CarH photoreceptor protein (CarH)

CarH photoreceptor protein is a DNA-binding, transcriptional regulatory protein found in various bacteria, such as *Thermus thermophilus* and *Myxococcus xanthus*. CarH is unique in using a derivative of vitamin B12—specifically adenosylcobalamin (coenzyme B12)—as a light-sensing chromophore. In darkness, adenosylcobalamin binding promotes CarH tetramerization, allowing it to attach to operator DNA and repress transcription of genes involved in carotenoid biosynthesis. Upon exposure to visible light, the protein’s chromophore undergoes photolysis (cleavage of the Co–C bond), leading to tetramer disassembly, loss of DNA binding, and activation of carotenoid production. This light-responsive system provides bacteria with a means to protect against photo-oxidative stress by producing photoprotective carotenoid pigments when needed. CarH photoreceptor proteins have inspired interest as optogenetic tools, but are not therapeutic targets in human disease contexts[1][3][5][6][7][9].

Other names

CarHCarH-type photoreceptorB12-dependent photoreceptorCarH transcriptional regulator

Not applicable for drugs; mechanism relates to light-induced conformational change driven by coenzyme B12 (adenosylcobalamin) acting as a chromophore