The algal PETC-Pro171-Leu suppresses electron transfer in the cytochrome b6f complex under acidic lumenal condition

Linear photosynthetic electron flow (LEF) produces NADPH and generates a proton electrochemical potential gradient across the thylakoid membrane used to synthesize ATP, both of which are required for CO2 fixation. As cellular demand for ATP and NADPH are variable, cyclic electron flow (CEF) between PSI and cytochrome b6f complex (b6f) produces extra ATP. The b6f regulates LEF and CEF via photosynthetic control, which is a pH-dependent b6f slowdown of plastoquinol oxidation at the lumenal site. This protection mechanism is triggered at more alkaline lumen pH in the pgr1 mutant of the vascular plant Arabidopsis thaliana, carrying Pro194Leu in the b6f Rieske Iron-sulfur protein. In this work, we introduced pgr1 mutation in the green alga Chlamydomonas reinhardtii (PETC-P171L). Consistent with pgr1 phenotype, PETC-P171L displayed an impaired NPQ induction along with slower photoautotrophic growth under high light conditions. Our data provides evidence that the {Delta}pH component in PETC-P171L is dependent on oxygen availability. Only under low oxygen conditions the {Delta}pH component was sufficient to trigger a phenotype in algal PETC-P171L where the mutant b6f was more restricted to oxidize the PQ pool and showed a diminished electron flow through the b6f complex. One sentence summaryChange of PETC to P171L via site directed mutagenesis alters the pH dependency of the photosynthetic control mechanism