An O-acetylserine(thiol)lyase homolog regulates cysteine homeostasis in Arabidopsis thaliana

Cysteine occupies a central position in plant metabolism due to its biochemical functions. Arabidopsis thaliana cells contain different O-acetylserine(thiol)lyase (OASTL) enzymes that catalyze the biosynthesis of cysteine. Because they are localized in the cytosol, plastids and mitochondria, this results in multiple subcellular cysteine pools. Much progress has been made on the most abundant OASTL enzymes; however, information on the less abundant OASTL-like proteins has been scarce. To unequivocally establish the enzymatic reaction catalyzed by the minor cytosolic OASTL isoform CS-LIKE (AT5G28030), we expressed this enzyme in bacteria and characterized the purified recombinant protein. Our results demonstrate that CS-LIKE catalyzes the desulfuration of L-cysteine to sulfide plus ammonia and pyruvate. Thus, CS-LIKE is a novel L-cysteine desulfhydrase (EC 4.4.1.1), and we propose to designate it DES1. The impact and functionality of DES1 in cysteine metabolism was revealed by the phenotype of the T-DNA insertion mutants des1-1 and des1-2. Mutation of the DES1 gene leads to premature leaf senescence, as demonstrated by the increased expression of senescence-associated genes and transcription factors. Also, the absence of DES1 significantly reduces the total cysteine desulfuration activity in leaves, and there is a concomitant increase in the total cysteine content. As a consequence, the expression levels of sulfur-responsive genes are de-regulated, and the mutant plants show enhanced antioxidant defenses and tolerance to conditions that promote oxidative stress. Our results suggest that DES1 from Arabidopsis is an L-cysteine desulfhydrase involved in maintaining cysteine homeostasis, mainly at late developmental stages or under environmental perturbations.

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Álvarez C, Bermúdez MÁ, Romero LC, Gotor C, García I