Primarily catalyzes the formation of cyanoalanine and hydrogen sulfide from cysteine and hydrogen cyanide. Can also catalyze, although less efficiently, the formation of cyanoalanine and hydrogen sulfide from either S-sulfocysteine or O-acetylserine and hydrogen cyanide and the formation of cysteine from either S-sulfocysteine or O-acetylserine and hydrogen sulfide (PubMed:24100226). By catalyzing the assimilation of cyanide produced by P.aeruginosa, mediates resistance to infection (PubMed:21840852). Involved in fertility, growth and aging (PubMed:24260346). Does not mediate survival in high levels of hydrogen sulfide (PubMed:21840852). hydrogen cyanide + L-cysteine = 3-cyano-L-alanine + H(+) + hydrogen sulfide hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Homodimer. By hydrogen sulfide or cyanide (PubMed:21840852). By morpholin-4-ium 4-methoxyphenyl (morpholino) phosphinodithioate (GYY4137) (PubMed:24260346). Belongs to the cysteine synthase/cystathionine beta-synthase family.