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Mutations in the Predicted Active Site of Xanthomonas oryzae pv. oryzae XopQ Differentially Affect Virulence, Suppression of Host Innate Immunity, and Induction of the HR in a Nonhost Plant

Xanthomonas oryzae pv. oryzae, the bacterial blight pathogen of rice, secretes a number of effectors through a type 3 secretion system. One of these effectors, called XopQ, is required for virulence and suppression of rice innate immune responses induced by the plant cell-wall-degrading enzyme lipase/esterase A (LipA). Bioinformatic analysis suggested that XopQ is homologous to inosine-uridine nucleoside hydrolases (NH). A structural model of XopQ with the protozoan Crithidia fasciculata purine NH suggested that D116 and Y279 are potential active site residues. X. oryzae pv. oryzae xopQ mutants (xopQ⁻/pHM1::xopQ^D116A and xopQ⁻/pHM1::xopQ^Y279A) show reduced virulence on rice compared with xopQ⁻/pHM1::xopQ. The two predicted XopQ active site mutants (xopQ⁻/pHM1::xopQ^D116A and xopQ⁻/pHM1::xopQ^Y279A) exhibit a reduced hypersensitive response (HR) on Nicotiana benthamiana, a nonhost. However, Arabidopsis lines expressing either xopQ or xopQ^Y279A are equally proficient at suppression of LipA-induced callose deposition. Purified XopQ does not show NH activity on standard nucleoside substrates but exhibits ribose hydrolase activity on the nucleoside substrate analogue 4-nitrophenyl β-D-ribofuranoside. The D116A and Y279A mutations cause a reduction in biochemical activity. These results indicate that mutations in the predicted active site of XopQ affect virulence and induction of the HR but do not affect suppression of innate immunity.