(Microbial infection) In addition to its role in cysteine synthesis, stimulates the tRNase activity of CdiA-CT from E.coli strain 536 / UPEC; stimulation does not require O-acetylserine sulfhydrylase activity. CdiA is the toxic component of a toxin-immunity protein module, which functions as a cellular contact-dependent growth inhibition (CDI) system. CDI modules allow bacteria to communicate with and inhibit the growth of closely related neighboring bacteria in a contact-dependent fashion (experiments done in strains BW25113 and X90, both K12 derivatives). This protein is not required for CDI of strain EC93, whose toxin may function by forming inner cell membrane pores (PubMed:22333533). CysK stabilizes CdiA-CT, allowing it to bind tRNA substrate; neither CdiA-CT nor CysK bind tRNA alone in vitro (PubMed:27531961). hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine 3-chloro-L-alanine + thioglycolate = chloride + H(+) + S-carboxymethyl-L-cysteine Optimum pH is 9-10.5. Optimum temperature is 50 degrees Celsius. Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer (Ref.6, PubMed:27531961). Forms a cysteine synthase complex with 1 copy of CysE (By similarity). (Microbial infection) Interacts with CdiA-CT from strain 536 / UPEC, this is blocked upon preincubation with O-acetyl-L-serine. CysK forms a complex with CdiA-CT/CdiI (PubMed:22333533). One CdiA toxin subunit binds to each subunit of the CysK homodimer, and one CdiI immunity protein binds to each toxin subunit; the immune complex is thus a dimer of trimers. The 4 C-terminal residues of CdiA fit into the active site of CysK (PubMed:27531961). Repressed by sulfate or cysteine. Significant decrease in tRNA nuclease activity of contact-dependent growth inhibitor CdiA-CT from strain 536 / UPEC (deletion in strain BW25113). Belongs to the cysteine synthase/cystathionine beta-synthase family.