Genetic and biochemical characterization of a high-affinity betaine uptakesystem (BusA) in Lactococcus lactis reveals a new functional organization within bacterial ABC transporters

The cytoplasmic accumulation of exogenous betaine stimulates the growth of Lactococcus lactis cultivated under hyperosmotic conditions. We report that L. lactis possesses a single betaine transport system that belongs to the ATP-binding cassette (ABC) superfamily of transporters. Through transposon mutagenesis, a mutant deficient in betaine transport was isolated. We ident ified two genes, busAA and busAB, grouped in an operon, busA (betaine uptak e system). The transcription of busA is strongly regulated by the external osmolality of the medium. The busAA gene codes for the ATP-binding protein, busAB encodes a 573-residue polypeptide which presents two striking featur es: (i) a fusion between the regions encoding the transmembrane domain (TMD ) and the substrate-binding domain (SBD) and (ii) a swapping of the SBD sub domains when compared to the Bacillus subtilis betaine-binding protein, Opu AC. BusA of L. lactis displays a high affinity towards betaine (K-m = 1.7 m u M) and is an osmosensor whose activity is tightly regulated by external o smolality, leading the betaine uptake capacity oft. lactis to be under dual control at the biochemical and genetic levels. A protein presenting the ch aracteristics predicted for BusAB was detected in the membrane fraction oft . lactis. The fusion between the TMD and the SBD is the first example of a new organization within prokaryotic ABC transporters.