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Updated: May 13, 2026
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Isolation and Characterization of Intact Phycobilisome in Cyanobacteria
Published on: November 10, 2021
Green/red cyanobacteriochromes regulate complementary chromatic acclimation via a protochromic photocycle
Yuu Hirose1, Nathan C Rockwell, Kaori Nishiyama
1Electronics-Inspired Interdisciplinary Research Institute, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan.
Summary
Cyanobacteriochromes (CBCRs) are light sensors in cyanobacteria. A new "protochromic photocycle" explains how they switch between green and red light absorption through bilin chromophore protonation changes.
Area of Science:
- Biochemistry
- Photobiology
- Microbiology
Background:
- Cyanobacteriochromes (CBCRs) are phytochrome-related photosensors in cyanobacteria.
- CBCRs utilize a bilin chromophore for light sensing, exhibiting diverse photocycles.
- The mechanism for green-to-red light absorption tuning in CBCRs remained unclear.
Purpose of the Study:
- Investigate the photocycle mechanism of RcaE, a CBCR involved in complementary chromatic acclimation.
- Elucidate the molecular basis for green and red light absorption switching in CBCRs.
- Characterize the light-regulated activity of RcaE.
Main Methods:
- Spectroscopic analysis of RcaE from Fremyella diplosiphon.
- Biochemical assays to determine kinase activity.
- Site-directed mutagenesis to identify key residues.
Main Results:
- RcaE exhibits a green/red photocycle, converting between green-absorbing (15Z)P(g) and red-absorbing (15E)P(r) states.
- The bilin chromophore is deprotonated in the (15Z)P(g) state and protonated in the (15E)P(r) state.
- Three conserved residues modulate the bilin protonation state, driving the color change.
Conclusions:
- Green/red CBCRs utilize a novel "protochromic photocycle" driven by bilin protonation changes, not solely photoisomerization.
- This mechanism allows cyanobacteria to adapt light-harvesting antennae to different light conditions.
- The findings provide a new understanding of light sensing in prokaryotes.
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