Magnetotactic bacteria (MTB) share the ability to biomineralize membrane-enclosed organelles consisting of either magnetite (Fe 3O 4) or greigite (Fe 3S 4) crystals, called magnetosomes, which enable the cells to navigate within EarthвЂ™s magnetic field ( 1). Sesbanimide R displayed strong cytotoxic activity against several carcinoma cell lines. In contrast to the case with sesbanimides D, E, and F, we were able to assign the stereocenter of the arginine moiety experimentally and two of the remaining three stereocenters by predictive biosynthetic tools. However, sesbanimide R exhibits an additional arginine moiety the presence of which reconciles inconsistencies in the previously proposed sesbanimide biosynthesis pathway observed when comparing the chemical structure and the potential biochemistry encoded in the BGC. Structure elucidation by mass spectrometry and nuclear magnetic resonance spectroscopy (NMR) revealed that this secondary metabolite resembles sesbanimides, which were very recently reported from other taxa. By using targeted deletion and transcriptional activation, we connected a large biosynthetic gene cluster (BGC) of the trans-acyltransferase polyketide synthase ( trans-AT PKS) type to the biosynthesis of a novel polyketide in the alphaproteobacterium Magnetospirillum gryphiswaldense. Genomic information from various magnetotactic bacteria suggested that besides their common ability to form magnetosomes, they potentially also represent a source of bioactive natural products.

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