Structural basis for gene regulation by a B12-dependent photoreceptor

Photoreceptor proteins enable organisms to sense and respond to light. The newly discovered CarH-type photoreceptors use a Vitamin B 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 visualizations 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 B and provide fundamental insight into a new mode of light-dependent gene regulation.This work was supported in part by the National Institutes of Health (NIH, grant GM069857 to C.L.D.), the Ministerio de Economía y Competitividad, Spain (grants BFU2012-40184-C02-01 co-financed by FEDER (Fondo Europeo de Desarrollo Regional) funds to M.E.-A.; BFU2012-40184-C02-02 to S.P.; PhD fellowship to J.F.-Z.), a CSIC-JAE-Predoc (Spain) fellowship (to J.M.O.-G.), and an MIT Poitras pre-doctoral fellowship (to M.J.). C.L.D. is a Howard Hughes Medical Institute Investigator. This work used the Northeastern Collaborative Access Team beamlines of the Advanced Photon Source, which are supported by National Institute of General Medical Sciences (NIGMS) grant P41GM103403 and the NIH. The Advanced Photon Source is a US Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-06CH11357. Use of the Stanford Synchrotron Radiation Lightsource is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract number DE-AC02-76SF00515, the Department of Energy Office of Biological and Environmental Research, and by the NIH and NIGMS (including P41GM103393).Peer Reviewe

Structural basis for gene regulation by a B12-dependent photoreceptor

Photoreceptor proteins enable organisms to sense and respond to light. The newly discovered CarH-type photoreceptors use a vitamin B[subscript 12] 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 visualizations 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 B[subscript 12] and provide fundamental insight into a new mode of light-dependent gene regulation.National Institutes of Health (U.S.) (Grant GM069857)Massachusetts Institute of Technology. Poitras Pre-Doctoral Fellowshi