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Putative protein sequences of the ORFs 5 and 6 are 3164 and 3021 amino acids in length, respectively. Both proteins shared significant identities to two NRPSs produced by a gene cluster in the genome of strain AMMD. ORF5 was predicted to have two NRPS catalytic modules for asparagine and serine, followed by a thioesterase domain at the C terminus (a.a. 2892–3159; E score: 2.3e-23). ORF6 was predicted to have two catalytic modules of NRPS, however, no amino BKM120 matches from the database were found. These results suggest that the two NRPS genes contribute to catalyzation of four amino acid residues of an unknown oligopeptide. Sequence analysis revealed that the transposons in the genomes of mutants MS14MT13 and MS14MT18 were located between nucleotides 7336 and 7337 of ORF5 and 4648–4649 of ORF6, respectively, relative to their translational start codons. These mutations resulted in a complete loss of antifungal activity.
NRPS genes are widely distributed in the bacterial genomes and frequently clustered together with transcriptional regulator genes and transporter genes [15] and [20]. The NRPS contribute to biosynthesis of oligopeptides or oligopeptide-based metabolites including various antibiotics [21]. The findings of this study suggest that the antifungal compound of MS14 is an oligopeptide synthesized via a nonribosomal peptide synthetase mechanism.