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 β-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 β-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. β-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.