Adenosylcobalamin (AdoCbl, coenzyme B<sub>12</sub>) is a complex organometallic enzyme cofactor and enables difficult radical chemistry through cleavage of its covalent cobalt-carbon bond. In a recently discovered bacterial pathway, however, AdoCbl is used in a completely new function: it is used as a light sensor in light-dependent gene regulation. Here, AdoCbl binds to the transcription factor CarH to regulate expression of the carotenoid biosynthetic cluster. Gene expression is only activated upon light exposure, leading to carotenoid production and protection of the cell from light-induced damage. This elegant response mechanism to light exposure is found in hundreds of bacterial species.<br/><br/>Here, we present a structure/function analysis of the light-sensing transcription factor CarH. In the dark, AdoCbl mediates formation of a CarH tetramer that positions the DNA-binding domains on the DNA operator, blocking transcription. Exposure to light cleaves AdoCbl’s Co-C bond, triggering a conformational change that causes tetramer disassembly, dissociation from DNA, and transcription activation. Thus, CarH harnesses the light sensitivity of AdoCbl to drive a light-dependent gene expression switch. Although exposure of AdoCbl to light normally leads to generation of reactive radical species, release of which would be harmful to the cell, we determined using HPLC as well as EPR, UV/Vis, and NMR spectroscopy that CarH mediates an organometallic reaction to quench these radical species, thereby safeguarding the use of AdoCbl as a light sensor. These studies, which combine in vitro DNA-binding with spectroscopy and crystallography, provide fundamental insight into a new mode of light-dependent gene regulation and provide a new biological role for AdoCbl.

Old cofactor in a new light: Adenosylcobalamin in light-dependent gene regulation

250th National Meeting (2015)

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