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 (PubMed:17277085). 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 (PubMed:17277085). a medium-chain 3-oxoacyl-[ACP] + dihydroxyacetone phosphate = a 2-oxo-3-(phosphooxy)propyl medium-chain 3-oxoalkanoate + holo-[ACP] kcat is 2.0 min(-1). Optimum pH is 6.5-7.5. Optimum temperature is 15 degrees Celsius. Monomer. Belongs to the AfsA family.