Catalyzes the acetylation of serine by acetyl-CoA to produce O-acetylserine (OAS). acetyl-CoA + L-serine = CoA + O-acetyl-L-serine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 1/2. Strains lacking this gene are shown to be attenuated in a mouse tuberculosis model. Was identified as a high-confidence drug target. Belongs to the transferase hexapeptide repeat family.