Proline Betaine Uptake in Sinorhizobium meliloti : Characterization of Prb, an Opp-Like ABC Transporter Regulated by both Proline Betaine and Salinity Stress

Abstract

Sinorhizobium meliloti uses proline betaine (PB) as an osmoprotectant when osmotically stressed and as an energy source in low-osmolarity environments. To fulfill this dual function, two separate PB transporters, BetS and Hut, that contribute to PB uptake at high and low osmolarity, respectively, have been previously identified. Here, we characterized a novel transport system that mediates the uptake of PB at both high and low osmolarities. Sequence analysis of Tn5-luxAB chromosomal insertions from several PB-inducible mutants has revealed the presence of a four-gene locus encoding the components of an ABC transporter, Prb, which belongs to the oligopeptide permease (Opp) family. Surprisingly, prb mutants were impaired in their ability to transport PB, and oligopeptides were not shown to be competitors for PB uptake. Further analysis of Prb specificity has shown its ability to take up other quaternary ammonium compounds such as choline and, to a lesser extent, glycine betaine. Interestingly, salt stress and PB were found to control prb expression in a positive and synergistic way and to increase Prb transport activity. At low osmolarity, Prb is largely implicated in PB uptake by stationary-phase cells, likely to provide PB as a source of carbon and nitrogen. Furthermore, at high osmolarity, the analysis of prb and betS single and double mutants demonstrated that Prb, together with BetS, is a key system for protection by PB.

Growing varieties durum wheat (Triticum durum) in response to the effect of osmolytes and inoculation by Azotobacter chroococcum under salt stress

“…In addition, the salinity reduced the number of root colonizing bacteria. The contribution of osmolytes (proline or glycine betaine) in the environment played an important role in the adaptation of these rhizobacteria to salt stress and as osmoprotectant or as carbon and nitrogen sources (Alloing et al, 2006).…”

Salt tolerance of a Sinorhizobium meliloti strain isolated from dry lands: growth capacity and protein profile changes

“…The growth of rhizobia under osmotic stress is linked to the ability to develop intrinsic mechanisms leading to the conservation of cell integrity. Salt-tolerant rhizobia exposed to increased salinity can maintain equilibrium across the membrane by exclusion of salts and via accumulation in cytoplasm of compatibles and nontoxic solutes called osmoprotectants (Csonka 1989;Boscari et al 2004;Alloing et al 2006). The combination of high resolution two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) with methods that have become standard in protein identification such as peptide mass fingerprinting provides an adequate strategy to analyze and understand the bacterial response to abiotic stresses (Becker 2009).…”

Characterization of the Osmoprotectant Transporter OpuC from Pseudomonas syringae and Demonstration that Cystathionine-β-Synthase Domains Are Required for Its Osmoregulatory Function

“…The Erwinia chrysanthemi OusB transporter is among the few identified in a gram-negative species and is similar to ProU in sequence, substrate range, expression, and activation profile (14). S. meliloti ABC transporters that function in the uptake of osmoprotectant compounds have also been identified (1,8,19,28), but the primary or sole function of each appears to be for catabolism rather than osmoprotection. Greater functional knowledge of osmoregulatory ABC transporters in gram-negative bacteria would improve our ability to identify such transporters among multiple candidates in a given organism, as has been attempted in studies examining structure-function relations within these transporters (45).…”