Dual-Cys bacteriophytochromes: intermediates in cyanobacterial phytochrome evolution?

Area of Science:

• Molecular Biology
• Biochemistry
• Microbiology

Background:

• Phytochrome photoreceptors are crucial for light sensing in cyanobacteria.
• Three families of knotted phytochromes have been previously identified.
• Understanding phytochrome diversity is key to deciphering light-dependent processes.

Purpose of the Study:

• To identify and characterize novel phytochrome photoreceptor families in cyanobacteria.
• To investigate the structural and functional properties of these new photoreceptors.
• To explore the evolutionary relationships between different phytochrome types.

Main Methods:

• Recombinant expression of dual-cysteine bacteriophytochromes (DCBs) in Escherichia coli.
• Spectroscopic analysis of bilin adducts and photocycles.
• X-ray crystallography to determine the structure of FiDCB bound to biliverdin IXα (BV).
• Site-directed mutagenesis to alter cysteine residues.
• Phylogenetic analysis of phytochrome sequences.

Main Results:

• A fourth phytochrome type, DCBs, was identified with unique cysteine residues in N-terminal extensions and GAF domains.
• Recombinant DCBs bind phycocyanobilin (PCB) and biliverdin IXα (BV), exhibiting distinct photocycles.
• Crystal structure revealed thioether linkages between cysteine residues and BV.
• DCBs display significant structural plasticity, allowing conversion to other phytochrome analogs via mutagenesis.
• Phylogenetic analysis suggests DCBs may represent an intermediate in the evolution of other cyanobacterial phytochrome families.

Conclusions:

• Dual-cysteine bacteriophytochromes represent a novel and structurally plastic class of cyanobacterial photoreceptors.
• The plasticity of DCBs offers insights into the evolutionary pathways of phytochrome diversification.
• DCBs provide a unique system for studying structure-function relationships in light sensing.