# Photoactivation in Myxobacterial Phytochrome: Early Events

## TL;DR

This study explores how myxobacteria sense red light through a protein called phytochrome, revealing early molecular changes triggered by light.

## Contribution

The paper provides new insights into the molecular mechanism of phytochrome activation in myxobacteria using time-resolved crystallography.

## Key findings

- At 3 ps after light activation, the BV ring-D adopts a configuration necessary for isomerization.
- At 100 ps, a mixture of Z and E configurations of BV is observed in one subunit, while the other remains in the Z configuration.
- These findings elucidate the early steps of phytochrome activation in myxobacteria.

## Abstract

Myxobacteria are non-photosynthetic, soil bacteria distinguished by a multicellular stage in their life cycle known as fruiting bodies. Stimulated by light, fruiting bodies from myxobacteria may rely on phytochrome function as protein photoreceptor responsible for the perception of red-light. The molecular mechanism how light is perceived and converted to a physiological response is unknown. Here, time-resolved serial femtosecond crystallographic (TR-SFX) experiments were conducted on microcrystals of the Stigmatella aurantiaca bacteriophytochrome 2 (SaBphP2). Initial events of the Z to E isomerization reaction of biliverdin, the covalently bound, open-chain tetrapyrrole, were explored. At 3 ps after light activation, the BV ring-D assumes a configuration needed for the isomerization. At 100 ps, a mixture of BV in the Z or E configuration is observed in subunit A, while in the other subunit the BV chromophore remains in the Z configuration. In conjunction with prior results, these BV structures shine light on the molecular mechanism of phytochrome activation involved in the photomorphogenesis of the myxobacteria and lay the molecular foundation to explain physiological responses to red light in other bacteria.