The role of plastidic cytochrome c in algal electron transport and photophosphorylation.

By an improved isolation procedure chloroplasts could be obtained from the alga Bumilleriopsis filiformis (Xanthophyceae) which exhibited high electron transport rates tightly coupled to ATP formation. Uncouplers both stimulate electron transport and inhibit photophosphorylation. These chloroplasts retain almost all soluble cytochrome c-553 besides a membrane-bound cytochrome c-554.5 (=f-554.5). Sonification or iron deficiency removed the soluble cytochrome only with a concurrent decrease of electron transport from water to methyl viologen or to NADP and decreased non-cyclic and cyclic photophosphorylation. However, photosynthetic control and the P/2e ratios remain unaltered. In Bumilleriopsis, which apparently has no plastocyanin, the soluble cytochrome c-553 seemingly links electron transport between the bound cytochrome c and P-700.