Transfers the 4'-phosphopantetheine moiety from coenzyme A to a Ser of acyl-carrier-protein (PubMed:16709676, PubMed:21697604). Involved in the post-translational modification of Fas-I and the AcpM subunit of Fas-II (PubMed:21697604). apo-[ACP] + CoA = adenosine 3',5'-bisphosphate + H(+) + holo-[ACP] Binds 2 Mg(2+) per subunit. Optimum pH is between 4.4 and 6.0. Homotrimer. Apo-AcpS adopts different conformations depending upon the pH conditions of the crystallization solution. Was identified as a high-confidence drug target. Belongs to the P-Pant transferase superfamily. AcpS family.