Involved in the biosynthesis of A factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone), a gamma-butyrolactone autoregulator that triggers secondary metabolism and morphogenesis in Streptomyces (By similarity). Catalyzes beta-ketoacyl transfer from 8-methyl-3-oxononanoyl-acyl carrier protein (ACP) to the hydroxyl group of dihydroxyacetone phosphate (DHAP), thus producing an 8-methyl-3-oxononanoyl-DHAP ester (By similarity). a medium-chain 3-oxoacyl-[ACP] + dihydroxyacetone phosphate = a 2-oxo-3-(phosphooxy)propyl medium-chain 3-oxoalkanoate + holo-[ACP] Belongs to the AfsA family.