Identification and Analysis of the Biosynthetic Gene Cluster for the Indolizidine Alkaloid Iminimycin in Streptomyces griseus

Abstract

Indolizidine alkaloids, which have versatile bioactivities, are produced by various organisms. Although the biosynthesis of some indolizidine alkaloids has been studied, the enzymatic machinery for their biosynthesis in Streptomyces remains elusive. Here, we report the identification and analysis of the biosynthetic gene cluster for iminimycin, an indolizidine alkaloid with a 6‐5‐3 tricyclic system containing an iminium cation from Streptomyces griseus. The gene cluster has 22 genes, including four genes encoding polyketide synthases (PKSs), which consist of eight modules in total. In vitro analysis of the first module revealed that its acyltransferase domain selects malonyl‐CoA, although predicted to select methylmalonyl‐CoA. Inactivation of seven tailoring enzyme‐encoding genes and structural elucidation of four compounds accumulated in mutants provided important insights into iminimycin biosynthesis, although some of these compounds appeared to be shunt products. This study expands our knowledge of the biosynthetic machinery of indolizidine alkaloids and the enzymatic chemistry of PKS.

Structure-Based Analysis of Transient Interactions between Ketosynthase-like Decarboxylase and Acyl Carrier Protein in a Loading Module of Modular Polyketide Synthase

“…Furthermore, comparisons of the amino acid sequences of the KS Q and ACP L domains revealed similarities in the amino acid residues involved in the GfsA KS Q domain–ACP L interactions, suggesting that KS Q domains have a similar recognition mode for ACP L domains to that observed in GfsA. Together with our previously reported functional analysis of KS Q domains, ,, the present study has revealed that the KS Q domain is indeed highly functionally similar to the modular type I PKS KS domain. Our findings are expected to help understand the complicated functions of the KS domain of modular type I PKS, which plays a vital role in polyketide biosynthesis.…”

“…Although several PKS modules have different domain organizations, only the structures of full-length modules consisting of KS−AT−KR−ACP have been determined. 24,25,28,29 As a dynamic property of a KS−AT−KR− ACP module, it is proposed that the dimeric KS−AT−KR− ACP module adopts an asymmetric conformation with the binding of a single ACP domain to the KS homodimer when the KS domain functions (Figure S20B). 25 An artificial state that is crosslinked with the separated ACP L domain may cause a symmetric conformation of the GfsA loading module.…”