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  • 1
    ISSN: 1617-4623
    Keywords: Key words Regulation of gene expression ; Sinorhizobium meliloti ; Rhizobium meliloti ; Exopolysaccharide biosynthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Sinorhizobium meliloti (Rhizobium meliloti) is able to produce two different exopolysaccharides, succinoglycan and galactoglucan. Mutations in the mucR gene of S. meliloti result in the stimulation of galactoglucan synthesis, while the type of succinoglycan produced is modified. In culture supernatants of a mucR mutant, low-molecular-weight succinoglycan is present, whereas no high-molecular-weight succinoglycan could be detected. The biosynthesis of succinoglycan is directed by the products of the exo gene cluster. Two DNA fragments from this cluster, one located in front of the exoH gene and one in the intergenic region between the divergently transcribed genes exoX and exoY, were shown to represent effective binding sites for MucR. Whereas the latter binding site contains an inverted repeat motif, the former does not. However, the binding of MucR did not strongly modify the transcription of the exo genes involved. In the mucR mutant the expression levels of exoH-lacZ and exoX-lacZ transcriptional fusions were found to be increased 1.5- and 1.7-fold, respectively. On the other hand, the expression level of an exoY-lacZ transcriptional fusion was found to be 1.5-fold lower in the mucR mutant than in the wild-type background. Comparison of the DNA sequences of MucR-binding sites provides insight into the structural requirements for binding of MucR.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1617-4623
    Keywords: Key wordsRhizobium-legume symbiosis ; Rhizobium meliloti ; Succinoglycan degradation ; Repressor ; cAMP
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The mutant T482 of Sinorhizobium meliloti CXM1-105, which carries a Tn5 insertion on megaplasmid 1, exhibits an enhanced symbiotic efficiency phenotype. Three genes, eglC, cya3 and syrB2, were identified in the eff-482 region tagged by the Tn5 insertion in T482. The eglC gene encodes an endoglycanase which contributes to the depolymerization of the exopolysaccharide succinoglycan. The N-terminal region of the predicted cya3 gene product was similar to eukaryotic-type adenylate cyclases from Brevibacterium liquefaciens and Streptomyces coelicolor. Four contiguous tetratricopeptide repeats which are known to mediate protein-protein interactions were identified in the C-terminal portion of Cya3. Complementation analysis demonstrated that cya3 indeed encodes a functional adenylate cyclase. A central helix-turn-helix DNA-binding motif and a putative C-terminal coiled-coil structure implicated in protein oligomerization were found in SyrB2. Extra copies of the syrB2 gene negatively affect transcription of both syrB2 itself and cya3. The Tn5 insertion in T482 was localized between the divergently transcribed genes eglC and syrB2. It eliminated eglC function and slightly stimulated transcription of both syrB2 and cya3, which lies downstream of syrB2. Mutants carrying insertions of the lacZ-Gm interposon in the genes eglC, syrB2 and cya3 exhibit the same phenotype as mutant T482, indicating that these three genes influence symbiotic efficiency.
    Type of Medium: Electronic Resource
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