Inability to Catabolize Rhamnose by Sinorhizobium meliloti Rm1021 Affects Competition for Nodule Occupancy

strains unable to grow on rhamnose as a sole carbon source are less competitive for nodule occupancy. To determine if the ability to use rhamnose as a sole carbon source affects competition for nodule occupancy in , Tn mutants unable to use rhamnose as a sole carbon source were isolated. mutations affecting rhamnose utilization were found in two operons syntenous to those of . Although the Tn mutants were complemented using an cosmid that contains the entire wild-type rhamnose catabolic locus, complementation did not occur if the cosmids carried Tn insertions within the locus. Through a series of heterologous complementation experiments, enzyme assays, gene fusion, and transport experiments, we show that the regulator, RhaR, is dominant to its counterpart. In addition, the data support the hypothesis that the kinase is capable of directly phosphorylating rhamnose and rhamnulose, whereas the kinase does not possess rhamnose kinase activity. In nodule competition assays, mutants incapable of rhamnose transport were shown to be less competitive than the wild-type and had a decreased ability to bind plant roots in the presence of rhamnose. The data suggests that rhamnose catabolism is a general determinant in competition for nodule occupancy that spans across rhizobial species.