σS-controlling Networks in Escherichia coli
Eberhard Klauck and Regine Hengge
from: Bacterial Regulatory Networks (Edited by: Alain A.M. Filloux). Caister Academic Press, U.K. (2012)
The σS (RpoS) sigma subunit is the master regulator of the general stress response in Escherichia coli, which controls the expression of more than 500 genes during entry into stationary phase or upon exposure to many different stress conditions. σS is present at very low levels only in rapidly growing cells, but multiple stress signals are integrated in a way that results in strong σS accumulation and efficient σS-containing RNAP holoenzyme (EσS) formation. The first part of this review summarizes the molecular control mechanisms of switching from the "low-σS" to "high-σS" state, which operate at the levels of rpoS transcription, rpoS mRNA turnover and translation, σS proteolysis and EσS formation, and outlines multiple stress signal integration into these highly interconnected regulatory processes. We then show that, despite its complexity, the σS control network essentially is an intricate combination of a few typical network motifs. These are several key feedforward loops that control σS expression, a central and homeostatic negative feedback loop that integrates post-transcriptional σS control mechanisms, mutual inhibition of sigma factors competing for RNAP core enzyme governing σS activity control, and a series of smaller positive feedback loops that seem to stabilize the "high-σS" state read more ...