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

Abstract

Rhizobium leguminosarum 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 Sinorhizobium meliloti, Tn5 mutants unable to use rhamnose as a sole carbon source were isolated. S. meliloti mutations affecting rhamnose utilization were found in two operons syntenous to those of R. leguminosarum. Although the S. meliloti Tn5 mutants were complemented using an R. leguminosarum cosmid that contains the entire wild-type rhamnose catabolic locus, complementation did not occur if the cosmids carried Tn5 insertions within the locus. Through a series of heterologous complementation experiments, enzyme assays, gene fusion, and transport experiments, we show that the S. meliloti regulator, RhaR, is dominant to its R. leguminosarum counterpart. In addition, the data support the hypothesis that the R. leguminosarum kinase is capable of directly phosphorylating rhamnose and rhamnulose, whereas the S. meliloti kinase does not possess rhamnose kinase activity. In nodule competition assays, S. meliloti 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.

Genome Analysis and Potential Ecological Functions of Members of the Genus Ensifer from Subsurface Environments and Description of Ensifer oleiphilus sp. nov.

“…Members of the Ensifer / Sinorhizobium clade are known for their ability to form extracellular polysaccharides. The eps and exo gene clusters have been identified as playing an essential role in exopolysaccharide production, which is vital for symbiotic rhizobial bacteria, since polysaccharides are directly involved in root nodule occupancy [ 72 ]. Bacteria of the genus Ensifer are typically nonsymbiotic species, but they are also capable of releasing extracellular polysaccharides.…”

“…The epsF and exoFQZ genes encoding glycosyltransferase responsible for exopolysaccharide biosynthesis and the exopolysaccharide production protein were found in the genomes of strains HO-A22 T and SHC 2-14, which were therefore able to form these compounds and increase the viscosity of the cultivation media. In addition, the studied strains possessed the genes rhaIMS and rfbCD , responsible for L-rhamnose isomerization (EC: 5.3.1.14) and mutarotation (EC: 5.1.3.32) [ 72 ] and dTDP-4-dehydrorhamnose reduction (EC: 1.1.1.133) and epimerization (EC: 5.1.3.13) [ 73 , 74 ], respectively. These processes are known to be the key stages in the synthesis of succinoglycan exopolysaccharides [ 9 , 75 , 76 ], and, therefore, it becomes possible to suggest that strains HO-A22 T and SHC 2-14 produced this type of extracellular polysaccharide.…”