An acyltransferase domain of FK506 polyketide synthase recognizing both an acyl carrier protein and coenzyme A as acyl donors to transfer allylmalonyl and ethylmalonyl units.

Acyltransferase (AT) domains of polyketide synthases (PKSs) usually use coenzymeA (CoA) as an acyl donor to transfer common acyl units to acyl carrier protein (ACP) domains, initiating incorporation of acyl units into polyketides. Two clinical immunosuppressive agents, FK506 and FK520, are biosynthesized by the same PKSs in several Streptomyces strains. In this study, characterization of AT4(FkbB) (the AT domain of the fourth module of FK506 PKS) in transacylation reactions showed that AT4(FkbB) recognizes both an ACP domain (ACP(TcsA)) and CoA as acyl donors for transfer of a unique allylmalonyl (AM) unit to an acyl acceptor ACP domain (ACP4(FkbB)), resulting in FK506 production. In addition, AT4(FkbB) uses CoA as an acyl donor to transfer an unusual ethylmalonyl (EM) unit to ACP4(FkbB), resulting in FK520 production, and transfers AM units to non-native ACP acceptors. Characterization of AT4(FkbB) in self-acylation reactions suggests that AT4(FkbB) controls acyl unit specificity in transacylation reactions but not in self-acylation reactions. Generally, AT domains of PKSs only recognize one acyl donor; however, we report here that AT4(FkbB) recognizes two acyl donors for the transfer of different acyl units. DatabaseNucleotide sequence data have been submitted to the GenBank database under accession numbers and .