The D-ring, not the A-ring, rotates in Synechococcus OS-B' phytochrome

Area of Science:

• Biochemistry
• Molecular Biology
• Spectroscopy

Background:

• Phytochromes are photoreceptors controlling biological processes via photochromic switching.
• The tetrapyrrole chromophore's phototransformation mechanism is debated, with proposed rotation of either ring A or ring D.
• Previous studies suggested A-ring rotation, challenging the canonical D-ring rotation model.

Purpose of the Study:

• To investigate the phototransformation mechanism of the GAF domain of Synechococcus OS-B' phytochrome.
• To determine whether A-ring or D-ring rotation occurs during phytochrome photochromic switching.
• To elucidate the structural changes associated with phytochrome photoconversion.

Main Methods:

• Magic angle spinning (MAS) NMR spectroscopy applied to the GAF domain of Synechococcus OS-B' phytochrome.
• Analysis of the two parent states of the phytochrome GAF domain.
• Denaturation experiments to confirm structural changes upon photoconversion.

Main Results:

• Major changes observed at the A-ring linkage to the protein and at the protein-ring D contact.
• Conserved contacts around the A-ring nitrogen exclude A-ring photoflip.
• Loss of contact of the D-ring nitrogen implies D-ring movement, consistent with Z/E isomerization at C15=C16.

Conclusions:

• Findings disprove A-ring rotation in Synechococcus phytochrome.
• The study supports D-ring movement, likely involving tilting and Z/E isomerization, as the primary photochromic mechanism.
• Results contribute to understanding photochromicity and structural dynamics across the phytochrome family.