Identification of the incednine biosynthetic gene cluster: characterization of novel β-glutamate-β-decarboxylase IdnL3

A biosynthetic gene cluster for the 24-membered macrolactam antibiotic incednine was identified from the producer strain, Streptomyces sp. ML694-90F3. Among the putative incednine biosynthetic enzymes, a novel pyridoxal 5′-phosphate (PLP)-dependent β-glutamate-β-decarboxylase, IdnL3, was functionally characterized in vitro by demonstrating its (S)-3-aminobutyrate-forming activity with β-glutamate in the presence of PLP. Because (S)-3-aminobutyrate is known for the direct precursor of incednine, this enzyme supplies the unique β-amino acid starter unit. The identified gene cluster encodes five characteristic β-amino acid carrying enzymes, consisting of a pathway-specific ATP-dependent ligase, a discrete acyl carrier protein (ACP), β-aminoacyl-ACP β-amino group-protecting ATP-dependent ligase, dipeptidyl-ACP:PKS-loading ACP dipeptidyltransferase and a terminal amino acid peptidase, which are completely conserved in β-amino acid-containing macrolactam biosynthetic gene clusters. Overall, a plausible biosynthetic pathway for incednine was proposed.