- Bathy phytochromes are a subclass of bacterial biliprotein
photoreceptors that carry a biliverdin IXα chromophore. In
contrast to prototypical phytochromes that adopt a red-light–
absorbing Pr ground state, the far-red light–absorbing Pfrform
is the thermally stable ground state of bathy phytochromes.
Although the photobiology of bacterial phytochromes
has been extensively studied since their discovery in
the late 1990s, our understanding of the signal transduction
process to the connected transmitter domains, which are often
histidine kinases, remains insufficient. Initiated by the analysis
of the bathy phytochrome PaBphP from Pseudomonas aeruginosa,
we performed a systematic analysis of five different
bathy phytochromes with the aim to derive a general statement
on the correlation of photostate and autokinase output. While
all proteins adopt different Pr/Pfr-fractions in response to red,
blue, and far-red light, only darkness leads to a pure or highly
enriched Pfr-form, directly correlated with the lowest level of
autokinase activity. Using this information, we developed a
method to quantitatively correlate the autokinase activity of
phytochrome samples with well-defined stationary Pr/Pfrfractions.
We demonstrate that the off-state of the phytochromes
is the Pfr-form and that different Pr/Pfr-fractions
enable the organisms to fine-tune their kinase output in
response to a certain light environment. Furthermore, the
output response is regulated by the rate of dark reversion,
which differs significantly from 5 s to 50 min half-life. Overall,
our study indicates that bathy phytochromes function as sensors
of light and darkness, rather than red and far-red light, as
originally postulated.
