Redundancy in Periplasmic Binding Protein-Dependent Transport Systems for Trehalose, Sucrose, and Maltose in Sinorhizobium meliloti

Abstract

We have identified a cluster of six genes involved in trehalose transport and utilization (thu) in Sinorhizobium meliloti. Four of these genes, thuE, -F, -G, and -K, were found to encode components of a binding proteindependent trehalose/maltose/sucrose ABC transporter. Their deduced gene products comprise a trehalose/ maltose-binding protein (ThuE), two integral membrane proteins (ThuF and ThuG), and an ATP-binding protein (ThuK). In addition, a putative regulatory protein (ThuR) was found divergently transcribed from the thuEFGK operon. When the thuE locus was inactivated by gene replacement, the resulting S. meliloti strain was impaired in its ability to grow on trehalose, and a significant retardation in growth was seen on maltose as well. The wild type and the thuE mutant were indistinguishable for growth on glucose and sucrose. This suggested a possible overlap in function of the thuEFGK operon with the aglEFGAK operon, which was identified as a binding protein-dependent ATP-binding transport system for sucrose, maltose, and trehalose. The K m s for trehalose transport were 8 ؎ 1 nM and 55 ؎ 5 nM in the uninduced and induced cultures, respectively. Transport and growth experiments using mutants impaired in either or both of these transport systems show that these systems form the major transport systems for trehalose, maltose, and sucrose. By using a thuE-lacZ fusion, we show that thuE is induced only by trehalose and not by cellobiose, glucose, maltopentaose, maltose, mannitol, or sucrose, suggesting that the thuEFGK system is primarily targeted toward trehalose. The aglEF-GAK operon, on the other hand, is induced primarily by sucrose and to a lesser extent by trehalose. Tests for root colonization, nodulation, and nitrogen fixation suggest that uptake of disaccharides can be critical for colonization of alfalfa roots but is not important for nodulation and nitrogen fixation per se.

Mapping the Sinorhizobium meliloti 1021 solute-binding protein-dependent transportome

“…A plasmid fusion to SMb20325 was constitutive, but a fusion containing the upstream regulator (SMb20324) and the proposed promoter of SMb20325 was induced 6-fold by trehalose. This result agrees with Jensen et al (19), who found that although the Thu system of S. meliloti transported the ␣-glucosides trehalose and maltose, it was only induced by trehalose. In contrast, the S. meliloti ␣-glucoside transporter, Agl (20), transports sucrose, maltose, and trehalose and was induced by sucrose and to a lesser extent trehalose (19).…”

“…This result agrees with Jensen et al (19), who found that although the Thu system of S. meliloti transported the ␣-glucosides trehalose and maltose, it was only induced by trehalose. In contrast, the S. meliloti ␣-glucoside transporter, Agl (20), transports sucrose, maltose, and trehalose and was induced by sucrose and to a lesser extent trehalose (19). In our hands, a reporter gene fusion integrated into aglE, (SMc03061) encoding the SBP, was induced by sucrose and maltose but also by the previously undescribed inducers maltotriose (Glc1-4Glc1-4Glc) and turanose (Glc3-4Fruc) ( Table 2), with the caveat that the trisaccharide may either contain, or be converted to, a disaccharide.…”

The High-Affinity Maltose/Trehalose ABC Transporter in the Extremely Thermophilic Bacterium Thermus thermophilus HB27 Also Recognizes Sucrose and Palatinose

“…A similar redundancy was found in Sinorhizobium meliloti, where two maltose/trehalose/sucrose ABC transporters were characterized (23,44). One of them (encoded by thuEFGK) is primarily targeted toward trehalose, whereas the second one (encoded by aglEFGAK) is induced primarily by sucrose and to a lesser extent by trehalose (23).…”

“…In the genome of the hyperthermophilic bacterium T. maritima, two putative maltose-binding proteins sharing very high sequence similarity (around 80%) were found (31). A similar redundancy was found in Sinorhizobium meliloti, where two maltose/trehalose/sucrose ABC transporters were characterized (23,44). One of them (encoded by thuEFGK) is primarily targeted toward trehalose, whereas the second one (encoded by aglEFGAK) is induced primarily by sucrose and to a lesser extent by trehalose (23).…”

“…Binding protein-dependent systems that are able to recognize maltose and trehalose are not restricted to thermophiles or hyperthermophiles. In S. meliloti, the binding protein encoded by thuE has high affinity for maltose, trehalose and sucrose, while a second transporter, encoded by aglEFGK, recognizes cellobiose in addition to those three disaccharides (23). Whether the proteins from S. meliloti are also able to bind palatinose, as MalE1 from HB27, is not known since this substrate was not tested (23).…”