Interrelations between Glycine Betaine Catabolism and Methionine Biosynthesis in Sinorhizobium meliloti Strain 102F34

Abstract

Methionine is produced by methylation of homocysteine. Sinorhizobium meliloti 102F34 possesses only one methionine synthase, which catalyzes the transfer of a methyl group from methyl tetrahydrofolate to homocysteine. This vitamin B 12 -dependent enzyme is encoded by the metH gene. Glycine betaine can also serve as an alternative methyl donor for homocysteine. This reaction is catalyzed by betaine-homocysteine methyl transferase (BHMT), an enzyme that has been characterized in humans and rats. An S. meliloti gene whose product is related to the human BHMT enzyme has been identified and named bmt. This enzyme is closely related to mammalian BHMTs but has no homology with previously described bacterial betaine methyl transferases. Glycine betaine inhibits the growth of an S. meliloti bmt mutant in low-and high-osmotic strength media, an effect that correlates with a decrease in the catabolism of glycine betaine. This inhibition was not observed with other betaines, like homobetaine, dimethylsulfoniopropionate, and trigonelline. The addition of methionine to the growth medium allowed a bmt mutant to recover growth despite the presence of glycine betaine. Methionine also stimulated glycine betaine catabolism in a bmt strain, suggesting the existence of another catabolic pathway. Inactivation of metH or bmt did not affect the nodulation efficiency of the mutants in the 102F34 strain background. Nevertheless, a metH strain was severely defective in competing with the wild-type strain in a coinoculation experiment.

Homeostasis and Catabolism of Choline and Glycine Betaine: Lessons from Pseudomonas aeruginosa

“…The widespread use of GB has been explained, in part, by the superior osmoprotection offered by GB as a compatible solute for many organisms (21). GB functions as a compatible solute in these organisms and also plays important roles in methyl group metabolism (22)(23)(24). Soluble GB is at low concentrations in most animal fluids (ϳ35 M in serum [23]), and concentrations are also thought to be low in most environments, although no data could be found for soluble GB concentrations in soil or water.…”

“…In the Rhizobiaceae, a GB methyl transferase (GBMT) mechanism has been proposed for GB demethylation and enzyme activity was shown to correspond to growth on GB (24). In the pseudomonads, there is evidence for GBMT enzymatic activity in P. aeruginosa (80,83), and one group showed evidence for its effects on betaine growth in a nonstandard isolate of P. aeruginosa (83).…”

A nonpyrrolysine member of the widely distributed trimethylamine methyltransferase family is a glycine betaine methyltransferase

“…One of these homologs is under quorum-sensing control within S. meliloti 1021 (47), supporting a potential role during some stage of plant symbiosis. The betaine:homocysteine methyltransferase was suggested as a first step in the demethylation of GB for Rhizobiales (15); however, it has now been shown by mutagenesis of S. meliloti that this methyltransferase is likely to be an anabolic enzyme (48), leaving the possibility that, at least under anaerobic conditions, MtgB homologs might serve this purpose in Rhizobiales. 5.…”

Phaeobacter gallaeciensis genomes from globally opposite locations reveal high similarity of adaptation to surface life

“…Only one ORF (PGA1_c13370/PGA2_c13310) was found in each of the Phaeobacter genomes encoding a homocysteine S-methyltransferase domain (PF02574), which is required for the enzymatic activity of BHMT (Garrow, 1996). These ORFs share homology with the metH gene of Escherichia coli and with the S. meliloti betaine-homocysteine methyltransferase gene bmt, which represents a link between methionine biosynthesis and glycine betaine degradation in this organism (Barra et al, 2006). The Phaeobacter ORFs have only one-third of the size of metH and lack the cobalamin binding, the activation and the pterin-binding domains.…”