acetyl-CoA + L-serine = CoA + O-acetyl-L-serine Sensitive to feedback inhibition by L-cysteine. Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 1/2. Part of the cysteine synthase complex formed at a ratio of 1 copy of this protein and 2 copies of O-acetylserine sulfhydrylase (cysK). The complex reversibly dissociates in the presence of O-acetyl-L-serine in the absence of hydrogen sulfide. Belongs to the transferase hexapeptide repeat family.