Biophysical studies on bacterial biliverdin-binding photoreceptors

Bacterial photoreceptors binding open-chain tetrapyrroles (bilins) as chromophores are related to plant phytochromes (phy) as they are photochromic and their primary photochemistry consists of a Z/E isomerisation around the bilin 15=16 double bond. The chromophore is embedded in all cases within a so-called GAF domain with a typical α/β fold. Different to the canonical plant phys which invariably bind phytochromobilin and switch between a red (R) and a far red (FR) absorbing form, the bacterial bilin-binding photoreceptors exhibit a much wider variety of spectroscopic and functional properties, and bind diverse bilin chromophores, e.g., phycocyanobilin (PCB) and biliverdin (BV). In particular, BV-binding photoreceptors present the most red-shifted spectrum, reaching the near infra-red (NIR) range in the photoactive form. This makes these phytochromes very well suited for biomedical applications (1). Here we report steady-state and time-resolved spectroscopic measurements on selected bacterial BV-binding photoreceptors, representatives for four variations of this photoreceptor family: a. a phy and a bathy-phy from Pseudomonas strains with R/FR photochromism; b. a “bacterio” phytochrome from the fungus Aspergillus nidulans, a eukaryotic organism with photochemistry akin to the Pseudomonas syringae protein; c. a novel phy from Methylobacterium radiotolerans with FR/NIR photochromism. In particular, nanosecond time-resolved absorption spectroscopy has revealed kinetics and spectral features of transient species after photoactivation for both the directions of conversion: the conversions of all these BV-phytochromes in the time range 1 μs – 400 ms seem to be more simple than those from plant phytochromes (oat phyA) or from cyanobacteria (Cph1, CphA) (2)(3), in some cases travelling through only one observable intermediate in the R to FR conversion. References (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.