Carrier of the growing fatty acid chain in fatty acid biosynthesis. Lipid metabolism; fatty acid biosynthesis. Expression requires either cleavage of the mRNA to liberate the ribosome binding site (RBS), or rapid translation before the hairpin in its 5' regulatory region can form. RNase 3 (rnc) is probably cleaves the RBS, while RNases J1/J2 (rnj1, rnj2) are involved in mRNA degradation. 4'-phosphopantetheine is transferred from CoA to a specific serine of apo-ACP by AcpS. This modification is essential for activity because fatty acids are bound in thioester linkage to the sulfhydryl of the prosthetic group. Belongs to the acyl carrier protein (ACP) family.