A novel lysozyme from Xanthomonas oryzae phage ϕXo411 active against Xanthomonas and Stenotrophomonas

In this study, a bacteriophage of Xanthomonas oryzae pv. oryzae designated as ϕXo411 was isolated. Random sequencing of its genome revealed that it is closely related to another X. oryzae phage, Xp10. A cloned fragment carries the lysozyme gene, lys411. The deduced protein, Lys411, shares 92% identity with Xp10 lysozyme, which contains an extra 46 aa at the N-terminus. Lys411 shows over 40% identities to several other phage lysozymes. The His-tagged protein, Lys411H, expressed in Escherichia coli largely formed as inclusion bodies. The insoluble protein was solubilized in urea and purified by passing through a His-bind column, and the lytic activity was then restored by a refolding process. The optimal assay conditions determined for Lys411H are in 0.1 M potassium phosphate buffer, pH 6.6 containing 1  mM CuCl2 at 25 °C. Lysis assays using different bacterial cells as the substrates indicate that Lys411H is the first lysozyme active against both Xanthomonas and Stenotrophomonas maltophilia. This suggests that Lys411 can be a candidate to be developed into a therapeutic agent for treating S. maltophilia infections, in addition to the potential use in control of the plant diseases caused by Xanthomonas. By analogy to the situation in Xp10, we predict that ϕXo411 has no holin, the protein required for lysozyme export, and the N-terminal signal-arrest-release sequence of Lys411 can accommodate its own export to the periplasm.