Identification of the phosphocarrier protein enzyme IIIgut: essential component of the glucitol phosphotransferase system in Salmonella typhimurium

Effect of replacing the general energy-coupling proteins of the PEP:sugar phosphotransferase system of Salmonella typhimurium with their fructose-inducible counterparts on utilization of the PTS sugar glucitol

A strain of Salmonella typhimurium in which the genes encoding the general phosphoenolpyruvate:sugar phosphotransferase system (PTS) proteins HPr and Enzyme I have been deleted, the normally cryptic gene encoding the fructose-inducible Enzyme I (EI* or EI(fructose)) is expressed, and the fructose repressor protein is inactive (fruR or cra mutant) was studied. This strain lacks HPr and EI, but expresses FPr (DTP) and EI(fructose) constitutively. Since FPr and EI(fructose) can substitute for HPr and EI, the strain grew in minimal liquid medium supplemented with the PTS sugars glucose, fructose, N-acetylglucosamine, mannitol or mannose. However, it showed very poor to negligible growth on the PTS sugar glucitol. It also grew very poorly on the non-PTS sugars maltose, melibiose and especially glycerol. Adding cAMP to the medium allowed growth on glucitol, but did not affect growth on glycerol. We suggest that poor phosphorylation of the regulatory molecule Enzyme IIA(glucose) by FPr is responsible for these effects.

Temperature-dependent fermentation of D-sorbitol in Escherichia coli O157:H7

The influence of growth temperature on the ability to ferment D-sorbitol was investigated in Escherichia coli O157:H7. It was found that O157:H7 strains have a temperature-sensitive sorbitol phenotype. D-Sorbitol transport and sorbitol-6-phosphate dehydrogenase activities were expressed in sorbitol-fermenting cells grown at 30 degrees C but only at a low level at 40 degrees C. Sorbitol-positive variants able to transport D-sorbitol were easily selected at 30 degrees C from culture of Sor(-) E. coli O157:H7 strains.

Virulence of an Escherichia coli O157:H7 sorbitol-positive mutant

Virulence and pathogenicity of an Escherichia coli O157:H7 sorbitol-positive mutant were investigated with an infant rabbit animal model as well as a battery of in vitro assays. Total cell lysate protein profiles, outer membrane protein profiles, plasmid profiles, and levels of cytotoxic activity against Vero cells were similar in the wild-type and mutant strains. Both adhered to intestinal epithelial cells in culture and reacted with fluorescein isothiocyanate-labeled antiserum against E. coli O157:H7. The mutant appeared to be similar to the wild type in all respects except in its ability to ferment sorbitol. [14C]sorbitol uptake and sorbitol-6-phosphate dehydrogenase activities were notably increased in the mutant strain. Diarrhea developed in rabbits administered the wild-type strain and in those fed the sorbitol-positive mutant. There was greater bacterial attachment and mucosal damage in the cecum and large intestine than in the small intestine. Scanning electron microscopy revealed bacteria adhering as single cells and as aggregates closely associated with mucus. Mucosal lesions consisted of areas of tissue necrosis with sloughing of epithelial cells. By transmission electron microscopy, electron-dense necrotic epithelial cells were visible in areas where bacteria were present, and epithelial cell debris containing bacteria was observed between the villar luminal surfaces. Light microscopy of epithelial cells of intestinal sections of infected rabbits revealed noticeable vacuolation and spherical, pyknotic nuclei. These data indicate that the sorbitol-negative phenotype is not associated with the pathogenicity of E. coli O157:H7.

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.

Physical and genetic characterization of the glucitol operon in Escherichia coli

The glucitol (gut) operon has been identified in the colony bank of Clark and Carbon (A. Sancar and W. D. Rupp, Proc. Natl. Acad. Sci. USA 76:3144-3148, 1979). We subcloned the gut operon by using paCYC184, pACYC177, and pBR322. The operon, which is encoded in a 3.3-kilobase nucleotide fragment, consists of the gutC, gutA, gutB, and gutD genes. The repressor of the gut operon seemed to be encoded in the region downstream from the operon. The gene products of the gut operon were identified by using maxicells. The apparent molecular weights of the glucitol-specific enzyme II (product of the gutA gene), enzyme III (product of the gutB gene), and glucitol-6-phosphate dehydrogenase (product of the gutD gene) were about 46,000, 13,500, and 27,000, respectively, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Evidence against direct involvement of cyclic GMP or cyclic AMP in bacterial chemotactic signaling

Defects in phosphotransferase chemotaxis in cya and cpd mutants previously cited as evidence of a cyclic GMP or cyclic AMP intermediate in signal transduction were not reproduced in a study of chemotaxis in Escherichia coli and Salmonella typhimurium. In cya mutants, which lack adenylate cyclase, the addition of cyclic AMP was required for synthesis of proteins that were necessary for phosphotransferase transport and chemotaxis. However, the induced cells retained normal phosphotransferase chemotaxis after cyclic AMP was removed. Phosphotransferase chemotaxis was normal in a cpd mutant of S. typhimurium that has elevated levels of cyclic GMP and cyclic AMP. S. typhimurium crr mutants are deficient in enzyme III glucose, which is a component of the glucose transport system, and a regulator of adenylate cyclase. After preincubation with cyclic AMP, the crr mutants were deficient in enzyme II glucose-mediated transport and chemotaxis, but other chemotactic responses were normal. It is concluded that cyclic GMP does not determine the frequency of tumbling and is probably not a component of the transduction pathway. The only known role of cyclic AMP is in the synthesis of some proteins that are subject to catabolite repression.

The bacterial phosphotransferase system: kinetic characterization of the glucose, mannitol, glucitol, and N-acetylglucosamine systems

The kinetic mechanisms by which the glucose, glucitol, N-acetylglucosamine, and mannitol enzymes II catalyze sugar phosphorylation have been investigated in vitro. Lineweaver-Burk analyses indicate that the glucose and glucitol enzymes II catalyze sugar phosphorylation by a sequential mechanism when the two substrates are phospho-enzyme III and sugar. The N-acetylglucosamine and mannitol enzymes II, which do not function with an enzyme III, catalyze sugar phosphorylation by a ping-pong mechanism when the two substrates are phospho-HPr and sugar. These results, as well as previously published kinetic characterizations, suggest a common kinetic mechanism for all enzymes II of the system. It is suggested that all enzymes II and enzyme II-III pairs arose from a single (fused) gene product containing two sites of phosphorylation and that phosphoryl transfer from the second phosphorylation site to sugar can only occur when the enzyme II-III pair is present in the associated state.