B12-based photoreceptors: from structure and function to applications in optogenetics and synthetic biology

Abstract

Vitamin B12-based photoreceptor proteins sense ultraviolet (UV), blue or green light using 5´-deoxyadenosylcobalamin (AdoCbl). The prototype of this widespread bacterial photoreceptor family, CarH, controls light-dependent gene expression in photoprotective cellular responses. It represses transcription in the dark by binding to operator DNA as an AdoCbl-bound tetramer, whose disruption by light relieves operator binding to allow transcription. Structures of the "dark" (free and DNA-bound) and "light" CarH states and studies on the unusual AdoCbl photochemistry have provided fundamental insights into these photoreceptors. We highlight these, the plasticity within a conserved mode of action among CarH homologs, their distribution, and their promising applications in optogenetics and synthetic biology.