The accumulation of compatible solutes is a common defense of bacteria against the detrimental effects of high osmolarity. Uptake systems for these compounds are cornerstones in cellular osmostress responses because they allow the energy preserving scavenging of osmostress protectants from environmental sources. is well studied with respect to the import of compatible solutes and its five transport systems (OpuA, OpuB, OpuC, OpuD, OpuE) for these stress protectants have previously been comprehensively studied. Building on this knowledge and taking advantage of the unabated appearance of new genome sequences of members of the genus, we report here the discovery, physiological characterization, and phylogenomics of a new member of the Opu family of transporters, OpuF (OpuFA-OpuFB). OpuF is not present in but it is widely distributed in members of the large genus. OpuF is a representative of a sub-group of ABC transporters in which the substrate-binding protein (SBP) is fused to the trans-membrane domain (TMD). We studied the salient features of the OpuF transporters from and by functional reconstitution in a chassis strain lacking known Opu transporters. A common property of the examined OpuF systems is their substrate profile; OpuF mediates the import of glycine betaine, proline betaine, homobetaine and the marine osmolyte dimethylsulfoniopropionate (DMSP). An model of the SBP domain of the TMD-SPB hybrid protein OpuFB was established. It revealed the presence of an aromatic cage, a structural feature commonly present in ligand-binding sites of compatible solute importers. The high affinity import of compatible solutes from environmental sources is an important aspect of the cellular defense of many and against the harmful effects of high external osmolarity. The accumulation of these osmostress protectants counteracts high osmolarity instigated water efflux, drop in turgor to non-physiological values, and an undue increase in molecular crowding of the cytoplasm; they thereby foster microbial growth under osmotically unfavorable conditions. Importers for compatible solutes allow the energy-preserving scavenging of osmoprotective and physiologically compliant organic solutes from environmental sources. We report here the discovery, exemplary physiological characterization, and phylogenomics of a new compatible solute importer (OpuF) widely found in members of the genus. The OpuF system is a representative of a growing sub-group of ABC transporters in which the substrate-scavenging function of the substrate-binding protein (SBP) and the membrane-embedded substrate trans-locating subunit (TMD) are fused into a single polypeptide chain.

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