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First published online September 5, 2008; 10.1105/tpc.108.200910

The Plant Cell 20:2281

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IN BRIEF

Conservation and Redundancy of Serine Acetyltransferases

Nancy R. Hofmann

Science Editor

nhofmann{at}aspb.org

In the final stages of inorganic sulfur assimilation, plants fix sulfur into Cys, the major intermediate for the formation of sulfur-containing compounds in the cell (reviewed in Saito, 2004Go). Two enzymes catalyze the last steps of Cys formation, Ser acetyltransferase (SERAT) and OAS (thiol) lyase (OASTL; also called Cys synthase). Both SERAT and OASTL are found in multiple isoforms and are present in each compartment of the cell in which protein synthesis takes place. 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., 2008Go; Watanabe et al., 2008aGo).

Watanabe et al. (2008bGo; pp. 2484–2496) have now performed a comprehensive analysis of the Arabidopsis SERAT family with results that neatly complement those recent reports and further strengthen the notion that Cys can cross cellular membranes. First, phylogenetic analysis shows that each species analyzed, including both monocots and dicots, contains SERAT proteins from all three compartments (cytosol, plastid, and mitochondrion), emphasizing the importance of all three isoforms. In Arabidopsis, the plastid and mitochondrial isoforms each are encoded by one gene, while three other genes encode cytosolic forms. Watanabe et al. went on to analyze knockout mutants in all five SERAT genes.

Interestingly, no phenotype is seen in any of the single knockout mutants, even in the plants entirely lacking SERAT protein in mitochondria or plastids. In fact, quadruple mutant plants are also viable, though some are stunted. Furthermore, the quadruple mutant analysis indicates that the presence of any one of the SERAT isoforms is sufficient for survival, suggesting that Cys formed in any compartment can be used elsewhere in the cell. The quintuple mutant was found to be embryo lethal (see figure ), demonstrating that the SERAT proteins are required and that no other enzymes carry out SERAT activity in their absence.


Figure 1
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  		The quintuple SERAT mutant is embryo lethal. The panels show a wild-type silique (top) and a silique from progeny of a self-fertilized heterozygous quadruple mutant, in an attempt to produce quintuple SERAT knockout mutants (bottom). No viable quintuple mutants were obtained, and the ratio of viable:nonviable seed was ~3:1, suggesting that the quintuple mutant is embryo lethal.

 
The authors then characterized the contribution of each isoform to total SERAT activity in the plant and found that, in most tissues, the mitochondrial form is responsible for the bulk of activity. This supports the conclusion of Heeg et al. (2008)Go 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)Go 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. Understanding why this is so may lead to a better understanding of the regulation of sulfur metabolism in plants.

Footnotes

www.plantcell.org/cgi/doi/10.1105/tpc.108.200910

REFERENCES

Heeg, C., Kruse, C., Jost, R., Gutensohn, M., Ruppert, T., Wirtz, M., and Hell, R. (2008). Analysis of the Arabidopsis O-acetylserine(thiol)lyase gene family demonstrates compartment-specific differences in the regulation of cysteine synthesis. Plant Cell 20: 168–185.[Abstract/Free Full Text]

Saito, K. (2004). Sulfur assimilatory metabolism. The long and smelling road. Plant Physiol. 136: 2443–2450.[Free Full Text]

Watanabe, M., Kusano, M., Oikawa, A., Fukushima, A., Noji, M., and Saito, K. (2008a). Physiological roles of the β-substituted alanine synthase gene family in Arabidopsis. Plant Physiol. 146: 310–320.[Abstract/Free Full Text]

Watanabe, M., Mochida, K., Kato, T., Tabata, S., Yoshimoto, N., Noji, M., and Saito, K. (2008b). Comparative genomics and reverse genetics analysis reveal indispensable functions of the serine acetyltransferase gene family in Arabidopsis. Plant Cell 20: 2484–2496.[Abstract/Free Full Text]


Related articles in Plant Cell:

Comparative Genomics and Reverse Genetics Analysis Reveal Indispensable Functions of the Serine Acetyltransferase Gene Family in Arabidopsis
Mutsumi Watanabe, Keiichi Mochida, Tomohiko Kato, Satoshi Tabata, Naoko Yoshimoto, Masaaki Noji, and Kazuki Saito
Plant Cell 2008 20: 2484-2496. [Abstract] [Full Text]  




This Article
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