Biosynthèse de l'albicidine, une molécule antibiotique et une pathotoxine de Xanthomonas albilineans
Albicidin is a major pathogenicity factor of Xanthomonas albilineans, the causal agent of leaf scald disease of sugarcane. This pathotoxin, that inhibits chloroplast différenciation and exhibits antibiotic activity, is produced at very low level. Its structure and its chemical composition remain unknown. Three genetic regions involed in albicidin biosynthesis (XALB1, XALB2 and XALB3) were sequenced. XALB1 cluster encodes three megasynthases belonging to polyketide synthetases (PKS) and non ribosomal peptide synthases (NRPS) families, along with several putative resistance, regulation and modification genes. XALB2 region encodes an enzyme responsible for PKS and NRPS post transcriptional modification. Based on these data, an albicidin biosynthesis model was proposed, as well as a putative chemical composition and a theroretical backbone of the toxin. XALB3 region also harbours a single albicidin biosynthesis gene. This gene was characterised by functional complementation using E. coli HtpG, a heat shock protein homologous to the eucaryotic Hsp90 molecular chaperone. The whole set of identified biosynthesis genes was transferred to a heterologous host (Xanthomonas axonopodis pv. vesicatoria), driving the biosynthesis of albicidin activity. These data confirmed that all the albicidin biosynthetic genes were identified and should result in production of substantial amounts of the toxin in order to characterize its chemical structure and biological properties. These results should in turn allow us to obtain structural analogues of this potent antibiotic by metabolic engineering.