Electron transfer components of wild-type and photosynthetic mutant strains of Scenedesmus obliquus D3

To investigate the possible alteration of various components of the photosynthetic electron transport system of certain mutants of Scenedesmus techniques were developed for their extraction and purification from whole cells of this alga. The components identified in the normal alga were cytochrome c 549, cytochrome b 562, a cytochrome c 551, flavoprotein-ferredoxin reductase, plastocyanin, cytochrome c 552, and ferredoxin. Lamellar-bound cytochrome c 552 and cytochromes b were also detected. Application of the extraction and purification techniques to two photosynthetic mutants revealed that Mutants 26 and 50 lacked cytochrome f in both the bound and soluble forms (Mutant 50) or in only the bound form (Mutant 26). Chloroplasts prepared from either of these mutants lacked Hill reaction activity with a variety of oxidants with water as the electron donor but photoreduced NADP⁺ with 2,6-dichlorophenolindophenol and ascorbate as the electron donor system. No photophosphorylation in vivo was detected with either mutant, but isolated chloroplasts performed a cyclic photophosphorylation with phenazine methosulphate as cofactor. Fluorescence analysis revealed that both mutants possess a measurable Photosystem II activity.

It was concluded that the loss of cytochrome f prevents the normal flow of electrons from Photosystem II to NADP and also to a variety of other Hill reaction oxidants. Furthermore, cytochrome f is not required for the reduction of NADP with electron donor systems other than water nor is it an essential component of the mechanism of cyclic photophosphorylation with phenazine methosulphate as cofactor.