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Flavin-binding LOV domains are blue-light photosensory modules that are conserved in a number of developmental and circadian regulatory proteins in plants, algae, and fungi. LOV domains are also present in bacterial genomes, and are commonly located at the amino termini of sensor histidine kinases. Genes predicted to encode LOV-histidine kinases are conserved across a broad range of bacterial taxa, from aquatic oligotrophs to plant and mammalian pathogens. However, the function of these putative prokaryotic photoreceptors remains largely undefined. The differentiating bacterium, Caulobacter crescentus, contains an operon encoding a two-component signaling system consisting of a LOV-histidine kinase, LovK, and a single-domain response regulator, LovR. LovK binds a flavin cofactor, undergoes a reversible photocycle, and displays increased ATPase and autophosphorylation activity in response to …