A putative type III secretion system effector encoded by the MA20_12780 gene in Bradyrhizobium japonicum Is-34 causes incompatibility with Rj₄ genotype soybeans

The nodulation of Bradyrhizobium japonicum Is-34 is restricted by Rj₄ genotype soybeans (Glycine max). To identify the genes responsible for this incompatibility, Tn5 mutants of B. japonicum Is-34 that were able to overcome this nodulation restriction were obtained. Analysis of the Tn5 mutants revealed that Tn5 was inserted into a region containing the MA20_12780 gene. In addition, direct disruption of this gene using marker exchange overcame the nodulation restriction by Rj₄ genotype soybeans. The MA20_12780 gene has a tts box motif in its upstream region, indicating a possibility that this gene encodes a type III secretion system (T3SS) effector protein. Bioinformatic characterization revealed that the MA20_12780 protein contains the small ubiquitin-like modifier (SUMO) protease domain of the C48 peptidase (ubiquitin-like protease 1 [Ulp1]) family. The results of the present study indicate that a putative T3SS effector encoded by the MA20_12780 gene causes the incompatibility with Rj₄ genotype soybeans, and they suggest the possibility that the nodulation restriction of B. japonicum Is-34 may be due to Rj₄ genotype soybeans recognizing the putative T3SS effector (MA20_12780 protein) as a virulence factor.