Insights into the biosynthesis of icumazole unveiling a distinctive family of crotonyl-CoA carboxylase/reductase

Summary Icumazoles are potent antifungal polyketides with intriguing structural features. Here, we present the polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) hybrid biosynthetic gene cluster of icumazoles. Surprisingly, an unusual non-terminal thioesterase domain divides the PKS/NRPS assembly line. The succeeding PKS modules potentially form a rare precursor 4-methyl-2-hexenoyl-ACP thus deviating from the previously proposed polyoxypeptin pathway. The 4-methyl-2-hexenoyl-ACP is further reductive carboxylated to 2-methylbutylmalonyl-ACP essential for icumazole biosynthesis by IcuL, representing a new type of crotonyl-CoA carboxylase/reductase (CCR). We characterize IcuL and its homologs TgaD and Leu10 in vitro , suggesting a stricter substrate specificity of this new family of CCRs than found in canonical ones. Intriguingly, we also find that TgaD unprecedently utilizes both NADPH and NADH as cofactors with similar efficiency, diverging from the NADPH-specific characteristic of canonical CCRs. Furthermore, a sequence similarity network-based bioinformatic survey reveals that the IcuL-like CCRs are evolutionarily separated from canonical CCRs.