Published Research - Department of Chemistry


Date of this Version



Angew Chem Int Ed Engl. 2014 July 14; 53(29): 7524–7530.


Copyright (c) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


HSAF (1) was isolated from the biocontrol agent Lysobacter enzymogenes (Figure 1).[1-4]

This bacterial metabolite belongs to polycyclic tetramate macrolactams (PTM) that are

emerging as a new class of natural products with distinct structural features. [5, 6] HSAF

exhibits a potent antifungal activity and shows a novel mode of action.[1-4] The HSAF

biosynthetic gene cluster contains only a single-module hybrid polyketide synthasenonribosomal

peptide synthetase (PKS-NRPS), although the PTM scaffold is apparently

derived from two separate hexaketide chains and an ornithine residue.[1-4] This suggests that

the same PKS module would act not only iteratively, but also separately, in order to link the

two hexaketide chains with the NRPS-activated ornithine to form the characteristic PTM

scaffold. Recently, the Gulder group reported heterologous expression of the ikarugamycin

(4) biosynthetic gene cluster in E. coli,[7] and the Zhang group reported the enzymatic

mechanism for formation of the inner 5-memebered ring and demonstrated the polyketide

origin of the ikarugamycin skeleton.[8] Ikarugamycin is a Streptomyces-derived PTM which

has a 5,6,5-tricyclic system (Figure 1). Both the Gulder and Zhang groups showed that a

three-gene cluster is sufficient for ikarugamycin biosynthesis. Despite the progress, this

iterative polyketide biosynthetic mechanism had not been demonstrated using purified PKS

and NRPS. In addition, HSAF has a 5,5,6-tricyclic system, and its gene cluster contains at

least six genes.[3] Finally, unlike most PTM compounds, HSAF is produced by a Gramnegative

bacterium, L. enzymogenes. Here, we report the heterologous production of HSAF

analogs in Gram-positive Streptomyces hosts, in which the native PKS have been deleted.

We also obtained evidence for the formation of the polyene tetramate intermediate in

Streptomyces when only the single-module hybrid PKS-NRPS gene was expressed. Finally,

we showed the in vitro production of the polyene tetramate using the individually purified

PKS and NRPS. The results provide direct evidence for this iterative polyketide biosynthetic

mechanism that is likely general for the PTM-type hybrid polyketide-peptides.