SummaryPhotoreceptor proteins enable organisms to sense and respond to light. The newly discovered CarH-type photoreceptors use a vitamin B12 derivative, adenosylcobalamin, as the light-sensing chromophore to mediate light-dependent gene regulation. Here, we present crystal structures of Thermus thermophilus CarH in all three relevant states: in the dark, both free and bound to operator DNA, and after light exposure. These structures provide a visualization of how adenosylcobalamin mediates CarH tetramer formation in the dark, how this tetramer binds to the promoter −35 element to repress transcription, and how light exposure leads to a large-scale conformational change that activates transcription. In addition to the remarkable functional repurposing of adenosylcobalamin from an enzyme cofactor to a light sensor, we find that nature also repurposed two independent protein modules in assembling CarH. These results expand the biological role of vitamin B12 and provide fundamental insight into a new mode of light-dependent gene regulation.

Cobalamin (B 12 ) typically functions as an enzyme cofactor but can also regulate gene expression via RNA-based riboswitches. B 12 -directed gene regulatory mechanisms via protein factors have, however, remained elusive. Recently, we reported down-regulation of a light-inducible promoter in the bacterium Myxococcus xanthus by two paralogous transcriptional repressors, of which one, CarH, but not the other, CarA, absolutely requires B 12 for activity even though both have a canonical B 12 -binding motif. Unanswered were what underlies this striking difference, what is the specific cobalamin used, and how it acts. Here, we show that coenzyme B 12 (5′-deoxyadenosylcobalamin, AdoB 12 ), specifically dictates CarH function in the dark and on exposure to light. In the dark, AdoB 12 -binding to the autonomous domain containing the B 12 -binding motif foments repressor oligomerization, enhances operator binding, and blocks transcription. Light, at various wavelengths at which AdoB 12 absorbs, dismantles active repressor oligomers by photolysing the bound AdoB 12 and weakens repressor-operator binding to allow transcription. By contrast, AdoB 12 alters neither CarA oligomerization nor operator binding, thus accounting for its B 12 -independent activity. Our findings unveil a functional facet of AdoB 12 whereby it serves as the chromophore of a unique photoreceptor protein class acting in light-dependent gene regulation. The prevalence of similar proteins of unknown function in microbial genomes suggests that this distinct B 12 -based molecular mechanism for photoregulation may be widespread in bacteria.carotenogenesis | Thermus thermophilus | antirepressor | MerR | TtCarH