Cyclic beta-glucans of members of the family Rhizobiaceae.

Cyclic beta-glucans are low-molecular-weight cell surface carbohydrates that are found almost exclusively in bacteria of the Rhizobiaceae family. These glucans are major cellular constituents, and under certain culture conditions their levels may reach up to 20% of the total cellular dry weight. In Agrobacterium and Rhizobium species, these molecules contain between 17 and 40 glucose residues linked solely by beta-(1,2) glycosidic bonds. In Bradyrhizobium species, the cyclic beta-glucans are smaller (10 to 13 glucose residues) and contain glucose linked by both beta-(1,6) and beta-(1,3) glycosidic bonds. In some rhizobial strains, the cyclic beta-glucans are unsubstituted, whereas in other rhizobia these molecules may become highly substituted with moieties such as sn-1-phosphoglycerol. To date, two genetic loci specifically associated with cyclic beta-glucan biosynthesis have been identified in Rhizobium (ndvA and ndvB) and Agrobacterium (chvA and chvB) species. Mutants with mutations at these loci have been shown to be impaired in their ability to grow in hypoosmotic media, have numerous alterations in their cell surface properties, and are also impaired in their ability to infect plants. The present review will examine the structure and occurrence of the cyclic beta-glucans in a variety of species of the Rhizobiaceae. The possible functions of these unique molecules in the free-living bacteria as well as during plant infection will be discussed.     

Fragmentations of the large-subunit rRNA in the family Rhizobiaceae.

A 130-nucleotide-long rRNA species corresponding to the 5' end of the 23S rRNA gene was found in 96 strains belonging to different Rhizobium, Bradyrhizobium, and Agrobacterium species. Additional fragmentation in the central region of the large-subunit rRNA occurred in all agrobacteria, except Agrobacterium vitis, and in most Rhizobium leguminosarum and Rhizobium etli strains but did not occur in any of the other rhizobia and bradyrhizobia studied.     

Degradation of the Herbicide Glyphosate by Members of the Family Rhizobiaceae

Several strains of the family Rhizobiaceae were tested for their ability to degrade the phosphonate herbicide glyphosate (isopropylamine salt of N-phosphonomethylglycine). All organisms tested (seven Rhizobium meliloti strains, Rhizobium leguminosarum, Rhizobium galega, Rhizobium trifolii, Agrobacterium rhizogenes, and Agrobacterium tumefaciens) were able to grow on glyphosate as the sole source of phosphorus in the presence of the aromatic amino acids, although growth on glyphosate was not as fast as on P_i. These results suggest that glyphosate degradation ability is widespread in the family Rhizobiaceae. Uptake and metabolism of glyphosate were studied by using R. meliloti 1021. Sarcosine was found to be the immediate breakdown product, indicating that the initial cleavage of glyphosate was at the C—P bond. Therefore, glyphosate breakdown in R. meliloti 1021 is achieved by a C—P lyase activity.     

Sequence diversity of the plasmid replication gene repC in the Rhizobiaceae

The repABC operon is essential for stable maintenance of some Rhizobiaceae plasmids and of pTAV320 from Paracoccus versutus. These plasmids are the largest described family of homologous, yet compatible replicons. The repC gene is essential for plasmid replication, and previous work identified four distinct sequence groups (repC1, repC2, repC3, and repC4) that appear to define different compatibility classes. Probes for these different groups were used to characterize plasmids in Rhizobium leguminosarum population studies and three new repC sequence groups, repC5, repC6, and repC7 were identified. The general repC primers were modified to amplify a wider range of repC sequences and repC sequences were identified in Sinorhizobium and Mesorhizobium type strains. We also showed that the repC3 group-specific primers described previously do not amplify all repC3 sequences and developed a new repC3 amplification strategy.     

Selection of Characters for an Adansonian Analysis of Mycobacterial Taxonomy

Tests which are most frequently employed to provide characters for numerical analysis of bacterial taxonomy provide little differentiation among the slow-growing members of the genus Mycobacterium. A special scheme was proposed to deal with this problem. Characters were based, to some extent, on quantitative reactions to tests, and were defined, for each test, in terms of natural clustering behavior of a large collection of assorted strains. Inter- and intrataxon analyses were based on comparisons of reaction patterns with tentative hypothetical median strain patterns. This permitted the analysis of larger numbers of cultures than can be practically studied in a conventional n x n table.     

Congo red absorption and cellulose synthesis by Rhizobiaceae

Congo red uptake by Rhizobium colonies from yeast extract-mannitol-mineral salts-Congo red-agar plates was related with the cellulose content in the cell capsule of the bacteria.     

Lipopolysaccharide structures from Agrobacterium and Rhizobiaceae species

This review reports and discusses the structural and the biological data available for the lipopolysaccharides from the Gram-negative bacterium Agrobacterium together with those of other related Rhizobiaceae species.     

Conjugation transfer of the bireplicon plasmid pSPA044 into Rhizobiaceae bacteria

We have demonstrated the possibility of hybrid plasmid pSPA044 conjugative transfer from E. coli cells into different Rhizobium species. The bireplicon plasmid, constructed earlier in our laboratory, consisting of pBR325 and HindIII fragment 13 of the nopaline plasmid pTiC58 was mobilized for transfer by the helper plasmid pRK2013 with the frequency about 10(-4). We conclude the hybrid plasmid pSPA044 to be able to replicate stably in Rhizobiaceae cells.