49 



Bessel Kok 



One could speculate that (some 20 molecules of) C700 func- 

 tion as energy collectors for (one molecule of) P700. Ab- 

 sence of CyOO in the mutant then would explain the lack of a 

 light induced P700 bleaching and EPR signal — energy transfer 

 being impeded. ( This hypothesis still allows the presence 

 of pyOO in the mutant — which remains to be proven). 



An alternate hypothesis identifies C70O with P700: One 

 could conceive that one of the oriented C700 molecules upon 

 excitation loses an electron to primary acceptor (X) and re- 

 gains it from cytochrome f or plastocyanin (6). If X, cyt. f 

 and P.C. were only present in 1/20 the concentration of C700 

 (1/400 chl.) one would observe at any time only one "P7OO" 

 per 400 chlorophylls. This hypothesis would fit sensibly with 

 a rigid orientation of C70O around the reaction loci (11). 



Ill, Sensitization of the various emission bands. 



Figure k shows the fluorescence emission spectra of the 

 two types of Scenedesmus. At room temperature the 685 fluo- 

 rescence of the mutant is as high or higher than that of the 

 normal cells (even if the latter are poisoned with DGMU to 

 stop energy flow in order to obtain comparable conditions). 

 We are not certain whether the anomalous emission between 700 

 and 760 mu in Fig. 4 is typical. At 77®K the mutant fluo- 

 resces stronger than the v/ild type at 730 mu, relatively weaic 

 at 685 mu, and the 698 band is practically absent. 



Figures 3 and k show a distinct correlation between C7OO 

 and the fluorescence at 698 mu; both are practicaJLly absent 

 in the mutant. The simplest explanation is that C7OO is the 

 emitter of F7OO. 



The data, furthermore, indicate that the (77*'K) fluores- 

 cence at 730 mu does not directly originate from either C7OO 

 or P700; F73O is high in the mutant lacking C70O. It actu- 

 ally appears as if in the normal alga, C7OO functions as a 

 quencher for F685 at room temperature and possibly for F730 

 at 77*'K. Quenching at room temperature could at least partly 

 be an indirect effect: if C70O and/or P7OO operated in photo- 

 system I they would provide substrate for photosystem II, e.g. 

 "Qox" in (3). However, quenching of F730 at low temperature 

 must be due to a competition for absorbed quanta between C7OO 

 and "C730" (the chloro:chyll responsible for F730). 



We now meet a difficulty: Butler (4) observed, and we con- 

 firmed, that in leaves, chloroplasts , and green algae (where 

 F73O is found at 7l6 mu) the sensitization of F730 shows a 



