589 



Lester Packer 



chloroplasts occur under conditions of non- cyclic as veil as cyc- 

 lic photophosphorylation. In non-cyclic systems, using either 

 MDP or ferricyanide as electron acceptors, scattering responses 

 induced "by actinic light are completely abolished by lO'^M 

 5-(3,i^dichlorophenyl)-l,l-dimethylurea (DCMU) vhich blocks the 

 first light reaction. However the ability to manifest scattering 

 changes can be restored by adding ascorbate and the dye 2,6 di- 

 chlorophenol indophenol vhich reinstitutes electron flow and 

 phosphorylation by passing the DCMU block. Scattering changes 

 under conditions of cvclic or non- cyclic photophosphorylation are 

 abolished by 1 mM Ml^^or by one of a number of uncoupling agents 

 such as m-chloro-carbonyl cyanide phenylhydrazone (CCP). The mag- 

 nitude of the scattering response also varies with the intensity 

 of the actinic light, although a careful study of this relation- 

 ship for several non- cyclic and cyclic systems which support this 

 phenomenon has not yet been made. In brief, all of the kno^ra 

 effects of actinic light, electron carriers, phosphate acceptors, 

 and inhibitor substances in the chloroplast system seem to promote 

 or prevent scattering responses in an exactly predictable manner. 



While these studies establish that light- scattering increases 

 in chloroplasts occur under conditions of photophosphorylation, 

 the puzzling observation was made that these structural changes 

 were lost in some cases even when chloroplasts were incubated with 

 the cofactors for photophosphorylation. These "aged" chloroplasts 

 developed a requirement for ATP to manifest scattering changes. 

 Figure 2 demonstrates this ATP requirement. Illumination of 

 chloroplasts under conditions for cyclic photophosphorylation led 

 to an increase in scattering of 2/o upon illumination with red 

 light. This small scattering increase was reversed after extin- ^ 

 guishing actinic light. The addition of 3.5 mM ATP in the "dark" 

 did not change the scattering level. However, when the red light 

 was restored, a rapid and extensive increase in scattering ensued, 

 reaching a steady state at a level 85/0 higher than the initial 

 scattering intensity. This increased scattering state could be 

 fully reversed by extinguishing the actinic light. A second light 

 and dark period led to a similar cycle of scattering increase and 

 decrease. The large reversible scattering responses, characteris- 

 tic of "fresh" chloroplasts, had been restored by ATP. A similar 

 restoration of the response can be obtained with ITP. 



This remarkable restoration of the scattering response induced 

 by red light in the presence of ATP suggested that this action of 

 ATP might bear some relation to the existence of a light- induced 

 ATPase in chloroplasts reported by Avron (16) and Petrack and Lip- 

 raann (l?). Petrack and Lipmann demonstrated that light-induced 

 ATPase of spinach chloroplasts was maximally activated in the 



