Traits of Bathy Phytochromes and Application to Bacterial Optogenetics

Phytochromes are photoreceptors sensitive to red and
far-red light,
found in a wide variety of organisms, including plants, fungi, and
bacteria. Bacteriophytochromes (BphPs) can be switched between a red
light-sensitive Pr state and a far-red light-sensitive Pfr state by
illumination. In so-called prototypical BphPs, the Pr state functions
as the thermally favored resting state, whereas Pfr is more stable
in bathy BphPs. The prototypical DrBphP from Deinococcus radiodurans has been shown to be compatible
with different output module types. Even though red light-regulated
optogenetic tools are available, like the pREDusk system based on
the DrBphP photosensory module, far-red light-modulated
variants are still rare. Here, we study the underlying contributors
to bathy over prototypical BphP behavior by way of various chimeric
constructs between pREDusk and representative bathy BphPs. We pinpoint
shared traits of the otherwise heterogeneous subgroup of bathy BphPs
and highlight the importance of the sensor-effector linker in light
modulation of histidine kinase activity. Informed by these data, we
introduce the far-red light-activated system “pFREDusk”,
based on a histidine kinase activity governed by a bathy photosensory
module. With this tool, we expand the optogenetic toolbox into wavelengths
of increased sample and tissue penetration.