The bacterial adaptive response gene, barA, encodes a novel conserved histidine kinase regulatory switch for adaptation and modulation of metabolism in Escherichia coli.

Histidine kinases are important prokaryotic determinants of cellular adaptation to environmental conditions, particularly stress. The highly conserved histidine kinase, BarA, encoded by the bacterial adaptive response gene, barA, is a member of the family of tripartite histidine kinases, and is involved in stress adaptation. BarA has been implicated to play a role during infection of epithelial cells. Homologues and orthologues of BarA have been found in pathogenic yeast, fungi, mould and in plants. The primary aim of this review is to assimilate evidence present in the current literature linking the role of BarA in stress response, and to support it with preliminary experimental evidence indicating that, it is indeed a global response regulator. In particular, the review focuses on the unusual domain structure of the BarA protein, its role in oxidative, weak acid, and osmotic stress responses and its role in biofilm formation. A preliminary genomic approach to identify downstream genes regulated by the BarA signaling pathway, using DNA microarray, is reported. The results demonstrate that BarA plays a global response regulatory role in cell division, carbon metabolism, iron metabolism and pili formation. The evolutionary significance of these types of histidine kinase sensors is reviewed in light of their roles in pathogenesis.