Characterization of Bacteriophages Cp1 and Cp2, the Strain-Typing Agents for Xanthomonas axonopodis pv. citri
ABSTRACTThe strains ofXanthomonas axonopodispv. citri, the causative agent of citrus canker, are historically classified based on bacteriophage (phage) sensitivity. Nearly allX. axonopodispv. citri strains isolated from different regions in Japan are lysed by either phage Cp1 or Cp2; Cp1-sensitive (Cp1s) strains have been observed to be resistant to Cp2 (Cp2r) andvice versa. In this study, genomic and molecular characterization was performed for the typing agents Cp1 and Cp2. Morphologically, Cp1 belongs to theSiphoviridae. Genomic analysis revealed that its genome comprises 43,870-bp double-stranded DNA (dsDNA), with 10-bp 3′-extruding cohesive ends, and contains 48 open reading frames. The genomic organization was similar to that ofXanthomonasphage phiL7, but it lacked a group I intron in the DNA polymerase gene. Cp2 resembles morphologicallyEscherichia coliT7-like phages ofPodoviridae. The 42,963-bp linear dsDNA genome of Cp2 contained terminal repeats. The Cp2 genomic sequence has 40 open reading frames, many of which did not show detectable homologs in the current databases. By proteomic analysis, a gene cluster encoding structural proteins corresponding to the class III module of T7-like phages was identified on the Cp2 genome. Therefore, Cp1 and Cp2 were found to belong to completely different virus groups. In addition, we found that Cp1 and Cp2 use different molecules on the host cell surface as phage receptors and that host selection ofX. axonopodispv. citri strains by Cp1 and Cp2 is not determined at the initial stage by binding to receptors.