A sensory kinase integrates light and redox signals to coordinate cellular response to external stress in Caulobacter crescentus

Bacteria have evolved a wide variety of protein domains to absorb visible light in order to utilize solar radiation for the production of chemical energy or as a regulatory signal. One such photosensitive domain, the LOV domain, is frequently coupled to sensory histidine kinases which control diverse cellular processes in response to intracellular and extracellular stimuli. LovK, a LOV-histidine kinase encoded in the genome of the oligotrophic bacterium Caulobacter crescentus, is positively regulated by light and together with its downstream response regulator protein, LovR, plays a role in regulating cellular attachment to biotic and abiotic surfaces. The signaling kinetics of the LovK LOV domain are modulated by non-conserved protein elements outside the domain. LOV regulation of the LovK kinase domain is likely accomplished by rigid rotation of a C-terminal histidine kinase rather than the allosteric unfolding mechanism proposed for eukaryotic LOV-kinases. LovK photosensitivity is affected by the surrounding redox environment, which in bacterial cytoplasm is often influenced by extracellular stress. Given that LovK and LovR have recently been identified as negative regulators of the extracellular stress response sigma factor sigmaT in C. crescentus, this suggests that LovK integrates light and redox signals to optimize cellular response to changing external conditions.