Cryo-trapping intermediate states in the CarH photoreceptor

CarH is a vitamin B12-dependent photoreceptor protein that binds to operator DNA in a tetrameric complex to inhibit gene expression. The tetramer is stable in dark conditions, while exposure to light induces tetramer dissolution and dissociation from DNA, thus allowing gene expression. The suppressed gene controls the biosynthesis of carotenoids that protects bacteria from excess of sunlight (Jost et al., 2015). CarH’s dissociation mechanism has been studied by time-resolved transient absorption spectroscopy. Two photoreaction pathways have been proposed, providing insight into a number of intermediates formed during the dissociation process (Kutta et al., 2015 ; Miller et al., 2020). In both mechanisms the cobalt (Co) atom contained in the corrin of the B12 moiety plays the most important role by sensing light. Co’s redox photodynamics involves the transient formation of intermediate states that result in CarH tetramer dissociation.