REACTION CENTERS IN PHOTOSYNTHESIS 383 



KINETICS OF THE PRINCIPAL LIGHT REACTIONS 

 IN BACTERIAL CHROMATOPHORES 



For simplicity let us consider first the behavior of chromatophores 

 in which the light- reacting cytochromes remain fully oxidized, so that 

 the Cyt reaction is absent. This situation prevails in washed chroma- 

 tophores under frequent illumination (29), In such preparations the 

 P870-890 reaction, the UQ reaction, and the reactions of Types E and 

 F (Table 1) occur reversibly with identical kinetics and in fixed pro- 

 portions. These reversible events persist in dried chromatophores at 

 77 °K. The P870-890 and Type E and F reactions occur reversibly at 

 1°K; the UQ reaction was not examined at this temperature. Recovery 

 in the dark for all of these reactions requires several seconds at 

 room temperature and about 20 msec at temperatures below 150°K 

 (26). In intact cells these reactions are conspicuous when, through the 

 use of inhibitors or aeration, the cytochrome components are kept in 

 an oxidized state (27,37), These light reactions are essentially the same 

 in Rhodopseudowoims , Rhodospirilluui, and Chromatiuw (27). Their 

 persistence under adverse conditions, with no temperature coefficient 

 and with high quantum efficiency (at least for the P870-890 and Cyt 

 reactions (28,36)), characterizes them as primary photochemical re- 

 actions. As a specific working hypothesis, the first observable photo- 

 chemical act is held to be the transfer of an electron from P870 or 

 P890 to UQ, This is the counterpart of a reaction involving P700 and 

 (provisionally) PQ in chloroplasts. 



A complete parallelism between the bacterial and green plant sys- 

 tems would require that the Cyt reaction in chromatophores is the 

 transfer of an electron from a cytochrome to oxidized P870 or P890. 

 There are circumstantial reasons for believing that this is the case. 

 First, the Cyt reaction is closely coupled to a primary photochemical 

 act. The light-induced oxidation of Cyt occurs reversibly (recovery 

 requiring about 2 hours) in dried Chroniatium chromatophores (29) 

 and persists, although irreversibly, in Chroniatium cells at77°K (35), 

 Under conditions in which the Cyt reaction is absent the P870-890 and 

 UQ reactions still occur reversibly, A comparable situation is found 

 in Kok's partially purified preparations of P700, where the light- 

 induced bleaching of P700 occurs but the oxidation of Cyt /has been 

 lost (16). These facts are consistent with the view that the oxidation of 

 Cyt is an immediate sequel to a primary light-induced oxidation of 

 P870 or P890. Other possibilities cannot be ruled out; among these is 

 a primary reaction in which Cyt is oxidized and P870 or P890 is re- 

 duced (B, Chance, verbal communication). In that case the oxidation of 

 P870-890 would be an alternative reaction that occurs only when the 

 Cyt is oxidized (Fig. 2). Nothing that suggests the formation of re- 

 duced P870-890 (or BChl) has yet been observed in photosynthetic 

 bacteria. 



