Catalyzes the formation of cysteine and acetate from O-acetylserine and hydrogen sulfide (PubMed:24100226, PubMed:22405203). By metabolizing hydrogen sulfide produced by cysl-2-mediated cyanide assimilation, mediates resistance to P.aeruginosa infection (PubMed:21840852). Mediates survival in high levels of hydrogen sulfide (PubMed:22405203). By sequestering egl-9, which in turn promotes hif-1-mediated transcription, regulates behavioral responses to hypoxia (PubMed:22405203). hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer (PubMed:24100226). Interacts with egl-9 (via C-terminus); the interaction is enhanced by hydrogen disulfide and activates hif-1-mediated transcription (PubMed:22405203). Expressed in AVM sensory neuron, BDU interneurons, pharyngeal I1 interneurons and M2 motor neurons. Belongs to the cysteine synthase/cystathionine beta-synthase family.