# Structures of myxobacterial phytochrome revealed by cryo-EM using the Spotiton technique and with x-ray crystallography

## TL;DR

This paper presents detailed cryo-EM structures of a bacterial phytochrome, revealing how light affects its structure and function.

## Contribution

The study provides new cryo-EM structures of a full-length bacterial phytochrome and compares them with X-ray structures.

## Key findings

- Cryo-EM structures show homodimers and heterodimers of phytochrome in Pr and Pfr states.
- X-ray crystallography reveals a fragmented phytochrome structure in the Pr-state.
- Structural data helps understand how light regulates phytochrome activity.

## Abstract

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 Pr form and a far-red light-absorbing Pfr form. This Pr/Pfr photoconversion controls the activity of a C-terminal enzymatic domain, typically a histidine kinase (HK). However, the molecular mechanisms underlying light-induced regulation of HK activity in bacteria remain poorly understood, as only a few structures of unmodified bacterial phytochromes with HK activity are known. Recently, cryo-EM structures of a wild-type bacterial phytochrome with HK activity are solved that reveal homodimers in both the Pr and Pfr states, as well as a heterodimer with individual monomers in distinct Pr and Pfr states. Cryo-EM structures of a truncated version of the same phytochrome—lacking the HK domain—also show a homodimer in the Pfr state and a Pr/Pfr heterodimer. Here, we describe in detail how structural information is obtained from cryo-EM data on a full-length intact bacteriophytochrome, and how the cryo-EM structure can contribute to the understanding of the function of the phytochrome. In addition, we compare the cryo-EM structure to an unusual x-ray structure that is obtained from a fragmented full-length phytochrome crystallized in the Pr-state.