Light-induced structural changes in a full-length cyanobacterial phytochrome probed by time-resolved X-ray scattering

Phytochromes are photoreceptor proteins that transmit a light signal from a photosensory region to an output domain. Photoconversion involves protein conformational changes whose nature is not fully understood. Here, we use time-resolved X-ray scattering and optical spectroscopy to study the kinetics of structural changes in a full-length cyanobacterial phytochrome and in a truncated form with no output domain. X-ray and spectroscopic signals on the µs/ms timescale are largely independent of the presence of the output domain. On longer time-scales, large differences between the full-length and truncated proteins indicate the timeframe during which the structural transition is transmitted from the photosensory region to the output domain and represent a large quaternary motion. The suggested independence of the photosensory-region dynamics on the µs/ms timescale defines a time window in which the photoreaction can be characterized (e.g. for optogenetic design) independently of the nature of the engineered output domain. Heyes et al. report that the conformational changes of a full-length Synechocystis phytochrome from the red- to the far-red-absorbing states extend to the seconds timescale. Interestingly, while the rapid structural changes are independent of the output domain the slower changes are not, providing useful insights on photosensory-region dynamics.