Isolation of salt-sensitive mutants of Sinorhizobium meliloti strain Rm1021

Mutational analysis of the Sinorhizobium meliloti short-chain dehydrogenase/reductase family reveals substantial contribution to symbiosis and catabolic diversity

The short-chain dehydrogenase/reductase (SDR) family is one of the largest and most ubiquitous protein families in bacterial genomes. Despite there being a few well-characterized examples, the substrate specificities or functions of most members of the family are unknown. In this study, we carried out a large-scale mutagenesis of the SDR gene family in the alfalfa root nodule symbiont Sinorhizobium meliloti. Subsequent phenotypic analysis revealed phenotypes for mutants of 21 of the SDR-encoding genes. This brings the total number of S. meliloti SDR-encoding genes with known function or associated phenotype to 25. Several of the mutants were deficient in the utilization of specific carbon sources, while others exhibited symbiotic deficiencies on alfalfa (Medicago sativa), ranging from partial ineffectiveness to complete inability to form root nodules. Five of the mutants had both symbiotic and carbon utilization phenotypes. These results clearly demonstrate the importance of the SDR family in both symbiosis and saprotrophy, and reinforce the complex nature of the interaction of S. meliloti with its plant hosts. Further analysis of the genes identified in this study will contribute to the overall understanding of the biology and metabolism of S. meliloti in relation to its interaction with alfalfa.

Characterization of Sinorhizobium meliloti triose phosphate isomerase genes

A Tn5 mutant strain of Sinorhizobium meliloti with an insertion in tpiA (systematic identifier SMc01023), a putative triose phosphate isomerase (TPI)-encoding gene, was isolated. The tpiA mutant grew more slowly than the wild type on rhamnose and did not grow with glycerol as a sole carbon source. The genome of S. meliloti wild-type Rm1021 contains a second predicted TPI-encoding gene, tpiB (SMc01614). We have constructed mutations and confirmed that both genes encode functional TPI enzymes. tpiA appears to be constitutively expressed and provides the primary TPI activity for central metabolism. tpiB has been shown to be required for growth with erythritol. TpiB activity is induced by growth with erythritol; however, basal levels of TpiB activity present in tpiA mutants allow for growth with gluconeogenic carbon sources. Although tpiA mutants can be complemented by tpiB, tpiA cannot substitute for mutations in tpiB with respect to erythritol catabolism. Mutations in tpiA or tpiB alone do not cause symbiotic defects; however, mutations in both tpiA and tpiB caused reduced symbiotic nitrogen fixation.