Identification of Cbp1, a c-di-GMP Binding Chemoreceptor in ORS571 Involved in Chemotaxis and Nodulation of the Host Plant.

Cbp1, a chemoreceptor containing a PilZ domain was identified in ORS571, a nitrogen-fixing free-living soil bacterium that induces nodule formation in both the roots and stems of the host legume . Chemoreceptors are responsible for sensing signals in the chemotaxis pathway, which guides motile bacteria to beneficial niches and plays an important role in the establishment of rhizobia-legume symbiosis. PilZ domain proteins are known to bind the second messenger c-di-GMP, an important regulator of motility, biofilm formation and virulence. Cbp1 was shown to bind c-di-GMP through the conserved RxxxR motif of its PilZ domain. A mutant strain carrying a deletion was impaired in chemotaxis, a feature that could be restored by genetic complementation. Compared with the wild type strain, the Δ mutant displayed enhanced aggregation and biofilm formation. The Δ mutant induced functional nodules when inoculated individually. However, the Δ mutant was less competitive than the wild type in competitive root colonization and nodulation. These data are in agreement with the hypothesis that the c-di-GMP binding chemoreceptor Cbp1 in is involved in chemotaxis and nodulation.