Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: 9/1/1997
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Sucrose phosphate synthase plays a major regulating role in the sucrose biosynthetic pathway. The enzyme is thought to be, under circadian regulation, largely dictated by the protein's phosphorylation state. Previous work has illustrated a shift in the circadian rhythm of cold sensitive plants, such as tomato, when subjected to low temperatures. The rhythm halts until rewarming occurs. In effect, it becomes mistimed relative to its environment, thus imparing photosynthetic performance. Inhibitor experiments show that okadaic acid, cycloheximide and cordycepin (phosphatase, translation and transcription inhibitors, respectively) prevent activation of SPS but did not otherwise inhibit the enzyme in the circadian cycle. This is indicative of the SPS protein phosphatase exhibiting a circadian rhythm, thereby altering the phosphorylation state and activation of SPS. To clarify the role SPS phosphatase plays in driving the circadian rhythm of SPS activity, and in explaining the chilling induced mistiming of the rhythm, an in vitro system can be established using a small segment of the much larger SPS protein. Casein kinase II was used to phosphorylate the SPS fragment in vitro. To confirm that the proposed active site serine of SPS was phosphorylated in this natural SPS phosphatase will be added to the system to isolate the effects of changes in protein phosphatase levels on the phosphorylation state of the active sites of SPS. This system should effectively distinguish phosphatase and induced changes from other factors affecting the activation of SPS, without the use of inhibitors.