RESUMEN
Xanthomonas albilineans, the causal agent of leaf scald disease of sugarcane, produces a highly potent polyketide-peptide antibiotic and phytotoxin called albicidin. Previous studies established the involvement of a large cluster of genes in the biosynthesis of this toxin. We report here the sub-cloning and sequencing of an additional gene outside of the main cluster and essential for albicidin biosynthesis. This gene encodes a 634-amino-acid protein that shows high identity with the Escherichia coli heat shock protein HtpG. Complementation studies of X. albilineans Tox- mutants confirmed the requirement of htpG for albicidin biosynthesis and revealed functional interchangeability between E. coli and X. albilineans htpG genes. HtpG was co-localised with albicidin in the cellular membrane, i.e., the cellular fraction where the toxin is most probably biosynthesised. Here we show the requirement of an HtpG protein for the biosynthesis of a polyketide-peptide antibiotic.
Asunto(s)
Proteínas Bacterianas/genética , Proteínas HSP90 de Choque Térmico/genética , Xanthomonas/metabolismo , Proteínas Bacterianas/análisis , Proteínas Bacterianas/fisiología , Membrana Celular/química , Secuencia Conservada , ADN Bacteriano/química , ADN Bacteriano/genética , Escherichia coli/genética , Eliminación de Gen , Orden Génico , Prueba de Complementación Genética , Proteínas HSP90 de Choque Térmico/análisis , Proteínas HSP90 de Choque Térmico/fisiología , Datos de Secuencia Molecular , Familia de Multigenes , Compuestos Orgánicos/metabolismo , Análisis de Secuencia de ADN , Homología de Secuencia de Aminoácido , Xanthomonas/genéticaRESUMEN
Xanthomonas albilineans, which causes leaf scald disease of sugarcane, produces a highly potent pathotoxin called albicidin. We report here sequencing and homology analysis of the major gene cluster, XALB1 (55,839 bp), and a second, smaller region, XALB2 (2,986 bp), involved in albicidin biosynthesis. XALB1 contains 20 open reading frames, including i) three large genes with a modular architecture characteristic of polyketide synthases (PKSs) and nonribosomal peptide synthases (NRPSs) and ii) several putative modifying, regulatory, and resistance genes. Sequencing and complementation studies of six albicidin-defective mutants enabled us to confirm the involvement of the three PKS and NRPS genes encoded by XALB1 in albicidin production. XALB2 contains only one gene that is required for post-translational activation of PKS and NRPS enzymes, confirming the involvement of these enzymes in albicidin biosynthesis. In silico analysis of these three PKS or NRPS enzymes allowed us to propose a model for the albicidin backbone assembly and to gain insight into the structural features of this pathotoxin. This is the first description of a complete mixed PKS-NRPS gene cluster for toxin production in the genus Xanthomonas.