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Estimation of promoter activity from time-lapse data
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Stochastic transcriptional pulses orchestrate flagellar biosynthesis in Escherichia coli

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Promoter activity was estimated as previously described (24, 65). Briefly, if the total area of the cell is A(t) and the average pixel intensity is C(t), then Ftotal = A(t)C(t). It follows that$1AdFtotaldt=C1AdAdt+dCdt$

The left term $1AdFtotaldt$, i.e., the “cell size normalized” production rate, was operationally defined as the promoter activity. To compute promoter activity, $1AdAdt$, i.e., the relative growth rate of the cell, was estimated by taking the log ratio of the initial and final area of the cell for each cell division. $dCdt$ was estimated by smoothing our fluorescence traces with a Savitzky-Golay filter and taking the numerical derivative. See the “Estimation of promoter activity” section in the Supplementary Materials for more details.

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