PGR5 is required for efficient Q cycle in the cytochrome b6f complex during cyclic electron flow

Area of Science:

• Photosynthesis research
• Plant molecular biology
• Algal physiology

Background:

• Proton gradient regulation 5 (PGR5) influences photosynthetic electron transfer, but its precise mechanism remains unclear.
• Existing models propose PGR5 regulates cyclic electron flow (CEF) around photosystem I (PSI) to mitigate photodamage and control proton motive force (pmf).

Purpose of the Study:

• To elucidate the mechanistic role of PGR5 in regulating photosynthetic electron transfer in Chlamydomonas reinhardtii.
• To investigate the function of the cytochrome b6f complex (b6f) in wild-type and pgr5 mutant strains under different electron flow conditions.

Main Methods:

• Comparative analysis of electron transfer rates and redox states in wild-type and pgr5 mutant Chlamydomonas reinhardtii.
• Investigation of the cytochrome b6f complex's Q cycle activity during linear electron flow and CEF.
• Biochemical assays to determine the ferredoxin-plastoquinone reductase activity of the b6f complex.

Main Results:

• The pgr5 mutant exhibits a dysfunctional cytochrome b6f complex, impacting its ability to regulate electron flow.
• The b6f complex operates in two distinct Q cycle modes, regulated by PGR5 and dependent on stromal electron carriers.
• In CEF conditions, the b6f complex becomes the electron transfer bottleneck in pgr5 mutants, potentially explaining the altered pmf phenotype.

Conclusions:

• PGR5 plays a critical role in modulating the activity of the cytochrome b6f complex, likely through regulating its Q cycle modes.
• The findings challenge current models of CEF by suggesting a ferredoxin-plastoquinone reductase activity for the b6f complex and highlighting the importance of PGR5 in its redox tuning.