LOV to Get Together When It’s Light Out

Abstract

Like plants and fungi, many bacteria contain proteins with a photosensory LOV (light, oxygen, or voltage) domain. In bacteria, LOV domains are frequently found in sensor histidine kinases; however, the function of presumably photosensory proteins in many bacteria that are neither photosynthetic, phototactic, nor pigmented is unclear. Purcell et al. investigated the properties and functions of a two-component signaling system found in Caulobacter crescentus , comprising the LOV-histidine kinase LovK and its response regulator LovR. Analysis of purified LovK indicated that it exhibited a reversible photocycle and that both its autophosphorylation activity and its ability to hydrolyze ATP were enhanced by light. Strains of C. crescentus that lacked LovR were defective in adhesion to glass, whereas the combined overexpression of LovK and LovR promoted enhanced cell-to-cell adhesion. Mutational analysis indicated that this phenotype of enhanced intercellular adhesion depended on conserved phosphorylation sites in both LovK and LovR. Combined overexpression of LovK and LovR promoted cell adhesion even in the dark. However, adhesion was markedly enhanced by exposure to blue light, an effect that depended on the ability of LovK to undergo photoactivation--a process that requires formation of a transient covalent bond between a cysteine in the LOV domain and a flavin cofactor--as did the light-dependent enhancement of LovK autophosphorylation. Thus, the authors conclude that light-dependent modulation of LovK activity regulates intercellular attachment in C. crescentus . E. B. Purcell, D. Siegal-Gaskins, D. C. Rawling, A. Fiebig, S. Crosson, A photosensory two-component system regulates bacterial cell attachment. Proc. Natl. Acad. Sci. U.S.A. 104 , 18241-18246 (2007). [Abstract] [Full Text]