Résumé
Xanthomonas albilineans is a xylem-invading pathogen that causes leaf scald disease of sugarcane. Unlike most other plant pathogenic bacteria, X. albilineans does not possess an Hrp-Type Three Secretion System. Therefore, pathogenicity must rely on other virulence factors. X. albilineans produces the toxin albicidin, a potent inhibitor of DNA gyrase which inhibits proplastic DNA replication, resulting in blocked chloroplast differentiation and disease symptoms. Sequencing and annotation of the entire genome of X. albilineans recently revealed that X. albilineans possesses 12 large genes encoding nonribosomal peptide synthetases (NRPSs) which are located in four gene clusters covering 4 % of the genome (1). One of these NRPS gene clusters corresponds to the previously identified albicidin biosynthesis locus (2). In silico analysis of the three other NRPS gene clusters resulted in partial prediction of the sequences of the precursor peptides synthesized by these clusters, which do not resemble any peptide, described to date. One of these NRPS gene clusters encodes a complete machinery predicted to be required for secretion and tailoring of small molecules: ABC transporter, MbtH like protein, isomerase, aminotransferase, acyltransferase, enoyl-CoA hydratase. Interestingly, this complete NRPS gene cluster was recently identified in two African strains of X. oryzae pv. oryzae which is also a xylem-invading pathogen and which causes bacterial leaf blight of rice. The transfer of a phosphopantetheinyl group to a NRPS is required for the posttranslational activation of this enzyme. The genome of X. albilineans contains a phosphopantetheinyl transferase (PPTase) gene which is required for albicidin biosynthesis (2). This gene, which is obviously required for posttranslational activation of all NRPSs encoded by X. albilineans, is also conserved in X. oryzae pv. oryzae. Therefore, the new NRPS gene clusters recently identified in the genomes of X. albilineans and African strains of X. oryzae pv. oryzae are putatively involved in the biosynthesis of virulence factors. To test this hypothesis, PPTase deletion mutants were produced in strain GPE PC73 of X. albilineans and in African strain Bai3 of X. oryzae pv. oryzae. In contrast to an albicidin NRPS mutant, the PPTase mutant of X. albilineans was affected in the colonization of young sugarcane stalks. Rice inoculation studies with the PPTase mutant of X. oryzae pv. oryzae are in progress and will provide further data regarding involvement of NRPS in pathogenicity of these two xylem-invading pathogens attacking monocotyledones. (Résumé d'auteur)