Structural basis for gene regulation by a B12-dependent photoreceptor

Abstract

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.

New light on vitamin B12: The adenosylcobalamin-dependent photoreceptor protein CarH

“…1 Each monomer has one domain that attaches to DNA at the N-terminal end and a second domain that binds the light-sensing AdoCbl molecule at the C-terminal end. The N-terminal domain consists of a DNA-recognition helix and a β-hairpin wing.…”

“…1 Vitamin B 12 was first discovered as cyanocobalamin (CNCbl), an inactive form of the vitamin, as an artefact of the isolation procedure, which utilised cyanide. (For reviews on vitamin B 12 , see [2][3][4][5] ).…”

“…However, until recently, the photosensitivity of B 12 coenzymes appeared to have no physiological function. 1,6,7 Humans have only two vitamin B 12 -dependent enzymes: methylmalonyl-coenzyme A (CoA) mutase which utilises AdoCbl as its coenzyme and methionine synthase which utilises MeCbl. Methylmalonyl-CoA mutase, an example of an isomerase enzyme, converts methylmalonyl-CoA to succinyl-CoA in the oxidation of odd-chain fatty acids from the degradation of isoleucine and valine.…”

B 12 -dependent photoresponsive protein hydrogels for controlled stem cell/protein release

“…The early work led by Murphy and coworkers (20,21) showed the success of synthesizing calmodulin-based protein hydrogels with dynamic properties responsive to Ca 2+ and the small-molecule ligand trifluoperazine. Recently, the CarH protein, a transcriptional regulator controlling bacterial carotenoid synthesis, has been shown to sense and respond to visible light through its C-terminal adenosylcobalamin binding domain (CarH C ) (22)(23)(24)(25). The CarH C domains tetramerize when binding to adenosylcobalamin (AdoB 12 ) in the dark and can readily dissociate into monomers accompanied with a drastic protein conformational change caused by the cleavage of the C-Co bond on exposure to green (522 nm) or white light (Fig.…”

“…The CarH C domains tetramerize when binding to adenosylcobalamin (AdoB 12 ) in the dark and can readily dissociate into monomers accompanied with a drastic protein conformational change caused by the cleavage of the C-Co bond on exposure to green (522 nm) or white light (Fig. 1A) (23,24). We envisioned that the light-sensing CarH C domains could be used to construct protein-based photoresponsive hydrogels.…”