The eff-482 locus of Sinorhizobium meliloti CXM1-105 that influences symbiotic effectiveness consists of three genes encoding an endoglycanase, a transcriptional regulator and an adenylate cyclase

The mutant T482 of Sinorhizobium meliloti CXM1-105, which carries a Tn5 ins ertion on megaplasmid 1, exhibits an enhanced symbiotic efficiency phenotyp e. 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 endoglycanas e which contributes to the depolymerization of the exopolysaccharide succin oglycan. The N-terminal region of the predicted cya3 gene product was simil ar to eukaryotic-type adenylate cyclases from Brevibacterium liquefaciens a nd 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 D NA-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 transcr iption of both syrB2 and cya3, which lies downstream of syrB2. Mutants carr ying insertions of the lacZ-Gm interposon in the genes eglC, syrB2 and cya3 exhibit the same phenotype as mutant T482, indicating that these three gen es influence symbiotic efficiency.