Enhancement of alpha-methylglucoside efflux by respiration in respiratory mutants of Escherichia coli K-12

Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria

Numerous gram-negative and gram-positive bacteria take up carbohydrates through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). This system transports and phosphorylates carbohydrates at the expense of PEP and is the subject of this review. The PTS consists of two general proteins, enzyme I and HPr, and a number of carbohydrate-specific enzymes, the enzymes II. PTS proteins are phosphoproteins in which the phospho group is attached to either a histidine residue or, in a number of cases, a cysteine residue. After phosphorylation of enzyme I by PEP, the phospho group is transferred to HPr. The enzymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to the carbohydrates. Biochemical, structural, and molecular genetic studies have shown that the various enzymes II have the same basic structure. Each enzyme II consists of domains for specific functions, e.g., binding of the carbohydrate or phosphorylation. Each enzyme II complex can consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity. The genetics of the PTS is complex, and the expression of PTS proteins is intricately regulated because of the central roles of these proteins in nutrient acquisition. In addition to classical induction-repression mechanisms involving repressor and activator proteins, other types of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins are involved in chemotaxis toward PTS carbohydrates. Furthermore, the IIAGlc protein, part of the glucose-specific PTS, is a central regulatory protein which in its nonphosphorylated form can bind to and inhibit several non-PTS uptake systems and thus prevent entry of inducers. In its phosphorylated form, P-IIAGlc is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By sensing the presence of PTS carbohydrates in the medium and adjusting the phosphorylation state of IIAGlc, cells can adapt quickly to changing conditions in the environment. In gram-positive bacteria, it has been demonstrated that HPr can be phosphorylated by ATP on a serine residue and this modification may perform a regulatory function.

Novel phosphoenolpyruvate-dependent futile cycle in Streptococcus lactis: 2-deoxy-D-glucose uncouples energy production from growth

The addition of 2-deoxy-D-glucose to cultures of Streptococcus lactis 133 that were growing exponentially on sucrose or lactose reduced the growth rate by ca. 95%. Inhibition did not occur with glucose or mannose as the growth sugar. The reduction in growth rate was concomitant with rapid accumulation of the analog in phosphorylated form (2-deoxy-D-glucose 6-phosphate) via the phosphoenolpyruvate-dependent mannose:phosphotransferase system. Within 5 min the intracellular 2-deoxy-D-glucose 6-phosphate concentration reached a steady-state level of greater than 100 mM. After maximum accumulation of the sugar phosphate, the rate of sucrose metabolism (glycolysis) decreased by only 30%, but the cells were depleted of fructose-1,6-diphosphate. The addition of glucose to 2-deoxy-D-glucose 6-phosphate preloaded cells caused expulsion of 2-deoxy-D-glucose and a resumption of normal growth. S. lactis 133 contained an intracellular Mg2+-dependent, fluoride-sensitive phosphatase which hydrolyzed 2-deoxy-D-glucose 6-phosphate (and glucose 6-phosphate) to free sugar and inorganic phosphate. Because of continued dephosphorylation and efflux of the non-metabolizable analog, the maintenance of the intracellular 2-deoxy-D-glucose 6-phosphate pool during growth stasis was dependent upon continued glycolysis. This steady-state condition represented a dynamic equilibrium of: (i) phosphoenolpyruvate-dependent accumulation of 2-deoxy-D-glucose 6-phosphate, (ii) intracellular dephosphorylation, and (iii) efflux of free 2-deoxy-D-glucose. This sequence of events constitutes a futile cycle which promotes the dissipation of phosphoenolpyruvate. We conclude that 2-deoxy-D-glucose functions as an uncoupler by dissociating energy production from growth in S. lactis 133.

Regulation of methyl-beta-d-thiogalactopyranoside-6-phosphate accumulation in Streptococcus lactis by exclusion and expulsion mechanisms

Starved cells of Streptococcus lactis ML3 (grown previously on galactose, lactose, or maltose) accumulated methyl-beta-D-thiogalactopyranoside (TMG) by the lactose:phosphotransferase system. More than 98% of accumulated sugar was present as a phosphorylated derivative, TMG-6-phosphate (TMG-6P). When a phosphotransferase system sugar (glucose, mannose, 2-deoxyglucose, or lactose) was added to the medium simultaneously with TMG, the beta-galactoside was excluded from the cells. Galactose enhanced the accumulation of TMG-6P. Glucose, mannose, lactose, or maltose plus arginine, was added to a suspension of TMG-6P-loaded cells of S. lactis ML3, elicited rapid expulsion of intracellular solute. The material recovered in the medium was exclusively free TMG. Expulsion of galactoside required both entry and metabolism of an appropriate sugar, and intracellular dephosphorylation of TMG-6P preceded efflux of TMG. The rate of dephosphorylation of TMG-6P by permeabilized cells was increased two-to threefold by adenosine 5'-triphosphate but was strongly inhibited by fluoride. S. lactis ML3 (DGr) was derived from S. lactis ML3 by positive selection for resistance to 2-deoxy-D-glucose and was defective in the enzyme IIMan component of the glucose:phosphotransferase system. Neither glucose nor mannose excluded TMG from cells of S. lactic ML3 (DGr), and these two sugars failed to elicit TMG expulsion from preloaded cells of the mutant strain. Accumulation of TMG-6P by S. lactis ML3 can be regulation by two independent mechanisms whose activities promote exclusion or expulsion of galactoside from the cell.