Osmotic control of glycine betaine biosynthesis and degradation in Rhizobium meliloti

Abstract

Intracellular accumulation of glycine betaine has been shown to confer an enhanced level of osmotic stress tolerance in Rhizobium meliloti. In this study, we used a physiological approach to investigate the mechanism by which glycine betaine is accumulated in osmotically stressed R. meliloti. Results from growth experiments, 14C labeling of intermediates, and enzyme activity assays are presented. The results provide evidence for the pathway of biosynthesis and degradation of glycine betaine and the osmotic effects on this pathway. High osmolarity in the medium decreased the activities of the enzymes involved in the degradation of glycine betaine but not those of enzymes that lead to its biosynthesis from choline. Thus, the concentration of the osmoprotectant glycine betaine is increased in stressed cells. This report demonstrates the ability of the osmolarity of the growth medium to regulate the use of glycine betaine as a carbon and nitrogen source or as an osmoprotectant. The mechanisms of osmoregulation in R. meliloti and Escherichia coli are compared.

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

“…These genes also have well-supported orthologues in all other pseudomonads (www.pseudomonas.com). Apart from the identification of a novel GB demethylase system based on a monooxygenase, this pathway is similar to that described for other choline degraders, particularly in the Rhizobiaceae (81,82).…”

“…The calculation presented above would be similar for Rhizobium; however, it does not grow well on GB under high-salt conditions (82). Therefore, the pseudomonads must have some way to maintain intracellular osmoprotectant at or above a critical threshold to survive the osmotic insult while still permitting growth.…”

“…These two transcriptional regulators are our starting point for understanding regulation of choline and GB metabolism, and therefore homeostasis, in P. aeruginosa. In Rhizobium, some of the enzymes in the GB catabolic pathway have decreased activity at high salt concentrations, presumably to drive accumulation of cytoplasmic GB (82). In P. aeruginosa, high salt concentrations do not appear to directly impact the catabolic pathway (65,80); therefore, I hypothesize that the regulation at all salt concentrations occurs via regulation either at the transcriptional or the translational level.…”

Choline oxidase, a catabolic enzyme in Arthrobacter pascens, facilitates adaptation to osmotic stress in Escherichia coli

“…Inclusion of a 0.9-kb segment upstream of the BstEIIEcoRI fragment present in pKR47 resulted in a 7.5-fold higher level of choline oxidase activity (pKR32). Since the parental vector for both pKR47 and pKR32 is pKK223-3, which does not provide a translation initiation codon (6), the enhanced expression may be due to posttranscriptional events such as formation of RNA structures that could favor efficient translation when the 0.9-kb leader is present (16 (1,14,15,27,39,45). Alternatively, the steady-state concentration of betaine derived by oxidation of choline or from direct uptake of betaine may not be large enough to provide protection from the osmotic stress, due to further utilization of betaine.…”

“…The osmoprotective role of the quaternary ammonium compound glycine betaine (N,N,N-trimethylglycine; betaine) is evident in a number of diverse microbial systems, including enteric bacteria (1), soil bacteria (45), halophilic bacteria (15), cyanobacteria (27), and methanogenic archaebacteria (39). Accumulation of betaine as an adaptive strategy has also been demonstrated in plants (51,53).…”