Introduction
In bacteria, coupling of gene expression with external conditions is achieved through two molecular functions: (i) binding of transcription factors (TFs) at specific sites in the genome and (ii) recognition of a relevant effector signal or metabolite.1,2 Typically, these functions are performed by different domains of a single polypeptide, but there are also cases where two interacting proteins are responsible for these functions, as in two-component systems.3
At the phenotypic level, there are evidences for the coexistence of multiple phenotypes in bacterial cultures, e.g., of cells with different morphological and physiological abilities such as motility, biofilm formation, drug resistance, etc.4,5 In particular, biofilm formation and chemotaxis are considered as multistage developmental processes, and in mature biofilms, a mixture of bacterial population from different developmental stages were found to coexist.6,7
Here we take advantage of the wealth of experimental data on transcriptional regulation for the best-characterized bacterium, Escherichia coli, to analyse the structure of the transcriptional network in light of different functional constraints.