Rhizobium meliloti Genes Encoding Catabolism of Trigonelline Are Induced under Symbiotic Conditions

Abstract

Rhizobium meliloti trc genes controlling the catabolism of trigonelline, a plant secondary metabolite often abundant in legumes, are closely linked to nif-nod genes on the symbiotic megaplasmid pSym [Boivin, C., Malpica, C., Rosenberg, C., Denarie, J., Goldman, A., Fleury, V., Maille, M., Message, B., and Tepfer, D. (1989). In Molecular Signals in the Microbe-Plant Symbiotic and Pathogenic Systems. (Berlin: Springer-Verlag), pp. 401-407]. To investigate the role of trigonelline catabolism in the Rhizobium-legume interaction, we studied the regulation of trc gene expression in free-living and in endosymbiotic bacteria using Escherichia coli lacZ as a reporter gene. Experiments performed with free-living bacteria indicated that trc genes were organized in at least four transcription units and that the substrate trigonelline was a specific inducer for three of them. Noninducing trigonelline-related compounds such as betaines appeared to antagonize the inducing effect of trigonelline. None of the general or symbiotic regulatory genes ntrA, dctB/D, or nodD seemed to be involved in trigonelline catabolism. trc fusions exhibiting a low basal and a high induced [beta]-galactosidase activity when present on pSym were used to monitor trc gene expression in alfalfa tissue under symbiotic conditions. Results showed that trc genes are induced during all the symbiotic steps, i.e., in the rhizosphere, infection threads, and bacteroids of alfalfa, suggesting that trigonelline is a nutrient source throughout the Rhizobium-legume association.

The soybean cultivar specificity gene noIX is present, expressed in a nodD-dependent manner, and of symbiotic significance in cultivar-nonspecific strains of Rhizobium (Sinorhizobium) fredii

“…The third method involved histochemical staining of sections from McCall nodules containing strain RfCB26. Seedlings were cultured as described by Krishnan & Pueppke (1991) and sections stained in situ as described by Boivin et al (1990).…”

Molecular characterization of the bet genes encoding glycine betaine synthesis in Sinorhizobium meliloti 102F34

“…By contrast, these two dehydrogenases share only 21% identical residues with the choline oxidase from A. globiformis (Deshnium et al, 1995), essentially in the C-and N-terminal regions. However, both the two dehydrogenases and the oxidase possess, at their N-terminus, a ' glycine box ' containing a conserved motif (GXGXXG) and a series of amino acids which are characteristic features of flavoproteins (Wierenga et al, 1986;Hanukoglu & Gutfinger, 1989;Lamark et al, 1991 Symbiotic proficiency of LTS23-1020 and genomic location of the betBA genes Besides the chromosome, S. meliloti contains two symbiotic megaplasmids called pSyma and pSymb (Burkhardt et al, 1987;Sobral et al, 1991). Genes essential for the catabolism of at least three betaines, carnitine, trigonelline and stachydrine (proline betaine) , map near the nod region of pSyma in S. meliloti RCR2011 (Goldman et al, 1991).…”

“…In E. coli and A. pascens, CDH and choline oxidase, respectively, catalyse both steps of betaine biosynthesis from choline and we predict that a similar situation occurs in S. meliloti. The differences in their cellular location, and in the specific catalytic mechanisms which differentiate general dehydrogenases from oxidases (Brouquisse et al, 1989), most likely account for the rather low sequence homology displayed by the two classes of bacterial choline-oxidizing enzymes (Fig. 4).…”

“…4). The structural relationship of these enzymes with the chloroplastic, ferredoxin-dependent, choline monooxygenase from higher plants (Brouquisse et al, 1989) cannot yet be established.…”

“…A soluble choline oxidase (EC 1.1.3.17) is found in the Gram-positive bacteria Arthrobacter pascens and Arthrobacter globiforrnis, and in the fungus Cylindrocarpon didyrnum (Tani et al, 1979;Rozwadowski et al, 1991;Deshnium et al, 1995). Higher plants utilize a choline monooxygenase (CMO) in combination with a betaine-aldehyde dehydrogenase (BADH, EC 1.2.1.8 ; Brouquisse et al, 1989;Weretilnyk & Hanson, 1989). cDNA clones for the BADH of spinach (Spinacia oleracea), sugar beet (Beta vulgaris) and barley (Hordeurn vulgare) have been characterized.…”