Xylan Utilization Regulon in Xanthomonas citri pv. citri Strain 306: Gene Expression and Utilization of Oligoxylosides

Xanthomonas citri pv. citri strain 306 (Xcc 306), a causative agent of citrus canker, produces endoxylanases that catalyze the depolymerization of cell wall-associated xylans. In the sequenced genomes of all plant-pathogenic xanthomonads, genes encoding xylanolytic enzymes are clustered in three adjacent operons. InXcc 306, these consecutive operons contain genes encoding the glycoside hydrolase family 10 (GH10) endoxylanases Xyn10A and Xyn10C, theagu67 gene, encoding a GH67 α-glucuronidase (Agu67), thexyn43E gene, encoding a putative GH43 α-l -arabinofuranosidase, and thexyn43F gene, encoding a putative β-xylosidase. Recombinant Xyn10A and Xyn10C convert polymeric 4-O -methylglucuronoxylan (MeGXn ) to oligoxylosides methylglucuronoxylotriose (MeGX3 ), xylotriose (X3 ), and xylobiose (X2 ).Xcc 306 completely utilizes MeGXn predigested with Xyn10A or Xyn10C but shows little utilization of MeGXn .Xcc 306 with a deletion in the gene encoding α-glucuronidase (Xcc 306 Δagu67 ) will not utilize MeGX3 for growth, demonstrating the role of Agu67 in the complete utilization of GH10-digested MeGXn . Preferential growth on oligoxylosides compared to growth on polymeric MeGXn indicates that GH10 xylanases, either secreted byXcc 306in planta or produced by the plant host, generate oligoxylosides that are processed by Xyn10 xylanases and Agu67 residing in the periplasm. Coordinate induction by oligoxylosides ofxyn10 ,agu67 ,cirA , thetonB receptor, and other genes within these three operons indicates that they constitute a regulon that is responsive to the oligoxylosides generated by the action ofXcc 306 GH10 xylanases on MeGXn . The combined expression of genes in this regulon may allow scavenging of oligoxylosides derived from cell wall deconstruction, thereby contributing to the tissue colonization and/or survival ofXcc 306 and, ultimately, to plant disease.