acetyl-CoA + L-serine = CoA + O-acetyl-L-serine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 1/2. Transcriptionally induced in the absence of sulfur, requires CysR for transcription. Part of the srpG-srpH-srpI operon. Belongs to the transferase hexapeptide repeat family.