Catalyzes the conversion of O-acetylserine to cysteine. Also acts as a sensor of cysteine availability in the signal transduction pathway modulating CymR activity. When cysteine is present, the pool of O-acetylserine (OAS) is low, which leads to the formation of a CymR-CysK complex and transcriptional repression of the CymR regulon occurs. In the absence of cysteine, the OAS pool is high and the CymR-CysK complex is mostly dissociated, leading to a faster dissociation of CymR from its DNA targets and the lifting of CymR-dependent repression. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer. Forms CymR(2):CysK(2) or CymR(4):CysK(4) complexes in the absence of O-acetylserine. Highly expressed in the presence of methionine, but poorly expressed in the presence of cystine. Also induced by superoxide. Repressed both by sulfate and cysteine. Grows very slowly with sulfate, butanesulfonate or cystine as sole sulfur source. Belongs to the cysteine synthase/cystathionine beta-synthase family.