Detalle de: Plasticity in oligomerization, operator architecture, and DNA binding in the mode of action of a bacterial B12-based photoreceptor

Newly discovered bacterial photoreceptors called CarH sense light by using 5'-deoxyadenosylcobalamin (AdoCbl). They repress their own expression and that of genes for carotenoid synthesis by binding in the dark to operator DNA as AdoCbl-bound tetramers, whose light-induced disassembly relieves repression. High-resolution structures of Thermus thermophilus CarH Tt have provided snapshots of the dark and light states and have revealed a unique DNA-binding mode whereby only three of four DNA-binding domains contact an operator comprising three tandem direct repeats. To gain further insights into CarH photoreceptors and employing biochemical, spectroscopic, mutational, and computational analyses, here we investigated CarH Bm from Bacillus megaterium We found that apoCarH Bm , unlike monomeric apoCarH Tt , is an oligomeric molten globule that forms DNA-binding tetramers in the dark only upon AdoCbl binding, which requires a conserved W- X 9 -EH motif. Light relieved DNA binding by disrupting CarH Bm tetramers to dimers, rather than to monomers as with CarH Tt CarH Bm operators resembled that of CarH Tt , but were larger by one repeat and overlapped with the -35 or -10 promoter elements. This design persisted in a six-repeat, multipartite operator we discovered upstream of a gene encoding an Spx global redox-response regulator whose photoregulated expression links photooxidative and general redox responses in B. megaterium Interestingly, CarH Bm recognized the smaller CarH Tt operator, revealing an adaptability possibly related to the linker bridging the DNA- and AdoCbl-binding domains. Our findings highlight a remarkable plasticity in the mode of action of B 12 -based CarH photoreceptors, important for their biological functions and development as optogenetic tools.