In plants, the activity is catalysed by a chloroplastic enzyme that plays an important role in chloroplast function and is essential for light-dependent redox regulation within the chloroplast. The bacterial enzyme also catalyses the activity of EC 2.5.1.47, cysteine synthase. cf. EC 2.8.5.1, S-sulfo-L-cysteine synthase (3-phospho-L-serine-dependent).