Light-dependent gene regulation by a coenzyme B ₁₂ -based photoreceptor

Abstract

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. Unanswered were what underlies this striking difference, what is the specific cobalamin used, and how it acts. Here, we show that coenzyme B 12 (5′-deoxyadenosylcobalamin, AdoB 12 ), specifically dictates CarH function in the dark and on exposure to light. In the dark, AdoB 12 -binding to the autonomous domain containing the B 12 -binding motif foments repressor oligomerization, enhances operator binding, and blocks transcription. Light, at various wavelengths at which AdoB 12 absorbs, dismantles active repressor oligomers by photolysing the bound AdoB 12 and weakens repressor-operator binding to allow transcription. By contrast, AdoB 12 alters neither CarA oligomerization nor operator binding, thus accounting for its B 12 -independent activity. Our findings unveil a functional facet of AdoB 12 whereby it serves as the chromophore of a unique photoreceptor protein class acting in light-dependent gene regulation. The prevalence of similar proteins of unknown function in microbial genomes suggests that this distinct B 12 -based molecular mechanism for photoregulation may be widespread in bacteria.carotenogenesis | Thermus thermophilus | antirepressor | MerR | TtCarH

Green‐Light‐Induced Inactivation of Receptor Signaling Using Cobalamin‐Binding Domains

“…For both fusion receptors, we observed induction in the dark that was comparable to that of constitutively dimerized receptor (mFGFR1‐IgG; see the Experimental Section in the Supporting Information). Induction required AdoCbl supplementation, in line with observations that the cofactor is required for domain interaction 8c. Upon green‐light illumination, we observed decreased pathway activity comparable with that in unsupplemented or untransfected cells, suggesting light‐induced dissociation of receptor complexes.…”

B 12 -dependent photoresponsive protein hydrogels for controlled stem cell/protein release

“…Although adenosine could lead to cytotoxicity at relatively high concentrations or after prolonged exposure, so far, there has been no direct evidence pointing to the cytotoxicity or any other side effect of its analog 4′,5′-anhydroadenosine on cell phenotypes (41). In addition, only trace amounts of unbound AdoB 12 (∼1 μM) exists in the CarH C hydrogel (8.3 wt %) given the dissociation constant K d for the AdoB 12 -CarH complex (0.8 μM) (24). Therefore, the radical species resulting from the photolysis of the free AdoB 12 is negligible and can hardly cause cytotoxicity.…”

“…The CarH C domains tetramerize when binding to adenosylcobalamin (AdoB 12 ) in the dark and can readily dissociate into monomers accompanied with a drastic protein conformational change caused by the cleavage of the C-Co bond on exposure to green (522 nm) or white light (Fig. 1A) (23,24). We envisioned that the light-sensing CarH C domains could be used to construct protein-based photoresponsive hydrogels.…”

“…The early work led by Murphy and coworkers (20,21) showed the success of synthesizing calmodulin-based protein hydrogels with dynamic properties responsive to Ca 2+ and the small-molecule ligand trifluoperazine. Recently, the CarH protein, a transcriptional regulator controlling bacterial carotenoid synthesis, has been shown to sense and respond to visible light through its C-terminal adenosylcobalamin binding domain (CarH C ) (22)(23)(24)(25). The CarH C domains tetramerize when binding to adenosylcobalamin (AdoB 12 ) in the dark and can readily dissociate into monomers accompanied with a drastic protein conformational change caused by the cleavage of the C-Co bond on exposure to green (522 nm) or white light (Fig.…”

LdrP, a cAMP receptor protein/FNR family transcriptional regulator, serves as a positive regulator for the light-inducible gene cluster in the megaplasmid of Thermus thermophilus

“…The photolysis of the AdoB 12 -bound CTt2 leads to the conversion of AdoB 12 into hydroxocobalamin (OHB 12 ), resulting in the loss of its DNA-binding activity. It was also reported that the full-length native protein TtCarH (a LitR homologue with a single-residue substitution) from T. thermophilus HB8 bound to its operator DNA in an AdoB 12 -and light-dependent manner (Ortiz-Guerrero et al, 2011, Díez et al, 2013.…”