# Phytochromes: Structural Perspectives on Photoactivation and Signaling

> **NIH NIH R01** · WASHINGTON UNIVERSITY · 2022 · $326,833

## Abstract

PROJECT SUMMARY
Almost all cellular organisms employ an array of photoreceptors to detect their light environment. Arguably the most
influential are the phytochromes (Phys), a diverse group essential for plant development and the behavior of many bacterial,
fungal, and algal species. By reversible photointerconversion of their bilin (or open-chain tetrapyrrole) chromophores
between red light-absorbing Pr and far-red light-absorbing Pfr states, Phys act as photoswitches in various signaling
cascades responsive to light intensity, duration, direction, and spectral quality. Moreover, through the thermal reversion of
Pfr back to Pr, some Phys sense temperature through enthalpic effects on this reaction, and perceive photoperiod through
the nighttime depletion of Pfr. The cumulative effects of this Pr/Pfr interconversion impact numerous physiological
processes important to agriculture and the biology of harmful plant and human pathogens. In addition, their unique
photochemistries provide invaluable optogenetic tools, including novel fluorophores for tissue imaging, and engineered
photoswitches for regulating cellular events with remarkable temporal and spatial precision.
Recently, we made great strides in understanding how Phys signal, with emerging structures suggesting that microbial and
plant Phys use two distinct output modalities. Both start with light-triggered isomerization of the bilin, which drives a -
stranded to -helical rearrangement of a hairpin loop that links the signature PHY and GAF domains. While photoactivated
microbial Phys then connect torsional strain generated within the dimer to regulate an appended output domain (typically
with histidine kinase activity), plant Phys have rearranged their domain organization to create a photosensitive dimeric
platform that likely enables reversible binding and eventual degradation of the family of PIF transcriptional repressors.
While current models helped illuminate gross changes required for endstate conversion, the intermediates of photoexcitation
and ensuing structural changes necessary for creating a signaling-competent Pfr state remain uncertain.
The objectives of this proposal are to complete these pictures through continued X-ray crystallographic and cryo-electron
microscopic approaches followed by informed biochemical analyses of representatives in their inactive and active states.
Specific aims are to: (1) exploit time-resolved serial X-ray crystallography to structurally define the intermediates generated
by Phys after photon absorption; (2) generate more comprehensive structures of bacterial Phys, including models of full-
length dimeric photoreceptors with their signal output modules; (3) develop a model for how plant Phys signal through
structural studies on Pfr; (4) apply steady-state and time-resolved protein surface mapping to support the Phy
photoconversion pathway(s) seen structurally; (5) develop models of Phys interacting with their downstream effectors, and
(6) appreciat...

## Key facts

- **NIH application ID:** 10520962
- **Project number:** 2R01GM127892-05
- **Recipient organization:** WASHINGTON UNIVERSITY
- **Principal Investigator:** RICHARD DAVID VIERSTRA
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $326,833
- **Award type:** 2
- **Project period:** 2018-09-01 → 2026-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10520962

## Citation

> US National Institutes of Health, RePORTER application 10520962, Phytochromes: Structural Perspectives on Photoactivation and Signaling (2R01GM127892-05). Retrieved via AI Analytics 2026-05-27 from https://api.ai-analytics.org/grant/nih/10520962. Licensed CC0.

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