Myxobacteria are swarming micro-organisms that, under starving conditions, form multicellular aggregates called fruiting bodies. The fruiting body formation is, in addition, light dependent.

Under red and far-red light the myxobacterial response is likely controlled by bacterial phytochromes (Bphs). Structures of myxobacterial Bphs were unknown. The myxobacterium Stigmatella aurantiaca harbors two Bphs.  BioXFEL researchers together with scientists at Northeastern Illinois University, who led the experiments, solved the structure of the first myxobacterial Bph, SaBphP1. Bph are red/far-red light receptors. The photo-response is initiated by a biliverdin (BV) chromophore whose ring D isomerizes between distinct configuration after red or far-red light absorption. The BV isomerizations trigger conformational changes between two species called Pr and Pfr which display absorption maxima near 700 nm and 750 nm, respectively. SaBphP1 lacks a conserved histidine near the BV and contains a threonine instead. Structures of several constructs of the SaBphP1 wild type and the T289H mutant were solved at cryogenic conditions at the Advanced Photon Source, beamline 19-ID-D (structural biology center). In addition, room temperature studies were conducted at beamline BL3 of the Japanese XFEL, the Spring-8 Angstroem Compact X-ray Laser (SACLA). The structures explain absorption spectra that were obtained after red and far-red light irradiation on wild-type and various mutants of SaBphP1. From this a mechanism for the Pr to Pfr transition after BV isomerization was deduced that is likely general for all Bphs.

The results of the experiments were published on September 1st 2018 in IUCrJ.