Physicochemical differences between the red- and the far-red-absorbing forms of phytochrome

Area of Science:

• Plant Physiology
• Photobiology
• Biochemistry

Background:

• Phytochrome is a crucial plant photoreceptor that exists in two forms: red-absorbing (Pr) and far-red-absorbing (Pfr).
• Understanding the molecular differences between Pr and Pfr is key to elucidating phytochrome's role in plant development.

Purpose of the Study:

• To chemically and physically characterize the differences between the Pr and Pfr forms of oat phytochrome.
• To investigate the structural and chemical basis of phytochrome's photoreversibility and conformational changes.

Main Methods:

• Immunoaffinity purification of undegraded oat phytochrome.
• Isoelectric focusing to assess surface charges.
• Circular dichroism spectroscopy to evaluate secondary structure.
• Chemical modification of specific amino acid residues (His, Cys, carboxyl, Tyr) to probe reactivity differences.

Main Results:

• Pr and Pfr exhibited identical surface charges and secondary structures.
• Differential modification rates of histidine and cysteine residues were observed between Pr and Pfr.
• Tyrosine modification showed faster initial rates on Pfr, with photoreversibility lost upon modification of two tyrosine residues per monomer.

Conclusions:

• The data support a hypothesis of a minor conformational change in phytochrome upon photoconversion from Pr to Pfr.
• A conformation-stabilization mechanism is proposed, where photoconversion stabilizes phytochrome in a specific conformation.