The coenzyme B12-dependent photoreceptor protein, CarH, is a bacterial transcriptional regulator that controls the biosynthesis of carotenoids in response to light. On binding of coenzyme B12 the monomeric apoprotein forms tetramers in the dark, which bind operator DNA thus blocking transcription. Under illumination the CarH tetramer dissociates, weakening its affinity for DNA and allowing transcription. The mechanism by which this occurs is unknown. Here we describe the photochemistry in CarH that ultimately triggers tetramer dissociation; it proceeds via a cob(III)alamin intermediate, which then forms a stable adduct with the protein. This pathway is without precedent and our data suggest it is independent of the radical chemistry common to both coenzyme B12 enzymology and its known photochemistry. It provides a mechanistic foundation for the emerging field of B12 photobiology and will serve to inform the development of a new class of optogenetic tool for the control of gene expression.

We report an improved model of the Drosophila cryptochrome structure that corrects errors in the original coordinates (3TVS.pdb). Further refinement of the structure with automated rebuilding algorithms in PHENIX 1 followed by manual building, indicated that a model of dCRY could be produced with excellent refinement statistics without taking into account the non-merohedral twinning originally reported (Table 1). The rebuilt structure has an RMSD on Cα positions of 2.4 Å compared to the deposited coordinates with most differences found in the conformation of surface loops (Fig. 1). However, the new analysis also indicates that the sequence register of the C-terminal tail helix (CTT) is displaced by two residues (Fig. 2). This change in sequence register offsets the invariant FFW motif along the helix axis such that Phe534, and not Trp536, approaches closest to the flavin ring (Fig. 3). In the new model, the three residues composing the FFW motif continue to make extensive interactions with the photolyase homology domain. This new position of the FFW motif is more consistent with the cellular data of Fig. S6, which shows that substitution of FFW to three alanine residues has a dramatic effect on dCRY stability, but that the W536A substitution alone, does not. The configuration of the flavin center is similar between the old and new models, with the largest difference in the angle of the ribityl-to-flavin (N10) bond (Fig. 4). Phosphorylation of Thr518 is not apparent in the new electron density maps despite identification of this modification by mass spectrometry. The errors in the original structure stemmed from model bias introduced during the detwinning procedure. Lower resolution data sets to which the original dCRY structure was built appear to suffer more from twinning than the 2.3 Å resolution data that the final model was refined against. Although the high-resolution data does contain indications of non-merohedral twinning, including intensity oscillations along the reciprocal space l axis and spurious Patterson peaks, a model that agrees well with the diffraction data as collected can be produced without compensation for these effects (Table 1). The new coordinates have been deposited in the PDB as 4GU5. B.R.C. apologizes for these errors.

All cryptochromes are currently classified as flavoproteins. In animals their best-described role is as components of the circadian clock. This circadian function is variable, and can be either light-dependent or -independent; the molecular origin of this difference is unknown. Type I animal cryptochromes are photoreceptors that entrain an organism’s clock to its environment, whereas Type II (including mammals) regulate circadian timing in a light-independent manner. Here, we reveal that, in contrast to Type I, Type II animal cryptochromes lack the structural features to securely bind the photoactive flavin cofactor. We provide a molecular basis for the distinct circadian roles of different animal cryptochromes, which also has significant implications for the putative role of Type II cryptochromes in animal photomagnetoreception.

Understanding the rules of life is one of the most important scientific endeavours and has revolutionised both biology and biotechnology. Remarkable advances in observation techniques allow us to investigate a broad range of complex and dynamic biological processes in which living systems could exploit quantum behaviour to enhance and regulate biological functions. Recent evidence suggests that these non-trivial quantum mechanical effects may play a crucial role in maintaining the non-equilibrium state of biomolecular systems. Quantum biology is the study of such quantum aspects of living systems. In this review, we summarise the latest progress in quantum biology, including the areas of enzyme-catalysed reactions, photosynthesis, spin-dependent reactions, DNA, fluorescent proteins, and ion channels. Many of these results are expected to be fundamental building blocks towards understanding the rules of life.