Biosynthesis of the Putative Siderophore Erythrochelin Requires Unprecedented Crosstalk between Separate Nonribosomal Peptide Gene Clusters

The genome of the erythromycin-producing bacterium Saccharopolyspora erythraea contains many orphan secondary metabolite gene clusters including two (nrps3 and nrps5) predicted to govern biosynthesis of nonribosomal peptide-based siderophores. We report here the production by S. erythraea, even under iron-sufficient conditions, of a 2,5-diketopiperazine siderophore candidate we have named erythrochelin. Deletion of the nonribosomal peptide synthetase (NRPS) gene ercD within the nrps5 cluster abolished erythrochelin production. The tetrapeptide backbone of erythrochelin (alpha-N-acetyl-delta-N-acetyl-delta-N-hydroxyornithine-serine-delta-N-hydroxyornithine-delta-N-acetyl-delta- N-hydroxyornithine) suggests an orthodox colinear model for erythrochelin assembly. Curiously, the delta-N-acetyltransferase required for erythrochelin biosynthesis is encoded within a remote NRPS-cluster (nrps1) whose own NRPS contains an inactivating mutation. Disruption of the nrps1 gene mcd abolished erythrochelin biosynthesis, which could then be restored by addition of synthetic L-delta-N-acetyl-delta-N-hydroxyornithine, confirming an unprecedented example of functional crosstalk between nrps clusters.