A spectroscopic analysis of low-fluorescent mutants of Chlamydomonas reinhardti blocked in their water-splitting oxygen-evolving apparatus

Abstract

1. 1. A spectral analysis of six mutant strains of the unicellular alga, Chlamydomonas reinhardti, having lesions associated with the water-splitting oxygen-evolving apparatus is described. 2. 2. The chloroplast fragments from these mutant strains have a functional Photosystem II reaction center as shown by the normal photoreduction of C-550 and the photooxidation of cytochrome b 559 at low temperature, and by their ability to carry out a 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-sensitive photoreduction of NADP + at room temperature in the presence of electron donors which donate electrons specifically to Photosystem II. 3. 3. The mutant chloroplast fragments contain equal amounts of the ascorbate-reducible cytochrome b 559 and C-550 with all of the cytochrome b 559 present participating in the low-temperature photoreaction. In contrast the wild-type chloroplast fragments contain twice the amount of high-potential cytochrome b 559 as C-550 with only half of the cytochrome b 559 present participating in the low-temperature photoreaction. 4. 4. The only component difference found between the mutant and the wild-type chloroplast fragments was that the lfd chloroplast fragments contained only half as much ascorbate-reducible cytochrome b 559 as the wild type. Equal proportions, respectively, of cytochrome c 553, low-potential cytochrome b 559, and cytochrome b 564 are found in both the mutants and wild-type strains. 5. 5. The data indicate that there exist two functionally distinct pools of ascorbate-reducible cytochrome b 559 both of which function in the water-splitting oxygen-evolving apparatus. The lfd mutant strains are unable to split water and evolve oxygen because they lack one of the two pools of ascorbate-reducible cytochrome b 559.