[14-16] Bongkrekic acid is a highly unsaturated tricarboxylic fatty acid, which inhibits oxidative phosphorylation by blocking the mitochondrial adenine nucleotide translocator. Recently, the biosynthesis of the deadly toxin catalysed by an unusual polyketide synthase (PKS) was elucidated allowing for a better understanding of the pathogenicity of the contaminating bacteria.[17, 18] Besides bongkrekic acid, B. gladioli pv. cocovenenans is also known to produce the azapteridine toxoflavin (2), which might as well contribute to the toxic properties of contaminated tempe bongkrek. Several recent studies indicated that Burkholderia
species are prolific producers of secondary metabolites with potent biological and pharmacological selleck chemicals properties.[20-28] Interestingly, some species were also found to be associated with mucoralean fungi and are of eminent metabolic importance for the fungi.[4,
29] A prominent example are the bacterial endosymbionts of R. microsporus. The bacteria, Burkholderia rhizoxinica, are producers of highly active antitumoural agents as well as a strong hepatotoxin.[32, 33] The discovery of these natural products is of importance as R. microsporus is not only a plant pathogen but also implicated with human infections. In this regard it should be noted that full genome sequencing of natural product producing selleck chemical bacteria indicated that their biosynthetic potential may even be much higher than expected. It is believed that the majority of secondary metabolite encoding second genes is only expressed under certain conditions and may require a specific trigger. To get an overview of the secondary metabolic capabilities
of the toxinogenic B. gladioli strain and to investigate its metabolic contribution to the bacterial–fungal interaction, we performed a systematic survey on its biosynthetic potential on a genomic and an analytical-chemical level. Here, we report the formation and the biosynthesis of a class of antibiotics previously not known to be produced by these fungus-associated bacteria. We also describe the context-dependent production of the antibiotics and of the toxin bongkrekic acid in the fungal–bacterial coculture. Rhizopus microsporus var. oligosporus HKI 0401 (CBS 337.62; ATCC 46348; NRRL514) and Burkholderia gladioli pv. cocovenenans HKI 10521 (DSM 11318; ATCC 33664) were grown on potato dextrose agar (PDA) at 30 °C. Genomic DNA of B. gladioli was isolated using the MasterPure™ DNA purification kit (Epicentre Biotechnologies, Hessisch Oldendorf, Germany) to perform 454 Shotgun sequencing combined with a 3 kb paired end library. An approximately 25-fold coverage including 10 scaffolds was obtained and subsequent correct assembly of the generated contigs were achieved using the Lasergene SeqMan software (DNA Star, Inc., Madison, WI, USA).