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¥. L. Butler and N. I. Bishop 



absorption spectra show that essentially the same amounts of 

 chlorophyll b, chlorophyll a-670 and chlorophyll a-680 are pres- 

 ent in all samples. Thus, the mutants carry a normal compliment 

 of pigments in systems 1 and 2. P is present in the xd.ld-type 

 cells and in the "O2" mutant, as shovm by the slight shoulder in 

 the absorption spectra on the long-wavelength side of the main 

 chlorophyll-absorption band and by the maximum in the derivative 

 spectra near 700 nm. The spectra for the "CO2" mutant, however, 

 do not show the presence of P. Thus, in these mutants, system-1 

 pigments cannot affect the redox state of Q because of the block 

 in the electron transport chain at P. System-2 pigments, how- 

 ever, can cause Q to be more reduced in the light. The high 

 fluorescence yield of these mutants in the absence of actinic 

 light suggests that the Q is largely reduced in the dark. It 

 would be of interest to determine if the cytochrome in these mu- 

 tants could be reduced in the light by system 2 and oxidized in 

 the dark as is Q. 



The block in the "O2" mutants has not been localized. The 

 absence of a light affect on the fluorescence yield shows that 

 Q is not being affected by light even though the absorption spec- 

 trum indicates that system-2 pigments are present. The high 

 fluorescence yield suggests either that Q is fully reduced in the 

 dark or that it is not present. The absence of Q in the "O2" 

 mutants would be analogous to the absence of P in the "CO2" 

 mutants . 



The fluorescence yield changes are consistent with the photo- 

 synthetic electron transport chain and with the photochemical 

 activities of the wild-type and mutant cells. These measure- 

 ments provide a rapid and convenient method to study metabolic 

 inhibitors and mutations which affect photosynthesis. 



