In bacteria, transcriptional regulation is a key step in cellular gene expression. All bacteria contain a core RNA
polymerase that is catalytically competent but requires an additional σ factor for specific promoter recognition and correct
transcriptional initiation. The RNAP core is not able to selectively bind to a given σ factor. In contrast, different σ factors
have different affinities for the RNAP core. As a consequence, the concentration of alternate σ factors requires strict regulation
in order to properly control the delicate interplay among them, which favors the competence for the RNAP core.
This control is archived by different σ/anti-σ controlling mechanisms that shape complex regulatory networks and cascades,
and enable the response to sudden environmental cues, whose global understanding is a current challenge for systems
biology. Although there have been a number of excellent studies on each of these σ/anti-σ post-transcriptional regulatory
systems, no comprehensive comparison of these mechanisms in a single model organism has been conducted. Here,
we survey all these systems in E. coli dissecting and analyzing their inner workings and highlightin their differences.
Then, following an integral approach, we identify their commonalities and outline some of the principles exploited by the
cell to effectively and globally reprogram the transcriptional machinery. These principles provide guidelines for developing
biological synthetic circuits enabling an efficient and robust response to sudden stimuli.