The pathway consists of two major parts, binding and initiation, each with multiple steps. First, the polymerase, directed by its bound σ factor, binds to the promoter. A closed complex (in which the bound DNA is intact) and an open complex (in which the bound DNA is intact but partially unwound near the −10 sequence) form in succession.

Second, transcription is initiated within the complex, leading to a conformational change that converts the complex to the elongation form, followed by movement of the transcription complex away from the promoter (promoter clearance).

Any of these steps can be affected by the specific makeup of the promoter sequences. The σ subunit dissociates stochastically (at random) as the polymerase enters the elongation phase of transcription. The protein NusA (Mr 54,430) binds to the elongating RNA polymerase, competitively with the σ subunit.

Once transcription is complete, NusA dissociates from the enzyme, the RNA polymerase dissociates from the DNA, and a σ factor (σ 70 or another) can again bind to the enzyme to initiate transcription.