Pseudomonas syringaeBetT Is a Low-Affinity Choline Transporter That Is Responsible for Superior Osmoprotection by Choline over Glycine Betaine

Abstract

The plant pathogenPseudomonas syringaederives better osmoprotection from choline than from glycine betaine, unlike most bacteria that have been characterized. In this report, we identified a betaine/carnitine/choline family transporter (BCCT) inP. syringaepv. tomato strain DC3000 that mediates the transport of choline and acetylcholine. This transporter has a particularly low affinity (K_mof 876 μM) and high capacity (V_maxof 80 nmol/min/mg of protein) for choline transport relative to other known BCCTs. Although BetT activity increased in response to hyperosmolarity, BetT mediated significant uptake under low-osmolarity conditions, suggesting a role in transport for both osmoprotection and catabolism. Growth studies with mutants deficient in BetT and other choline transporters demonstrated that BetT was responsible for the superior osmoprotection conferred toP. syringaeby choline over glycine betaine when these compounds were provided at high concentrations (>100 μM). These results suggest thatP. syringaehas evolved to survive in relatively choline-rich habitats, a prediction that is supported by the common association ofP. syringaewith plants and the widespread production of choline, but genus- and species-specific production of glycine betaine, by plants. Among the three putative BCCT family transporters inPseudomonas aeruginosaand six inPseudomonas putida, different transporters were predicted to function based on similarity toEscherichia coliBetT than toP. syringaeBetT. FunctionalP. putidaandP. aeruginosatransporters were identified, and their possession of a long C-terminal tail suggested an osmoregulatory function for this tail; this function was confirmed forP. syringaeBetT using deletion derivatives.