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bioRxiv · 10.64898/2026.01.14.699543

Client distribution between Chlamydomonas FDX1 and FDX2 in carbon, nitrogen and sulfur assimilation

Schmollinger, S.·Kusi-Appiah, G.·Villegas, D.·Stainbrook, S.·O'Halloran, T. V.·Strenkert, D.
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Abstract

Plant-type ferredoxins (Fd) comprise small, soluble protein families that distribute electrons from photosystem I to various client proteins within the chloroplast stroma. In Chlamydomonas reinhardtii, the major, constitutively expressed FDX1/PetF supports Fd-NADP+ reductase (FNR) in NADPH production. The highly similar FDX2 is present only when its preferred nitrogen (N) source ammonium is absent, supplying Fd-dependent nitrite reductase (NiR) for nitrate/nitrite assimilation. Surprisingly, despite accumulating to [~]10% of FDX1 abundance and preferential interaction with NiR, fdx2 mutants are asymptomatic when grown on nitrate, requiring to additionally deplete FDX1 for growth to be halted. A fdx1 knockout itself appears lethal, severe fdx1 knockdowns have reduced growth rates both in phototrophic and photoheterotrophic conditions, independent of the N source. Transcriptome analyses of fdx1 mutants revealed expression patterns similar to sulfur (S) deficient algae, and fdx1 strains have a reduced total cellular S content. S assimilation requires Fd-dependent sulfite reductase (SiR) activity, an enzyme distantly related to FDX2 client NiR. Expression defects are partially alleviated; growth and S content are less impacted with FDX2 expression. Our mutant analysis shows the two major Fds in Chlamydomonas focus on a specific subset of Fd-dependent metabolism, mostly supplying Fd-dependent enzymes involved in macronutrient assimilation (C/N/S).

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Schmollinger, S., Kusi-Appiah, G., Villegas, D., Stainbrook, S., O'Halloran, T. V., Strenkert, D.. 2026-01-15. Client distribution between Chlamydomonas FDX1 and FDX2 in carbon, nitrogen and sulfur assimilation. https://doi.org/10.64898/2026.01.14.699543

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