Acts as a cysteine synthase. The cysteine synthesis reaction is more efficient than the cyanoalanine synthase activity. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine hydrogen cyanide + L-cysteine = 3-cyano-L-alanine + H(+) + hydrogen sulfide Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Mainly expressed in flowers. No visible phenotype. Belongs to the cysteine synthase/cystathionine beta-synthase family.