Conversion of the closed complex to the open complex by sigma factors. (a) σ⁷⁰ induces strand opening and formation of the open complex, without the need for ATP or other factors. All other sigma factors except σ⁵⁴ function in the same way. (b) σ⁵⁴ requires an AAA+ protein to form the open complex at a promoter. The AAA+ protein is a hexamer that binds an upstream activator sequence (UAS), then also binds σ⁵⁴ in the closed complex, creating a DNA loop. Such DNA looping is often facilitated by auxiliary DNA-binding and bending proteins such as integration host factor (IHF). AAA+ hydrolyzes a bound ATP, using the favorable energetics of this reaction to drive opening of the complex.