transcription of the genes a cell needs all the time. But when conditions change, the cell can deploy an alternative sigma that redirects polymerase to a whole battery of genes at once. After transcription gets going, the sigma factor often drops off and can be reused. By choosing which sigma factor is active, a bacterium switches entire programs of genes on as a block.
Sigma factors are how bacteria mount large, coordinated responses fast. When E. coli is suddenly overheated, a heat-shock sigma factor switches on dozens of protective genes together. When Bacillus runs out of food, a cascade of sporulation sigma factors orchestrates the genes that turn the cell into a tough dormant spore. This is regulation at the level of whole gene sets rather than single operons, and it shows that controlling which promoters get read — by swapping the polymerase's targeting subunit — is one of the most powerful levers a bacterium has.
Heat an E. coli culture and the heat-shock sigma factor (sigma-32) quickly accumulates, steering RNA polymerase to the promoters of chaperone and protease genes. A whole protective program switches on at once — not gene by gene, but by changing which route card the polymerase is reading.
Swap the sigma factor, and the polymerase reads a whole new set of genes.
Sigma is part of the polymerase holoenzyme that recognizes promoters, not a free-standing repressor or activator. It usually leaves the polymerase soon after transcription begins, so its role is to choose where to start, not to control elongation.