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

Abstract

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 [ 35 S]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.

Conservation and Redundancy of Serine Acetyltransferases

“…This supports the conclusion of Heeg et al (2008) based on analysis of Arabidopsis OASTL genes that the mitochondria, and not plastids, are the most important site of Cys synthesis. The work of Watanabe et al (2008b) also shows that, although the various isoforms of SERAT are functionally redundant, they are highly conserved across plant species, including the conservation of exon-intron structures.…”

“…This has been hypothesized to indicate that Cys exchange between compartments is limited, necessitating the production of Cys at the point of its use. Recent analysis of OASTL mutants has cast doubt on this view, however, suggesting that Cys must be transported between cellular compartments (Heeg et al, 2008;Watanabe et al, 2008a). Watanabe et al (2008b;pp.…”

Mitochondrial Cysteine Synthase Complex Regulates O-Acetylserine Biosynthesis in Plants

“…In Arabidopsis leaves, ϳ90% of OAS-TL activity is provided by OAS-TL A in the cytosol and OAS-TL B in chloroplasts. The remaining activity comes from mitochondrial OAS-TL C (10,12). In contrast, ϳ80% of total SAT activity is found to originate from the mitochondrial isoform SAT3 (serat2;2) (11,13).…”

Dissection of the Control of Anion Homeostasis by Associative Transcriptomics in Brassica napus

“…Two genes with a direct relevance to sulfur metabolism were identified in the sulfate data set. Cys synthase C catalyzes the last step of primary sulfate assimilation, the synthesis of Cys (Heeg et al, 2008;Takahashi et al, 2011). The At3g48990 gene is annotated as functioning in response to cadmium, where sulfur compounds, such as phytochelatins, play a very important role (Sarry et al, 2006).…”