Gene cluster for ferric iron uptake in Agrobacterium tumefaciens MAFF301001

Agrobacterium tumefaciens harboring a Ti plasmid causes crown gall disease in dicot plants by transferring its T-DNA into plant chromosomes. Iron acquisition plays an important role for pathogenicity in animal pathogens and several phytopathogens and for growth in the rhizosphere and on plant surfaces. Under iron-limiting condition, bacteria produce various iron-chelating agents called siderophores. Agrobacterium strains have the diversity in producing siderophores and a certain strain produces a typical catechol-type siderophore, called agrobactin, although its biosynthesis genes have not been analyzed yet. Here we describe the cloning and characterization of a functional gene cluster involved in ferric iron uptake in A. tumefaciens strain MAFF301001. Four complete open reading frames (ORFs) were found in 5-kb region of a genomic library clone 1A3. We named these genes agb, after agrobactin. agbC, agbE, agbB and agbA genes were identified in this order, and narrow intergenic spaces suggested that these genes constitute an operon. Predicted agb gene products and their phylogenetic analysis showed sequence similarity with enzymes which are involved in ferric iron uptake in other bacteria. Southern hybridization analysis clearly indicated the location of agb genes on the linear chromosome in strain MAFF301001 but the complete lack in another A. tumefaciens strain C58. Mutation analysis of agbB revealed that it is essential for growth and production of catechol compounds in iron-limiting medium.