166 



R. P. Levine 



It was also demonstrated that both ac- 115 and ac- 141 had retained the 

 sharp, fast ESR signal but that the slow, broad signal was missing. The former 

 signal has been attributed to a long wave length form of chlorophyll a such as 

 P- 700^25, 30)^ whereas the latter signal may be associated with chlorophyll b, 

 for this signal is absent in a chlorophyll b-less mutant of Chlorella^23)_ j^ the 

 case of ac-115 and ac- 141 it is tempting to correlate the absence of this signal 

 with the inability of these mutant strains to carry out a reaction in system II. 



In contrast to both ac- 115 and ac- 141 , ac- 208 has Hill reaction activity 

 with all of the Hill oxidants tested except ferricyanide, and yet it cannot photo- 

 reduce TPN from DPIP and ascorbate. These results suggest that the block in 

 ac-208 lies at a side in the system subsequent to the point of entry of electrons 

 TFom DPIP and ascorbate. The block could lie in system I. However, there is 

 no direct evidence for this, and ac- 208 has both ESR signals. 



The two light-dependent reactions occur in ac-21. However, they must 

 be coupled by at least one light-independent reaction which is blocked in this 

 mutant strain. A photoreductant is produced by system II in ac-21 as evidenced 

 by the fact that there is Hill reaction activity. However, this photoreductant 

 apparently cannot be utilized, for DPIP and ascorbate must be provided in order 

 to obtain TPN photoreduction. These results can be best explained by assuming 

 that a block lies in a light-independent reaction between systems I and II. This 

 explanation is supported by the observation that both ESR signals are generated 

 in cells of ac-21. Thus, inasmuch as the two different ESR signals may reflect 

 systems I and IL the mutant strain is identical to wild type. 



Both ac-21 and ac-208 pose some interesting questions regarding the site 

 of action of ferricyanide into the electron transport system of C^. reinhardi . 

 Witt, Miiller, and Rumberg^"^^) suggest that ferricyanide belongs to a group of 

 oxidants, termed ox Sj, whose reduction is associated with the oxidation of Chlj. 



Accordingly, any block in the electron transport system of C. reinhardi lying 

 at a site after system II should result in the absence of a Hill reaction with 

 ferricyanide. Thus, since ac-21 and ac- 208 are blocked after system II, neither 

 of them should have a Hill reaction with ferricyanide. This is contrary to our 

 observations, for ac-21 does have Hill reaction activity with this oxidant, 

 albeit at a rate that is about three times lower than that of wild type. It is con- 

 ceivable that the Hill reaction with ferricyanide could proceed via system II 

 alone in ac-21. If this assumption is made, however, it would be expected that 

 ac-208 would also give a Hill reaction with ferricyanide. Of course, the dilemma 

 with ac-208 could be avoided by making the second assumption that there are 

 two blocks in ac-208; namely, that there is a block in electron transport be- 

 tween the poirvTof entry of electrons from DPIP and the reduction of TPN, and 

 that ac-208 lacks a component of the electron transport system unique to the 

 ferricyanide Hill reaction. 



These ad hoc assumptions, however, should not be considered significant 

 in the absence of experimental evidence. If the Hill reaction with ferricyanide 

 in ac-21 proceeds from system II alone then its action spectrum might be dif- 

 ferent from that obtained with wild type. We have recently begun, in collaboration 



