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. Homohexamer; dimer of a homotrimer (PubMed:10617639). Forms a cysteine synthase complex with 2 copies of CysK (By similarity). Belongs to the transferase hexapeptide repeat family.