Binding-protein-dependent lactose transport in Agrobacterium radiobacter

Abstract

, 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.

Role of maltase in the utilization of sucrose by Candida albicans

“…Transport of sucrose was shown by a time-dependent, sucrose-inducible incorporation of ['4C]sucrose at 25 'C but not at 0 'C and was absent from identically treated cells that had been grown in dextrose rather than sucrose. Kinetic analysis shows this transport to be saturable (and thus facilitated) exhibiting a Km of 7.1 ,M, similar to that of the lactose permease (1 #tM) of Agrobacterium radiobacter (Greenwood et al, 1990), but much lower than the sucrose permease (6 mM) described from a single strain of Saccharomyces (Santos et al, 1982). It appears that this uniquely adapted high-affinity transporter in Candida may be the result of a greater reliance of Candida on such a transport mechanism in comparison with Saccharomyces, which relies predominantly on periplasmic invertase for the assimilation of sucrose (Schekman and Novick, 1982).…”

In Silico and Transcriptional Analysis of Carbohydrate Uptake Systems of Streptomyces coelicolor A3(2)

“…A Lactose. An assembly of genes around ORF SC6D11.04c may encode a lactose-specific ABC system because the products of SC6D11.04c to SC6D11.06c exhibit 24, 29, and 31% protein identity to LacEFG of Agrobacterium radiobacter, respectively (13,62) (Fig. 4A).…”

Canonical and ECF-type ATP-binding cassette importers in prokaryotes: diversity in modular organization and cellular functions

“…Besides ABC transporters for the uptake of glucose, galactose and xylose, the gram-negative bacterium Agrobacterium radiobacter also possesses a binding protein-dependent transport system for lactose, LacEFGK, which is usually transported by pmf-driven porters or PTS (Greenwood et al, 1990). The system displays considerable sequence similarity to the MalEFGK transporter of E. coli, which is underscored by the observation that the ATPase subunit LacK can replace MalK in maltose transport (Wilken et al, 1997).…”