Energy and Electron Transfer Systems of Chlamydomonas reinhardi: II. Two Cyclic Pathways of Photosynthetic Electron Transfer in the Pale Green Mutant.

Light- and oxygen-induced changes of cytochromes f, b(563), and b(559) and ferredoxin-flavoprotein were studied by a double beam spectrophotometer with combinations of inhibitors and lowered temperatures in the whole cells of the pale green mutant of Chlamydomonas reinhardi (ATCC 18302). At room temperature, the steady state changes of cytochrome f and ferredoxin-flavoprotein are small, but at low temperature slightly above 0 C, they are clearly defined. Phenylmercuric acetate inhibits photoreduction of ferredoxin-flavoprotein and cytochrome f simultaneously but not that of cytochrome b(563). 2-Heptyl-4-hydroxyquinoline-N-oxide shows a crossover point between cytochromes f and b(563) and partially inhibits photoreduction of cytochrome f. Two cyclic pathways operating in C. remhardi are postulated: (a) photosystem I --> x --> b(563) --> f --> photosystem I; and (b) photosystem I --> x --> ferredoxin-flavoprotein --> f --> photosystem I.