MutL Interaction with RaxM Regulates raxX Expression in Xanthomonas oryzae pv. oryzae

Rice XA21 is the cognate resistance (R) protein of the avirulence protein AvrXA21 (RaxX) of Xanthomonas oryzae pv. oryzae (Xoo). The RaxX precursor must be modified by the peptidase RaxB and the tyrosine sulfotransferase RaxST to transform into an active form that has ability to bind and activate XA21. We previously found that a small protein, RaxM, regulates raxX expression and thus affects the XA21-mediated immune response. However, the mechanism through which RaxM regulates raxX expression remains unknown. Here, we show that RaxM can interact with MutL and enhance its endonuclease activity in Xoo. Mutation of mutL increased raxM expression but decreased raxX expression. In addition, MutL positively regulated the expression of raxST and raxR, which are required for raxX expression or modification. Therefore, mutL mutation increased Xoo virulence on XA21-expressing rice. These findings indicate that RaxM regulates raxX expression and modification by influencing MutL activity and thus regulates the RaxX-mediated immune response.