Before revision:  Although promoter activity is often governed by a single limiting step, we argue here that the size of the noise strongly depends on whether this step is the initial equilibrium binding or one of the subsequent steps. In particular, the multi-step transcription initiation process allows weak promoters and repressors having fast binding kinetics to have substantial noise. Therefore the architecture of the promoter has implications for estimating the on-rates for transcriptional repressors in living cells .
After revision:  Although promoter activity is often governed by a single limiting step, we argue here that the size of the noise strongly depends on whether this step is the initial equilibrium binding or one of the subsequent uni-directional steps. Overall, we show that non-equilibrium steps of transcription initiation systematically increase the cell to cell heterogeneity in bacterial populations. In particular, this allows also weak promoters to give substantial transcriptional noise .