PLANT PHYSIOLOGY , Vol 113, Issue 4 1167-1175, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Circadian Regulation of Sucrose Phosphate Synthase Activity in Tomato by Protein Phosphatase Activity
T. L. Jones and D. R. Ort
Department of Plant Biology (T.L.J., D.R.O.) and Photosynthesis Research Unit, United States Department of Agriculture/Agricultural Research Service (D.R.O.), University of Illinois, Urbana, Illinois 61801-3838
Sucrose phosphate synthase (SPS), a key enzyme in sucrose biosynthesis, is
regulated by protein phosphorylation and shows a circadian pattern of
activity in tomato. SPS is most active in its dephosphorylated state, which
normally coincides with daytime. Applying okadaic acid, a potent protein
phosphatase inhibitor, prevents SPS activation. More interesting is that a
brief treatment with cycloheximide, a cytoplasmic translation inhibitor,
also prevents the light activation of SPS without any effect on the amount
of SPS protein. Cordycepin, an inhibitor of transcript synthesis and
processing, has the same effect. Both of these inhibitors also prevent the
activation phase of the circadian rhythm in SPS activity. Conversely,
cycloheximide and cordycepin do not prevent the decline in circadian SPS
activity that normally occurs at night. These observations indicate that
SPS phosphatase activity but not SPS kinase activity is controlled,
directly or indirectly, at the level of gene expression. Taken together,
these data imply that there is a circadian rhythm controlling the
transcription of a protein phosphatase that subsequently dictates the
circadian rhythm in SPS activity via effects on this enzyme's
phosphorylation state.
