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Published on March 23, 2007; 10.1104/pp.106.095208

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Received January 2, 2007
Accepted March 11, 2007

Synergistic Substrate Inhibition of Ent-copalyl Diphosphate Synthase: A Potential Feed-forward Inhibition Mechanism Limiting Gibberellin Metabolism

Sladjana Prisic and Reuben J. Peters *

Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, IA 50011, USA

* Corresponding author; email: rjpeters{at}iastate.edu.

Gibberellins are ubiquitous diterpenoid phytohormones required for many aspects of plant growth and development, including repression of photosynthetic pigment production (i.e. de-etiolation) in the absence of light. The committed step in gibberellin biosynthesis is catalyzed in plastids by ent-copalyl diphosphate synthase (CPS), whose substrate, geranylgeranyl diphosphate (GGPP), is also a direct precursor of carotenoids and the phytol side chain of chlorophyll. Accordingly, during de-etiolation gibberellin production is repressed while flux towards these photosynthetic pigments through their common GGPP precursor is dramatically increased. How this is accomplished has been unclear, as no mechanism for regulation of CPS activity has been reported. We present here kinetic analysis of recombinant pseudo-mature CPS from Arabidopsis thaliana (rAtCPS) demonstrating that Mg2+ and GGPP exert synergistic substrate inhibition effects on CPS activity. These results suggest that gibberellin metabolism may be limited by feed-forward inhibition of CPS. In particular, the effect of Mg2+, as light induces increases in plastid Mg2+ levels over a similar range as that observed here to affect rAtCPS activity. Notably, this effect is most pronounced in the gibberellin specific AtCPS, as the corresponding activity of the resin acid biosynthetic enzyme abietadiene synthase is 100-fold less sensitive to [Mg2+]. Furthermore, Mg2+ allosterically activates the plant porphobilinogen synthase involved in chlorophyll production. Hence, Mg2+ may have a broad role in regulating plastidial metabolic flux during de-etiolation. Finally, the observed synergistic substrate/feed-forward inhibition of CPS also seems to provide the first example of direct regulation of enzymatic activity in hormone biosynthesis from any organism.




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F. M. Mann, S. Prisic, H. Hu, M. Xu, R. M. Coates, and R. J. Peters
Characterization and Inhibition of a Class II Diterpene Cyclase from Mycobacterium tuberculosis: IMPLICATIONS FOR TUBERCULOSIS
J. Biol. Chem., August 28, 2009; 284(35): 23574 - 23579.
[Abstract] [Full Text] [PDF]


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