Figure 2.

The impact of parallel and random variations in the expression of spoIIE and spoIIA genes on σ^F release. (A) The spoIIA operon comprises the genes for SpoIIAA, SpoIIAB, and σ^F. The genes for SpoIIAA and SpoIIAB overlap; the genes for SpoIIAB and σ^F are separated by 11 bp. (B) The regulatory network is robust to parallel variations in gene expression. The predicted concentration of σ^F-RNA polymerase holoenzyme before (dashed lines) and after septum formation (continuous lines) if either all (grey lines) or all protein expression rates except that of SpoIIE (black lines) were increased by the factor on the horizontal axis compared to the standard reference rates (6 × 10⁻⁹ M s⁻¹ for SpoIIAA and SpoIIAB dimers and 2 × 10⁻⁹ M s⁻¹ for σ^F and SpoIIE; Iber et al, 2006). (C, D) The expression rate combinations for which septation-dependent σ^F release is possible (between the lines) or not possible (outside the area marked by lines). (C) The impact of differential regulation of spoIIE and spoIIA expression. The vertical and horizontal axes indicate the fold variation in the spoIIE and spoIIA expression rates respectively, compared to the standard reference rates. (D) The impact of differential regulation of the expression of genes encoded in the spoIIA operon. The vertical axis indicates the fold variation in the expression of SpoIIAA (circles), σ^F (black lines), or SpoIIAA and σ^F (grey lines). The horizontal axis indicates the fold variation in the expression of SpoIIAB and of any other protein whose expression is coupled to that of SpoIIAB (which are those genes in the spoIIA operon not reported on the vertical axis). The sudden jump observed at a high SpoIIAB to σ^F ratio (lower black line) is the consequence of impaired σ^F release when the relative SpoIIAB concentration is too high.