Structural basis for gene regulation by a B12-dependent photoreceptor.

Photoreceptor proteins enable organisms to sense and respond to light. The newly discovered CarH-type photoreceptors use a vitamin B12 derivative, adenosylcobalamin, as the light-sensing chromophore to mediate light-dependent gene regulation. Here we present crystal structures of Thermus thermophilus CarH in all three relevant states: in the dark, both free and bound to operator DNA, and after light exposure.

The photochemical mechanism of a B12-dependent photoreceptor protein.

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.

Analytical ultracentrifugation studies of oligomerization and DNA-binding of TtCarH, a Thermus thermophilus coenzyme B12-based photosensory regulator.

Thermus thermophilus transcriptional factor TtCarH belongs to a newly discovered class of photoreceptors that use 5'-deoxyadenosylcobalamin (AdoB12) as the light-sensing chromophore. Photoregulation relies on the repressor activity of AdoB12-bound oligomers in the dark, which light counteracts by oligomer disruption due to AdoB12 photolysis. In this study, we investigated TtCarH self-association and binding to DNA in the dark and in the light using analytical ultracentrifugation (AUC) methods, both sedimentation velocity (SV) as well as equilibrium (SE).

Light-dependent gene regulation by a coenzyme B12-based photoreceptor.

Cobalamin (B(12)) typically functions as an enzyme cofactor but can also regulate gene expression via RNA-based riboswitches. B(12)-directed gene regulatory mechanisms via protein factors have, however, remained elusive. Recently, we reported down-regulation of a light-inducible promoter in the bacterium Myxococcus xanthus by two paralogous transcriptional repressors, of which one, CarH, but not the other, CarA, absolutely requires B(12) for activity even though both have a canonical B(12)-binding motif.