Identification of a Broad-Spectrum Peptidoglycan Hydrolase Associated with the Particle of Xanthomonas oryzae Phage Xop411.

Virion-associated peptidoglycan hydrolases (VAPGH) in bacteriophages are potential antimicrobials. Xop411 is a syphophage infecting the Gram-negative Xanthomonas oryzae pv. oryzae that causes bacterial leaf blight in rice plants. The Xop411 gp21 protein was identified here as a peptidoglycan glycohydrolase by Western blotting and zymogram assay, and localized to the phage tail by immunogold-labelling electron microscopy. This protein showed an apparent molecular mass of 17 kDa in SDS-polyacrylamide gels, larger than that calculated from the amino acid sequence, 15 kDa with 130 residues. The recombinant gp21 expressed in Escherichia coli formed inclusion bodies, which gained enzyme activity after in-gel renaturation. In contrast, the secreted recombinant protein (s-gp21His) expressed in Pichia pastoris was soluble and enzymatically active. Plate assays showed that s-gp21His was capable of killing 3 species of Xanthomonas, a genus containing 27 closely related plant pathogenic species, as well as the opportunistic Pseudomonas aeruginosa and Stenotrophomonas maltophilia causing nosocomial infections. These results indicate that the Xop411 gp21 has possible wide applications as an antimicrobial against xanthomonads and at least 2 opportunistic bacteria. Several other VAPGH from Xanthomonas phages were also identified by bioinformatic analysis, with 1 being confirmed by Western blotting.