The rationale for this work is based on the fact that all photosynthetic organisms must adapt to changes in environment light intensity in order to maximize photosynthetic efficiency. At high light, excess energy has to be dissipated to prevent photodamage, while at low light, photon capture has be maximized to allow photosynthesis to proceed efficiently. We have been studying the photosynthetic prokaryote, Prochlorothrix hollandica, as a model system to examine the light/shade adaptation of the photosynthetic apparatus. This bacterium is well-suited to these studies because it is structurally similar to higher plant chloroplasts, but can be manipulated in the lab more easily that higher plants or eukaryotic algae. Our work has characterized the events occurring during light/shade adaptation in both P. hollandica and plant chloroplasts, and we have shown that similar mechanisms occur in both systems. Overall, we believe that analyzing this process in P. hollandica will reveal in more detail the pathway required for plants to handle alterations in environmental light intensity. %%% Specifically, our work has shown that light/shade adaptation involves in part the interplay of two separate events. First, upon a shift to high light, the overall chlorophyll pigment system becomes reorganized in approximately 30 minutes; we calculate 40% of the accessory chlorophyll pigments are capable of changing their orientation such that energy capture and transfer is altered at high light. This mechanism appears to be controlled by a light-activated protein kinase. Second, as cells experience high light for several hours, a light-activated metalloprotease activity degrades a subset of the accessory chlorophyll-binding proteins. The function of this protease is probably to limit the amount of photon capture at high irradiances. Understanding precisely the mechanism(s) by which cells sense high light and trigger these responses will help us understand how photosynthesis is regulated in chloroplast (plant) systems. To date, we have cloned many of the genes encoding the components involved in these mechanisms; these include the genes encoding the protein kinase. We are currently examining how light affects the expression of these genes, and how the function of the kinase affects the function and organization of the photosynthetic light-harvesting antenna. Lastly, we are examining in detail how the proteolytic activity acts to destroy the accessory chlorophyll-proteins in high light; it is possible that the accessory chlorophyll-proteins are targeted for proteolysis by their phosphorylation by the thylakoid protein kinase.
