Structures of Myxobacterial Phytochrome Revealed by Cryo-EM

Phytochromes are red-light photoreceptors first identified in plants, with homologs found in bacteria and fungi that regulate a variety of critical physiological processes. They undergo a reversible photocycle between two distinct states: a red-light-absorbing phytochrome (Pr) form and a far-red light-absorbing phytochrome (Pfr) form. This Pr-to-Pfr photoconversion controls the activity of a C-terminal enzymatic domain, typically a histidine kinase (HK). The molecular mechanisms underlying light-induced regulation of HK activity in bacteria remain poorly understood, as structures of unmodified bacterial phytochromes with HK activity were unknown until recently. Here, we report details of using cryogenic electron microscopy (cryo-EM) to determine the structure of an intact (full-length) bacteriophytochrome.