Mean and variability in RNA polymerase numbers are correlated to the mean but not the variability in size and composition of Escherichia coli cells

Cell morphology differs with cell physiology in general and with gene expression in particular. We investigate the degree to which these relationships differ with medium richness. Using Escherichia coli cells with fluorescently tagged β' subunits, flow cytometry, and statistical analysis, we study at the single-cell level the correlation between parameters associated to cell morphology and composition (FSC, SSC, and Width channels) and GFP tagged RNA polymerase (RNAp) levels (FITC channel). From measurements in three media differing in richness (M63, LB, and TB) and, thus, cell growth rates, we find that the mean and cell-to-cell variability in RNAp levels are correlated to the mean values of FSC, SSC, and/or Width. Further, in all growth conditions considered, RNAp levels are positively correlated to FSC, SSC, and Width at the single-cell level, with the correlation decreasing for increasing medium richness. Overall, the results suggest that the mean and cell-to-cell variability in levels of RNAp, a master regulator of gene expression, are correlated to the mean values of the parameters assessing the cellular morphology and composition, as measured by flow cytometry, but they do not correlate to the degree of variability of these parameter values.