212 METABOLISM AND PHYSIOLOGY 



be explained in terms of an increase or decrease of electron flux at a 

 bottleneck point. This point is most likely the HOQNO- and antimycin- 

 sensitive site (between cytochromes b and c?). Evidence supporting 

 this conclusion is as follows: (a) the reaction between cytochromes b 

 and c is inhibited by HOQNO or antimycin A in light- and oxygen- 

 activated oxidation- reduction reactions (8), (b) photophosphorylation is 

 inhibited strongly by HOQNO or antimycin A (22,23), (c) inhibition of 

 photophosphorylation by HOQNO or antimycin A is largely diminished 

 in the presence of MFM (22,23), (d) the rate of photophosphorylation 

 increases in the presence of MPM under sufficiently high light in- 

 tensities, and (e) the decay of delayed photophosphorylation is acceler- 

 ated by MPM and is decelerated by HOQNO. 



The mean value for the maximum single flash yield of ATP syn- 

 thesis, 0.047 ATP/bacteriochlorophyll, is much higher than the amount 

 of total delayed photophosphorylation by a single flash of light intens- 

 ity of 32,000 lux (+ infrared filter) in the repeating flash experiments. 

 For the maximal amount of delayed photophosphorylation, the light 

 intensities used in the repeating flash work were apparently not suf- 

 ficient. Since ATP synthesis during the flash is negligible as compared 

 to the total delayed photophosphorylation when the flash is short and 

 the light intensity is high, the maximum single flash yield obtained by 

 the xenon flash can be regarded as equal to the amount of delayed 

 photophosphorylation. The use of the infrared flashes for the activa- 

 tion of chromatophores excluded the possibility of participation of 

 carotenoids as the primary light- absorbing pigments. Therefore, the 

 number of light quanta absorbed by chromatophores is limited by the 

 number of bacteriochlorophyll molecules (except for nonspecific ab- 

 sorption by chromatophore materials). If we assume that the maximal 

 yield of ATP/bacteriochlorophyll is attained when all the bacterio- 

 chlorophyll molecules are excited by the infrared flash, the minimum 

 quantum yield for the delayed photophosphorylation will take the same 

 value as the maximal value of single flash yield per bacteriochloro- 

 phyll, i.e,, ATP/ht^ = ATP/bacteriochlorophyll = 0.047. 



There have been many discussions concerning the primary photo- 

 chemical reaction in photosynthesis (6,26-31). Except for activated 

 electronic states of assimilatory pigments, the first chemical process 

 which takes place in bacterial photosynthesis is probably the light- 

 induced oxidation of cytochrome. The rapidity and the temperature 

 independence of the process suggest that the oxidation of cytochrome 

 takes place during the short illumination and the rest of the photo- 

 synthetic reactions proceeds in the dark. As the mechanism of photo- 

 phosphorylation in photosynthetic bacteria, the following scheme seems 

 most feasible. 



BChl + Cytochrome c (pe^ 7 ^BChf + Cytochrome c (Fe"^^) (l) 



