Full elucidation of the hitherto unknown structure of albicidin, a potent antibiotic produced by Xanthomonas albilineans

Albicidin is a potent DNA gyrase inhibitor produced by the sugarcane pathogenic bacterium Xanthomonas albilineans. As such, this molecule blocks the differentiation of chloroplasts, resulting in appearance of narrow white stripes on sugarcane leaves that are characteristic of leaf scald disease. Albicidin targets the bacterial gyrase by a mechanism that is different from the one of other DNA gyrases inhibitors like coumarins or quinolones [ 1). lt also exhibits antibacterial activity at nanomolar concentrations against Escherichia coti and to a lower extent against numerous Gram-negative and -positive human pathogenic bacteria [2]. A decade of intense work was necessary to decipher albicidin's biosynthetic pathway and to elucidate its astonishing never-seen-before structure. Albicidin is produced by a hybrid PKS/NRPS (polyketide synthase/non ribosomal peptide synthetase) system. Such ribosome- independent systems consist of modular megasynthetases which operate in an assembly-line fashion to activate, modify and link mostly unusual aminoacid bui lding blocks, finally resulting in complex bioactive peptide-like molecules. The structure of albicid in, which was predicted by former in silico sequence analyses of its PKS/N RPS gene cluster [3], was ascertained by means of mass spectrometry and nuclear magnetic resonance spectroscopy. We were able to demonstrate that albicidin exhibits a linear polyaromatic penta-peptid ic structure containing the rare aminoacids paraaminobenzoate and cyanoalanine [4]. The determination of the structure of albicidin allowed the development of a protocol for the chemical synthesis of this complex molecule. Consequently, new research, such as structure-activity relationship studies, will now be possible [5]. New insights into biosynthesis pathway and structural determination for albicidin will be presented. (Résumé d'auteur