Fig. 7: Comparison of noise scaling and bursting mechanisms of σ⁷⁰ and σ⁵⁴ controlled transcription.

In bacteria, the form of transcriptional bursting is determined by the route to transcription initiation. We observe that for σ⁷⁰ controlled promotors in which open complex formation occurs spontaneously, regulation occurs by relieving repression, corresponding to a reduction of the parameter ν. Low expression levels therefore consist of relatively frequent but small bursts that lead to low variability (a). For σ⁵⁴ controlled promotors in which open complex formation requires ATP-hydrolysis dependent activation, regulation occurs by increasing the rate at which this activation occurs. Low expression levels in this case correspond to large but infrequent bursts, leading to large variability (a). At high expression levels, both types of promotor may converge towards similar behaviour. The relationships between mean expression level and the Noise (b) and Fano factor (c) can be expressed mathematically in each of these cases, provided that regulation is via only one of burst size or burst frequency. These expressions are provided in materials and methods and plotted in (b) and (c) along with the measured cell behaviour. Data are presented as the statistic of the full set of data samples +/− the SEM obtained from n = 10,000 bootstrap resamples.