Abstract 2626 Photoreception and signaling in bacterial phytochrome revealed by single-particle cryo-EM

Phytochromes are red-light photoreceptors discovered in plants with homologs in bacteria and fungi that regulate a variety of physiological responses.They display a reversible photocycle between two distinct states: a red-light-absorbing Pr state and a far-red light-absorbing Pfr state.The photoconversion regulates the activity of an enzymatic domain, usually a histidine kinase (HK).Here, we report three cryoelectron microscopy structures of a wild-type bacterial phytochrome with HK activity determined as Pr and Pfr homodimers and as a Pr/Pfr heterodimer with individual subunits in distinct states.We propose that the Pr/Pfr heterodimer is a physiologically relevant signal transduction intermediate.To date, this is the only structure of a wild-type phytochrome captured as Pr/Pfr heterodimer.Our results offer insight into the molecular mechanism that controls the enzymatic activity of the HK as part of a bacterial two-component system that perceives and transduces light signals.