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Andrimid (Adm) synthase, which belongs to the type-II system of enzymes, produces Adm in Pantoea agglomerans. The adm biosynthetic gene cluster lacks canonical acyltransferases (ATs) to load the malonyl group to acyl carrier proteins (ACPs), suggesting that a malonyl-CoA: ACP transacylase (MCAT) from the producer organism's fatty acid synthase (FAS) complex provides the essential acyltransferase (AT) activity in Adm biosynthesis. Here we report that the MCAT is essential for catalysis of the transacylation of malonate from malonyl-CoA to AdmA polyketide synthase (PKS) ACP in vitro. Furthermore, a catalytic self-malonylation of the AdmA PKS ACP was not observed in reactions without MCAT, while many type II PKS ACPs are capable of catalyzing the self-acylation process. This lack of self-malonylation was validated by amino acid sequence analysis of the AdmA PKS ACP and the type II PKS ACPs. These results show that the MCAT from the producer organism's FAS complex provides the missing AT activity in trans, suggesting a functional protein-protein interaction between the fatty acid and polyketide synthases in the Adm assembly line.