Galactoside permease

In bacteria, accumulation of substrates against a concentration gradient can occur through two main classes of transport systems (see [30] for a summary). The prototype of the first class of transporters is the /3-galactoside permease of Escherichia coli (see [31]). It is a relatively simple system involving only a single membrane-bound protein. It catalyzes a lactose-H symport. Other transporters... 

Hidalgo, C., Reyes, J. and Goldschmidt, R. 1977. Induction and general properties of 0-galatosidase and /3-galactoside permease in Pseudomonas BAL-31. J. BacterioL 129, 821-829. 

The Y gene, which encodes a galactoside permease, the transport protein required for entry of lactose into the cell... 

The lactose (toe) operon (Fig. 28-7a) includes the genes for jS-galactosidase (Z), galactoside permease (F), and thiogalactoside transacetylase (A). The last of these enzymes appears to modify toxic galactosides to facilitate their removal from the cell. Each of the three genes is preceded by a ribosome binding site (not shown in Fig. 28-7) that independently directs the translation... 

FIGURE 28-6 Lactose metabolism in /. coli. Uptake and metabolism of lactose require the activities of galactoside permease and /3-galactosidase. Conversion of lactose to allolactose by transglycosyla-tion is a minor reaction also catalyzed by /3-galactosidase. 

Despite this elaborate binding complex, repression is not absolute. Binding of the Lac repressor reduces the rate of transcription initiation by a factor of 10s. If the 02 and 03 sites are eliminated by deletion or mutation, the binding of repressor to CL alone reduces transcription by a factor of about 102. Even in the repressed state, each cell has a few molecules of /3-galactosidase and galactoside permease, presumably synthesized on the rare occasions when the repressor transiently dissociates from the operators. This basal level of transcription is essential to operon regulation. 

The Permease Systems of Bacteria. The best defined of these is the galactoside permease of E. coli. This transport system mediates the active accumulation of galactosides in the presence of metabolic energy and the facilitated diffusion of these compounds when the energy system is blocked (8). A specific galactoside-binding protein has been implicated, but it seems clear that the system is different from the phosphotransferase system described above since no covalent intermediates of... 

Much of the work defining the galactoside permease systems of E. coli was done before the discovery of the ubiquitous phosphotransferases of bacteria. There is now much discussion as to the relative importance of the permease and phosphotransferase systems and on the possible re-interpretation of some of the models proposed for permease systems, because of the new information on the phosphotransferases. 

Kepes, A., Etudes Cinetiques sur La Galactoside Permease D Escherichia... 

A crucial clue to the mechanism of gene regulation was the observation that two other proteins are synthesized in concert with P-galactosidase—namely, galactoside permease and thiogalactoside transacetylase. The permease is required for the transport of lactose across the bacterial cell membrane. The transacetylase is not essential for lactose metabolism but appears to play a role in the detoxification of compounds that also may be transported by the permease. Thus, the... 

The lactose (lac ) operon (Fig. 28-7a) includes the genes for jS-galactosidase (Z), galactoside permease... 

Operon composition/induction - The lactose operon consists of three linked structural genes that encode enzymes of lactose utilization, plus adjacent regulatory sites. The three structural genes—z, y, and a—encode / -galactosidase, / -galactoside permease (a transport protein), and thiogalactoside transacetylase (an enzyme of still unknown metabolic function), respectively. 

FIGURE 8.26 An example of secondary active transport. Galactoside permease uses the higher concentration of H outside the cell to drive the concentration of lactose inside the cell. (Adapted from Lehninger, Principles of Biochemistry, Third Edition, by David L. Nelson and Michael M. Cox. 1982, 1992, 2000 by Worth Publishers. Used with permission of W. H. Freeman and Company.)... 

Explain the stoichiometric relationship among the concentrations of (3-galactosidase, galactoside permease, and thiogalactoside transacetylase in E. coli. 

The y gene codes for M protein, a protein associated with the P-galactoside transport system. This protein was formerly called galactoside permease, but that term has now been reserved to indicate the entire transport system. The mechanism of transport which facilitates entry of /S-galactosides into the cell has been reviewed by Kennedy [83] and Kepes [84]. The permease system has the ability to allow induced cells to concentrate galactosides 100-fold over their concentration in the external medium and requires the presence of a supply of unsaturated fatty acids for activity [84a]. 

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