, and isopropyl-,3-D-thiogalactoside and were subject to catabolite repression by glucose, galactose, and succinate which was not alleviated by cyclic AMP. We conclude that lactose is transported into A. radiobacter via a binding protein-dependent active transport system (in contrast to the H+ symport and phosphotransferase systems found in other bacteria) and that the expression of this transport system is closely linked to that of P-galactosidase.Agrobacterium radiobacter is a gram-negative aerobe that utilizes a wide range of sugars as sole carbon substrates for growth. Glucose is transported into the cell via two highaffinity active transport systems, which are synthesized maximally during growth under glucose limitation and require periplasmic glucose-binding proteins GBP1 and GBP2 (7). These GBP1-and GBP2-dependent systems, respectively, also transport galactose and xylose with high affinity, and their general properties indicate that they have much in common with the binding protein-dependent systems that transport sugars such as maltose, galactose, arabinose, and ribose into enteric bacteria (1,5,9,15). These latter systems have generally been shown to consist of a periplasmic binding protein, two transmembrane proteins, and at least one protein that is associated with the cytoplasmic surface of the membrane and binds ATP. It is likely, although not proven, that the energy for transport is provided by the hydrolysis of ATP (1,5,15,16).Bacteria can also transport sugars by using proton-or cation-linked systems or via phosphoenolpyruvate-dependent phosphotransferase systems (5,15,23). Lactose transport has been extensively investigated in Escherichia coli and other enteric bacteria, in which it is catalyzed by a single transmembrane protein at the expense of the proton motive force generated by respiration or ATP hydrolysis (H+-lactose symport) (15,17,22). The transported lactose is subsequently hydrolyzed to galactose and glucose by Pgalactosidase. The genes coding for the transport protein (lacl) and ,-galactosidase (lacZ) are part of the lac operon, which also codes for galactoside transacetylase (lacA) and the lac repressor (lad). Transcription is induced by lactose, melibiose, and isopropyl-p3-D-thiogalactoside (IPTG) (also * Corresponding author.