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

摘要

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. In Xcc 306, these consecutive operons contain genes encoding the glycoside hydrolase family 10 (GH10) endoxylanases Xyn10A and Xyn10C, the agu67 gene, encoding a GH67 α-glucuronidase (Agu67), the xyn43E gene, encoding a putative GH43 α-l-arabinofuranosidase, and the xyn43F gene, encoding a putative β-xylosidase. Recombinant Xyn10A and Xyn10C convert polymeric 4- O -methylglucuronoxylan (MeGX_( n )) to oligoxylosides methylglucuronoxylotriose (MeGX_(3)), xylotriose (X_(3)), and xylobiose (X_(2)). Xcc 306 completely utilizes MeGX_( n ) predigested with Xyn10A or Xyn10C but shows little utilization of MeGX_( n ). Xcc 306 with a deletion in the gene encoding α-glucuronidase ( Xcc 306 Δ agu67 ) will not utilize MeGX_(3) for growth, demonstrating the role of Agu67 in the complete utilization of GH10-digested MeGX_( n ). Preferential growth on oligoxylosides compared to growth on polymeric MeGX_( n ) indicates that GH10 xylanases, either secreted by Xcc 306 in 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 of xyn10 , agu67 , cirA , the tonB receptor, and other genes within these three operons indicates that they constitute a regulon that is responsive to the oligoxylosides generated by the action of Xcc 306 GH10 xylanases on MeGX_( n ). 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 of Xcc 306 and, ultimately, to plant disease.