Living organisms sense and respond to light, a crucial environmental factor, using photoreceptors, which rely on bound chromophores such as retinal, flavins, or linear tetrapyrroles for light sensing.

The discovery of photoreceptors that sense light using 5'-deoxyadenosylcobalamin, a form of vitamin B12 that is best known as an enzyme cofactor, expanded the number of known photoreceptor families and unveiled a new biological role of this vitamin. The prototype of these B12-dependent photoreceptors, the transcriptional repressor CarH, is widespread in bacteria and mediates light-dependent gene regulation in a photoprotective cellular response. CarH activity as a transcription factor relies on the modulation of its oligomeric state by 5'-deoxyadenosylcobalamin and light. This article surveys current knowledge of this new family of B12-dependent photoreceptors, their discovery, distribution and mode of action, and the structural and photochemical basis of how they orchestrate signal transduction and control gene expression. The main focus of the review is largely based on results stemming from the collaborative work by members of IQFR and the Dpto. de Genética-Universidad de Murcia (Unidad Asociada al IQFR), and more recently with groups in MIT (USA), and the Univ. Manchester (UK) that have been published in, among others, PNAS, Nature Communications and Nature. This article is an invited review by Prof. Roger Kornberg (Nobel Prize in Chemistry, 2006) on behalf of the Editorial Committee of Annu Rev Biochem.

S. Padmanabhan, Marco Jost, Catherine L. Drennan, and Montserrat Elías-Arnanz. “A New Facet of Vitamin B12: Gene Regulation by Cobalamin-Based Photoreceptors”. Annu Rev Biochem 86, 485–514 (2017).
DOI: 10.1146/annurev-biochem-061516-044500