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Plant Cell Advance Online Publication
Published on January 25, 2008; 10.1105/tpc.107.056747

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Received November 9, 2007
Returned for revision December 21, 2007
Accepted January 10, 2008

Analysis of the Arabidopsis O-Acetylserine(thiol)lyase Gene Family Demonstrates Compartment-Specific Differences in the Regulation of Cysteine Synthesis

Corinna Heeg 1, Cordula Kruse 1, Ricarda Jost 2, Michael Gutensohn 3, Thomas Ruppert 4, Markus Wirtz 1, and Rüdiger Hell 1*

1 Heidelberg Institute of Plant Sciences, University of Heidelberg, 69120 Heidelberg, Germany
2 Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
3 Institute for Plant Physiology, Martin-Luther-University Halle-Wittenberg, 06120 Halle, Germany
4 Center for Molecular Biology, University of Heidelberg, 69120 Heidelberg, Germany

* To whom correspondence should be addressed. E-mail: rhell{at}hip.uni-heidelberg.de.

Cys synthesis in plants takes place in plastids, cytosol, and mitochondria. Why Cys synthesis is required in all compartments with autonomous protein biosynthesis and whether Cys is exchanged between them has remained enigmatic. This question was addressed using Arabidopsis thaliana T-DNA insertion lines deficient in the final step of Cys biosynthesis catalyzed by the enzyme O-acetylserine(thiol)lyase (OAS-TL). Null alleles of oastlA or oastlB alone showed that cytosolic OAS-TL A and plastid OAS-TL B were completely dispensable, although together they contributed 95% of total OAS-TL activity. An oastlAB double mutant, relying solely on mitochondrial OAS-TL C for Cys synthesis, showed 25% growth retardation. Although OAS-TL C alone was sufficient for full development, oastlC plants also showed retarded growth. Targeted affinity purification identified the major OAS-TL–like proteins. Two-dimensional gel electrophoresis and mass spectrometry showed no compensatory changes of OAS-TL isoforms in the four mutants. Steady state concentrations of Cys and glutathione and pulse-chase labeling with [35S]sulfate indicated strong perturbation of primary sulfur metabolism. These data demonstrate that Cys and also sulfide must be sufficiently exchangeable between cytosol and organelles. Despite partial redundancy, the mitochondria and not the plastids play the most important role for Cys synthesis in Arabidopsis.






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