True about sigma factor?

The basic DNA-dependent RNA polymerase of the bacterium Escherichia coli exists as an approximately 400-kDa core complex consisting of two identical a subunits, two large b, and b' subunits, and a o subunit. The core RNA polymerase, bb'a2o, often termed E, associates with a specific protein factor (the sigma factor) to form holoenzyme, . Sigma factors have a dual role in the process of promoter recognition; s association with core RNA polymerase decreases its affinity for non-promoter DNA, while simultaneously increasing holoenzyme affinity for promoter DNA. REF: Lippincott book of biochemistry 6th ed.