Kinetics of conversion between Pr and Pfr states in three prototypical bacteriophytochromes

Bacteriophytochromes (PphPs) show strong structural similarity to canonical phytochromes, however, they employ as chromophore biliverdin IX instead of phytochromobilin (plant phytochromes) or phycocyanobilin (cyanobacterial phytochromes). This change of chromophore shifts the absorption maxima – compared to those of the plant phytochromes – for both Pr and Pfr state further into the red- / far red range of the spectrum (to 700 and 750 nm, respectively), and thereby makes these phytochromes very well suited for biomedical applications (1). Both photochromic states are reported, however, the dynamics of laser flash-induced conversion between both states has not been studied in detail. Here, we present the kinectics of Pr-to-Pfr conversion (and vice versa) for three prototypal bacteriophytochromes, as such phy1 from Pseudomonas (P.) syringae pv. tomato (PstBphP1), phytochrome from P. aeruginosa (PaBphP), and phytochrome from the fungus Aspergillus nidulans (FphA). Whereas the phytochrome from P. syringae and ist fungal ortholog are formed biosynthetically in the Pr state with absorption maxima at 700 nm, the phytochrome from P. aeruginosa is generated in its Pfr state (750 nm), thus being called a bathy-phytochrome. The conversions between both states of all three bacteriophytochromes in the time range of ca. 1 us up to 20 ms are much more simple than comparable kinetics found for canonical phytochromes from plants (oat phyA) or from cyanobacteria (Cph1, CphA, 2,3). Whereas in the latter proteins a sequence of intermediates can be clearly identified, the bacteriophytochromes run through one intermediate, or in some cases a direct formation of the final photoproduct is immediately observed. 1) Chernov, K.G. et al. (2017) Chem. Rev. 117 6423-6446. 2) Gärtner, W. and Braslavsky, S.E. (2003) In: Photoreceptors and light signalling, Batschauer, A. (ed.). Compr. Series Photochem. Photobiol. Sci., Vol. 3, Batschauer, A. (ed.), Häder, D.-P. and Jori, G. (series eds.), Royal Soc. Chemistry, Cambridge, UK, pp. 136-180. 3) Remberg, A. et al. (1997) Biochemistry 36 13389-13395.