Feedback loops have been identified in a variety of regulatory systems and organisms. While feedback loops of the same type (negative or positive) tend to have properties in common, they can play distinctively diverse roles in different regulatory systems, where they can affect virulence in a pathogenic bacterium, maturation patterns of vertebrate oocytes and transitions through cell cycle phases in eukaryotic cells. This review focuses on the properties and functions of positive feedback in biological systems, including bistability, hysteresis and activation (...) surges. BioEssays 30:542–555, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

Horizontal gene transfer advances bacterial evolution. To benefit from horizontally acquired genes, enteric bacteria must overcome silencing caused when the widespread heat‐stable nucleoid structuring (H‐NS) protein binds to AT‐rich horizontally acquired genes. This ability had previously been ascribed to both anti‐silencing proteins outcompeting H‐NS for binding to AT‐rich DNA and RNA polymerase initiating transcription from alternative promoters. However, we now know that pathogenic Salmonella enterica serovar Typhimurium and commensal Escherichia coli break down H‐NS when this silencer is not bound to (...) DNA. Curiously, both species use the same protease – Lon – to destroy H‐NS in distinct environments. Anti‐silencing proteins promote the expression of horizontally acquired genes without binding to them by displacing H‐NS from AT‐rich DNA, thus leaving H‐NS susceptible to proteolysis and decreasing H‐NS amounts overall. Conserved amino acid sequences in the Lon protease and H‐NS cleavage site suggest that diverse bacteria degrade H‐NS to exploit horizontally acquired genes. (shrink)

The facultative intracellular pathogen Salmonella enterica serovar Typhimurium faces multiple environments during infection, including different cell types as well as extracellular fluids. We propose that Salmonella ascertains its cellular location by assessing the Mg2+ concentration of its milieu. A signal transduction system, PhoP/PhoQ, signals Salmonella its presence in a intracellular (low Mg2+) or extracellular (high Mg2+) environment, thereby promoting transcription of genes required for survival within or entry into host cells. The PhoP/PhoQ system is high in a regulatory hierarchy that (...) controls other signal transduction systems that respond to different host cues, enabling the microorganism to determine its precise tissue and cellular location. BioEssays 20:96–101, 1998. © 1998 John Wiley & Sons, Inc. (shrink)