External input information defines the last steps of the developmental processes
Downstream of biofilm and chemotaxis developmental cascades, transcriptional autoactivations and cross-inhibitions become more frequent (Figs. 1 and 2a). This part of the network comprises several two-component systems, enabling the sensing of external information. In particular, OmpR is responsible for switching the phenotype between biofilm and motility (activating the first and repressing the latter) depending on external stresses. During stressing conditions as well as in mature biofilms, cells tend to be smaller compared to rapidly growing cells. This phenotypic transition depends on the expression of BolA (cell morphogenetic regulator), which is transcribed by RpoS and repressed by OmpR.51
The activation of the master gene for motility, FlhCD, is further controlled by the quorum-sensing detector QseB, while the activity of the master regulator for biolfilm formation, CsgD, is repressed in the presence of extracellular stressing conditions, detected by CpxR. In addition, the BaeSR system controls the expression of export complexes, conferring multidrug resistance phenotypes,52 while RcsB mediates the glutamate-dependent acid resistance.53,54 QseB, CpxR, BaeR, OmpR and RcsAB are all members of two-component systems that are modified by protein–protein phosphorylation, enabling quick response in comparison to de novo protein production along transcriptional cascades. Strikingly, almost all of these two-component TFs (except OmpR) autoactivate their expression, an observation fitting the contention that positive feedback circuits are necessarily found at the core of all differentiation switches. Indeed, such TF autoactivations likely enable persistent TF expression in two stable states: “on” or “off”. Thus, they constitute a potentially robust machinery for all-or-nothing output response depending on transient exogenous signals.55–62