Apr 9, 1976

Evidence against proton gradient formation being the cause of chlorophyll fluorescence quenching by N-methylphenazonium methosulfate

Biochimica Et Biophysica Acta
R E Slovacek, T T Bannister


In strong illumination, 3-(3, 4-dichlorophenyl)-1,1-dimethylurea (DCMU)-poisoned chloroplasts exhibit a high yield of chlorophyll fluorescence while P-700 turnover, proton uptake, and phosphorylation are inhibited and a pH gradient is undectectable. When 10muM N-methylphenazonium methosulfate (PMS) is included, the fluorescence yield in light is substantially reduced, and when 100 muM ascorbate is also included, the yield is diminished approximately to the level in darkness. Only very slight increases in P-700 turnover and proton uptake (but no detectable pH gradient) accompany the fluorescence yield decline. When 10muM PMS and 15 mM ascorbate are added to poisoned chloroplasts (the oxygen concentration being greatly reduced), P-700 turnover, proton uptake, the pH gradient and phosphorylation all reach high levels. In this case, the yield of chlorophyll fluorescence is low and is the same in both light and dark. Further addition of an uncoupler eliminates proton uptake, the pH gradient and phosphorylation but does not significantly elevate the fluorescence yield. From these observations we suggest that, in DCMU-poisoned chloroplasts, the fluorescence quenching with PMS occurrs by a mechanism unrelated to the generation of a pho...Continue Reading

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Mentioned in this Paper

Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone
Electron Transport
Protein Phosphorylation
Fluorescence Spectroscopy
Chlorophyll Fluorescence

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