Cysteine synthesis and cysteine desulfuration in Arabidopsis plants at different developmental stages and light conditions

The last step in cysteine biosynthesis is catalysed by the pyridoxal 5'-phosphate-dependent enzyme O-acetyl-L-serine(thiol)lyase (OAS-TL). Several isoforms are localized in different compartments of the cell. OAS-TL and OAS-TL-like proteins also catalyse the formation of beta -cyanoalanine and sulfide; the release of sulfide and of an unknown product was also observed. In Arabidopsis plants of different age, the OAS-TL activity decreased with increasing age whereas the enzymatical sulfide release from cysteine increased in older plants. The beta -cyanoalanine synthase showed two activity peaks during the time course investigated. During a 12-h light/12-h dark rhythm, the OAS-TL and the sulfide-releasing activities ran parallel with a maximum in the light phase and a decrease of enzyme activity in the dark phase. beta -Cyanoalanine synthase activities did not follow a special pattern. Three genes coding for the OAS-TL isoforms A, B, and C, which are presumably localized in the cytoplasm, in plastids, and in mitochondria, respectively, were differentially expressed. The mRNA steady-state levels of the three proteins varied at different developmental stages. The results are discussed with respect to the physiological relevance in the plant organism. (C) 2001 Editions scientifiques et medicales Elsevier SAS.