Crystal Structures of a Bacteriophytochrome from Myxobacteria Stigmatella aurantiaca

Bacteriophytochromes (BphPs) are red‐light photoreceptors that regulate light responses in bacteria via reversible photoconversion between red (Pr) and far‐red (Pfr) light‐absorbing states. BphPs consist of a photosensory module that is composed of the PAS (Per‐ARNT‐Sim), GAF (cGMP phosphodiesterase/adenyl cyclase/FhlA), and PHY (phytochrome) domains. PHY domain is covalently linked to a signaling‐effector module such as histidine kinase. Wild type protein from non‐photosynthetic myxobacterium Stigmatella aurantiaca (SaBphP1) was purified, crystallized, and the structure of the complete photosensory module was determined at 2.35 Å resolution. Unlike classical BphPs, SaBphP1 undergoes limited Pr/Pfr photoconversion in the presence of the PHY domain. Wild‐type SaBphP1 lacks a conserved His that forms a hydrogen bond with the chromophore in the Pr state in the classical BphPs. A single Thr289His mutation in the chromophore‐binding pocket of SaBphP1 results in a complete Pr/Pfr photoconversion. Consequently, we determined the crystal structure of the complete photosensory module for SaBphP1 Thr289His mutant at 2.85 Å resolution. Both SaBphP1 crystal structures, solved in the Pr state, reveal how interdomain interactions bring together PAS, GAF and PHY domains to achieve Pr/Pfr photoconversion. We identify key amino acids that stabilize the chromophore in the Pr state and provide structural evidence to support the essential role of the PHY domain in Pr/Pfr photoconversion. Our results highlight local structural changes from the photosensory domain that significantly affect the long, signaling helices at the dimer interface and may impact the spatially distant histidine kinase domain and its activity. Support or Funding Information NSF MCB RUI 1413360