The structure of rhizonin, in particular the presence of nonproteinogenic amino acids, suggests that its biosynthesis is probably carried out by a nonribosomal peptide synthetase (NRPS). Fungal NRPSs are involved in biosynthesis of penicillin [27] and cyclopeptides such as the immunosuppressive cyclosporin 28, 29 and 30. NRPSs typically consist of giant multidomain enzymes with a modular architecture. A minimal set of adenylation (A) and condensation (C) domains, and a peptidyl carrier protein is present in each module. The A and C domains are highly conserved and allow the design of specific degenerate primers for targeting NRPS gene fragments by PCR 31 and 32. In analogy to previous findings regarding rhizoxin biosynthesis, fungal NRPS genes could not be detected by PCR in DNA extracted from R. microsporus, whereas fragments of bacterial NRPS genes were amplified [21].
The presence of endobacteria in the rhizonin-positive Rhizopus strain was unequivocally proven by transmission electron microscopy and the amplification of bacterial 16S rRNA gene sequences. The rod-shaped bacterial symbionts within the mycelium could be isolated and grown in pure culture. It turned out that the rhizonin producer is a close relative of B. rhizoxinica and is also capable of producing rhizoxin. Despite high similarity on the 16S rRNA sequence level, genome–genome hybridization experiments suggest defining a new species named Burkholderia endofungorum
The structure of rhizonin, in particular the presence of nonproteinogenic amino acids, suggests that its biosynthesis is probably carried out by a nonribosomal peptide synthetase (NRPS). Fungal NRPSs are involved in biosynthesis of penicillin [27] and cyclopeptides such as the immunosuppressive cyclosporin 28, 29 and 30. NRPSs typically consist of giant multidomain enzymes with a modular architecture. A minimal set of adenylation (A) and condensation (C) domains, and a peptidyl carrier protein is present in each module. The A and C domains are highly conserved and allow the design of specific degenerate primers for targeting NRPS gene fragments by PCR 31 and 32. In analogy to previous findings regarding rhizoxin biosynthesis, fungal NRPS genes could not be detected by PCR in DNA extracted from R. microsporus, whereas fragments of bacterial NRPS genes were amplified [21].The presence of endobacteria in the rhizonin-positive Rhizopus strain was unequivocally proven by transmission electron microscopy and the amplification of bacterial 16S rRNA gene sequences. The rod-shaped bacterial symbionts within the mycelium could be isolated and grown in pure culture. It turned out that the rhizonin producer is a close relative of B. rhizoxinica and is also capable of producing rhizoxin. Despite high similarity on the 16S rRNA sequence level, genome–genome hybridization experiments suggest defining a new species named Burkholderia endofungorum
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