Characterization of a red-light photoreceptor in the life-cycle of myxobacterium Stigmatella aurantiaca

Bacteriophytochromes (Bphs) are red-light photoreceptors found in various photosynthetic and non-photosynthetic bacteria. They are unique in their ability to undergo reversible photoconversion between spectrally distinct red (Pr) and far-red (Pfr) light absorbing forms.  Bphs require biliverdin (BV) as an organic cofactor for photoactivity. We cloned and purified an unusual Bph from a non-photosynthetic and fruiting myxobacterium, Stigmatella aurantiaca (SaBphP1). Interestingly, light greatly stimulates fruiting body formation and synthesis of a pheromone that promotes cell aggregation of S. aurantiaca. Unlike classical Bphs, SaBphP1 undergoes limited Pr/Pfr photoconversion.  SaBphP1 has a threonine (Thr) instead of a highly conserved histidine (His), which stabilizes the D-ring of BV in the Pr state. Based on structural and sequence analysis of SaBphP1, we propose that Thr is too far away to form a hydrogen bond to the D-ring of BV and therefore results in the partial Pr-Pfr photoconversion.  When Thr is converted to His (Thr289His mutant), increased photoactivity is observed due to His ability to hydrogen bond with the D-ring.  Two highly conserved residues, Asp208 and Tyr262, also work in conjunction to stabilize BV during photoconversion.  Asp208 forms a hydrogen bond with Tyr262 and BV. Mutations to either of these residues (Asp208His and Tyr262Phe) result in disrupted hydrogen bonding and eliminate all Pr/Pfr photoconversion in SaBphP1, as observed with steady-state UV-vis absorption spectroscopy. Our comparative structural and sequence analyses of wild-type SaBphP1 and mutants Thr289His, Asp208Thr and Tyr262Phe, highlight the role of conserved amino acids that hydrogen bond to BV during Pr/Pfr photoconversion.