591 



Lester Packer 



centrations equal to that of ATP (2 nW) broiight about a rapid com- 

 pletion of the reversal of the scattering response in the dark. 

 Abolition of the original scattering increase by imi^.Cl was vir- 

 tually complete, where as ADP led to only a partial reversal. 

 When the ADP concentration vas raised in a stepwise fashion, to 

 twice or three times the concentration of ATP, however, the cumu- 

 lative reversal was as great as that with NHj^Cl. The kinetics of 

 the reversal of scattering brought about by ADP and NHi^'*', show 

 that the rate of the dark return was more rapid with ADP than with 

 Mi^Cl. 



In agreement with the action of ADP on reversal of scattering 

 changes, it is found that this substance inhibits nucleoside tri- 

 phosphatase activity under the same conditions that it brings 

 about reversal of the scattering level mairr^ained in the presence 

 of ATP. Furthermore, in the absence of Mg or a thiol compound, 

 reversal of scattering is complete without any additions and under 

 these conditions, the corresponding values for ATPase are low. It 

 is therefore concluded that high ATPase is correlated with incom- 

 plete reversal of scattering. The action of NHi^Cl on the other 

 hand, must be different from ADP since Mi^Cl is not an inhibitor 

 of ATPase under these conditions (18-I9). The action of NHi^Cl on 

 scattering responses deserves further consideration since it is a 

 powerful inhibitor of photophosphorylation. In the absence of 

 thiol compounds, Mj^Cl added before illumination prevents scatter- 

 ing responses brought about by actinic light; in the presence of 

 a thiol compound however, scattering responses can still proceed. 

 In the presence of cysteine and ATP, for example, scattering 

 increases brought about by actinic light were approximately the 

 same whether lIHi^'*' was present or not; with lipoic acid, the scat- 

 tering increases in the presence of ammonia were hOf^o of those in 

 its absence. The action of KHi^Cl on scattering and its lack of 

 effect on nucleoside triphosphatase activity, suggest that this 

 substance acts at different site in the dark energy transfer 

 pathway in chloroplasts, to that involved in nucleoside triphos- 

 phate hydrolysis. 



Since these results suggested an involvement of light 

 actiA'D.ted nucleoside triphosphatase activity in the control of 

 scattering changes geared to photosynthetic electron transport, 

 the action of ATP and the requirement for actinic liglit were 

 therefore further examined. Recently (l8,20) it was discovered 

 that the actinic liglit requirement for the light activated nucleo- 

 side triphosphatase is indirect; the optimum conditions for stimu- 

 lating this activity were found to be a brief period of pre-illu- 

 mination in the -presence of a thiol compound, a redox factor such 



