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Review
. 2013 Apr;79(7):2112-20.
doi: 10.1128/AEM.03565-12. Epub 2013 Jan 25.

Homeostasis and catabolism of choline and glycine betaine: lessons from Pseudomonas aeruginosa

Matthew J Wargo  1
Affiliations
Review

Homeostasis and catabolism of choline and glycine betaine: lessons from Pseudomonas aeruginosa

Matthew J Wargo. Appl Environ Microbiol. 2013 Apr.

Abstract

Most sequenced bacteria possess mechanisms to import choline and glycine betaine (GB) into the cytoplasm. The primary role of choline in bacteria appears to be as the precursor to GB, and GB is thought to primarily act as a potent osmoprotectant. Choline and GB may play accessory roles in shaping microbial communities, based on their limited availability and ability to enhance survival under stress conditions. Choline and GB enrichment near eukaryotes suggests a role in the chemical relationships between these two kingdoms, and some of these interactions have been experimentally demonstrated. While many bacteria can convert choline to GB for osmoprotection, a variety of soil- and water-dwelling bacteria have catabolic pathways for the multistep conversion of choline, via GB, to glycine and can thereby use choline and GB as sole sources of carbon and nitrogen. In these choline catabolizers, the GB intermediate represents a metabolic decision point to determine whether GB is catabolized or stored as an osmo- and stress protectant. This minireview focuses on this decision point in Pseudomonas aeruginosa, which aerobically catabolizes choline and can use GB as an osmoprotectant and a nutrient source. P. aeruginosa is an experimentally tractable and ecologically relevant model to study the regulatory pathways controlling choline and GB homeostasis in choline-catabolizing bacteria. The study of P. aeruginosa associations with eukaryotes and other bacteria also makes this a powerful model to study the impact of choline and GB, and their associated regulatory and catabolic pathways, on host-microbe and microbe-microbe relationships.

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Figures

Fig 1
Fig 1
Schematic representation of choline and glycine betaine homeostasis and metabolism in P. aeruginosa under conditions of (A) low salt and (B) high salt. In P. aeruginosa, choline and GB import are mediated by the BCCT-family transporters BetT1, BetT2, and BetT3 and the ABC-family transporter CbcWV, but the importance of each changes based on the external osmolarity. The full choline catabolic pathway is depicted in Fig. 2 and simplified here. The potential for bacterial phosphatidylcholine (bPC) to be used as a source of choline is discussed in the text. There may be multiple export systems, but export has not been examined in P. aeruginosa. The outer membrane has been left out for clarity, but choline and betaine are predicted to access the periplasm via outer membrane porins. Abbreviations: Pcs = phosphatidylcholine synthase; PLC = phospholipase C.
Fig 2
Fig 2
The P. aeruginosa choline catabolic pathway and schematic representation of the loci encoding the associated catabolic enzymes. The colored arrows in the metabolic pathway correspond to the similarly colored arrows in the diagrams of the loci; a color match indicates the genes encoding required components of the catabolic enzyme. Similar, but lighter, colors indicate genes associated with the process that do not participate directly in catabolism or whose exact function is unknown. The indicated identities of the reactants and products of GbcAB-dependent demethylation are not known (hence the question mark) but are based on predictions of homologous monooxygenases. For transcriptional control, BetI is a repressor whose repression of the betI and betT1 promoters is released by binding choline; i.e., choline represses the BetI repressor function. GbdR is an activator that induces transcription from the gbcA, gbcB, PA5396, and glyA1 promoters in response to sensing GB and dimethylglycine (DMG). The soxA gene is labeled, and the other genes coding for the SoxBDAG heterotetrameric sarcosine demethylase are noted only by the corresponding letters.
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