Algal PETC-Pro171-Leu suppresses electron transfer in cytochrome b6f under acidic lumenal conditions

P171L substitution in the Rieske FeS PETC subunit of cytochrome b(6)f alters the pH dependency of the photosynthetic control mechanism. Linear photosynthetic electron flow (LEF) produces NADPH and generates a proton electrochemical potential gradient across the thylakoid membrane to synthesize ATP, both of which are required for CO2 fixation. As cellular demand for ATP and NADPH varies, cyclic electron flow (CEF) between Photosystem I and the cytochrome b(6)f complex (b(6)f) produces extra ATP. b(6)f regulates LEF and CEF via photosynthetic control, which is a pH-dependent b(6)f slowdown of plastoquinol oxidation at the lumenal site. This protection mechanism is triggered at more alkaline lumen pH in the pgr1 (proton gradient regulation 1) mutant of the vascular plant Arabidopsis (Arabidopsis thaliana), which contains a Pro194Leu substitution in the b(6)f Rieske Iron-sulfur protein Photosynthetic Electron Transfer C (PETC) subunit. In this work, we introduced the equivalent pgr1 mutation in the green alga Chlamydomonas reinhardtii to generate PETC-P171L. Consistent with the pgr1 phenotype, PETC-P171L displayed impaired NPQ induction along with slower photoautotrophic growth under high light conditions. Our data provide evidence that the Delta pH component in PETC-P171L depends on oxygen availability. Only under low oxygen conditions was the Delta pH component sufficient to trigger a phenotype in algal PETC-P171L where the mutant b(6)f was more restricted to oxidize the plastoquinol pool and showed diminished electron flow through the b(6)f complex. These results demonstrate that photosynthetic control of different stringency are established in C. reinhardtii depending on the cellular metabolism, and the lumen pH-sensitive PETC-P171L was generated to read out various associated effects.

Automated summary

The substitution of P171L in the Rieske FeS PETC subunit of cytochrome b(6)f alters the pH dependency of the photosynthetic control mechanism. This study demonstrates that different stringency of photosynthetic control is established in Chlamydomonas reinhardtii depending on cellular metabolism, and the lumen pH-sensitive PETC-P171L mutant can be used to read out various associated effects. The Delta pH component in PETC-P171L depends on the availability of oxygen.