The chejuenolide biosynthetic gene cluster harboring an iterative trans-AT PKS system in Hahella chejuensis strain MB-1084

Hahella chejuensis MB-1084 is a Gram-negative marine bacterial strain that produces unusual 17-membered carbocyclic tetraenes, chejuenolide A and B. Two fosmid clones responsible for chejuenolide production were identified from the genomic DNA library of the MB-1084 strain. Systematic inactivation of the open reading frames (ORFs) in the sequenced region defines the boundaries of the chejuenolide (che) biosynthetic gene cluster (24.9 kbp) that encodes one non-ribosomal peptide synthase (NRPS)-polyketide synthase (PKS) hybrid protein, three modular PKSs, two PKS domains, and an amine oxidase homolog. Based on the results, we found that the che PKSs have non-canonical features such as trans-AT system and insufficient number of KS domains (five KS domains) for chejuenolide production (requires eight rounds of Claisen condensation reaction). Heterologous expression of the che PKSs in the E. coli BAP1 strain provides strong evidence of the iterative characteristic of the modular PKSs. Additionally, the phylogenetic relatedness of the KS domains of che PKSs and other trans-AT PKSs was analyzed to propose a possible pathway for chejuenolide biosynthesis.