Salmonella locus affecting phosphoenolpyruvate synthase activity identified by a deletion analysis

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.

Cloning and nucleotide sequence of the Escherichia coli K-12 ppsA gene, encoding PEP synthase

We have cloned and sequenced the Escherichia coli K-12 ppsA gene. The ppsA gene codes for PEP synthase, which converts pyruvate into phosphoenolpyruvate (PEP), an essential step in gluconeogenesis when pyruvate or lactate are used as a carbon source. The open reading frame consists of 792 amino acids and shows homology with other phosphohistidine-containing enzymes that catalyze the conversion between pyruvate and PEP. These enzymes include pyruvate, orthophosphate dikinases from plants and Bacteroides symbiosus and Enzyme I of the bacterial PEP:carbohydrate phosphotransferase system.

Characterization of phosphoenolpyruvate synthase mutants in Salmonella typhimurium

The enteric bacteria are able to grow by utilizing three-carbon compounds (pyruvate, lactate, and alanine) as sole carbon sources only if they have a functional phosphoenolpyruvate synthase (PEP synthase). PEP synthase catalyzes the phosphorylation of pyruvate to PEP with the hydrolysis of ATP to AMP. This anaplerotic reaction is needed for the synthesis of carbohydrates and citric acid cycle intermediates that are essential for continued cell growth. Insertion mutants were isolated in Salmonella typhimurium that specifically lack the ability to grow on three-carbon compounds. These mutants also fail to utilize acetate as a sole carbon source. Enzyme assays were performed and the results showed that these mutants contain no PEP synthase activity. By using bacteriophage P22, the pps mutations isolated in this study were found to be contransducible with genetic markers in both the aroD and btuC genes. Three-factor crosses pinpointed the order of these genes and their distances with respect to each other. One of the mutants carries a pps::lac operon fusion. This fusion was used to explore the transcriptional regulation of the pps gene. A functional copy of the pps gene is required for its own induction. The pps gene is also under catabolite repression, but the addition of adenosine 3',5'-cyclic monophosphate (cyclic AMP) to cells grown in the presence of glucose does not relieve this repression. These results indicate that the synthesis of PEP synthase is regulated in a more complex manner than has been previously recognized.

The repressor of the PEP:fructose phosphotransferase system is required for the transcription of the pps gene of Escherichia coli

We have cloned the pps gene, coding for PEP synthase, of Escherichia coli. PEP synthase catalyses the ATP-dependent conversion of pyruvate into phosphoenol-pyruvate and is required for gluconeogenesis. The pps gene was cloned by an in vivo cloning method using a mini-Mulac bacteriophage containing a plasmid replicon. Upon expression of the cloned pps gene in the maxicell system a protein with an apparent molecular weight of 84 kDa was synthesized. The position of the pps gene of the plasmid was localized by restriction analysis of isolated transposon insertions and the determination of the PEP synthase activities of the different clones. An operon fusion between the pps gene and the galK gene was constructed. Measurements of the galactokinase activity in Salmonella typhimurium galK and galK fruR mutants showed that the transcription of the pps gene requires the presence of FruR, the repressor of the PEP: fructose phosphotransferase system (PTS) in E. coli and S. typhimurium. To test whether the components of the Fructose PTS, in particular FPr, are involved in the expression of the pps gene, we investigated a S. typhimurium galK strain, containing the fusion plasmid, in which the chromosomal fru operon was inactivated by a transposon insertion. Measurements of the galactokinase activity showed that the absence of the Fructose PTS proteins has no significant influence on the regulation of the pps gene.

Genetic expression of enzyme I activity of the phosphoenolpyruvate:sugar phosphotransferase system in ptsHI deletion strains of Salmonella typhimurium

Mutants expressing a novel enzyme I of the phosphoenolpyruvate:sugar phosphotransferase system, termed enzyme I, were isolated from strains of Salmonella typhimurium which were deleted for the HPr and enzyme I structural genes. The mutations lay in a newly defined gene, termed ptsJ, which mapped on the S. typhimurium chromosome between the ptsHI operon and the cysA gene.

Evidence for regulation of gluconeogenesis by the fructose phosphotransferase system in Salmonella typhimurium

A genetic locus designated fruR, previously mapped to min 3 on the Salmonella typhimurium chromosome, gave rise to constitutive expression of the fructose (fru) regulon and pleiotropically prevented growth on all Krebs cycle intermediates. Regulatory effects of fruR were independent of cyclic AMP and its receptor protein and did not prevent uptake of Krebs cycle intermediates. Instead, the phosphotransferase system appeared to regulate gluconeogenesis by controlling the activities of phosphoenolpyruvate carboxykinase and phosphoenolpyruvate synthase.

Relationship between pseudo-HPr and the PEP: fructose phosphotransferase system in Salmonella typhimurium and Escherichia coli

We have studied in Salmonella typhimurium and Escherichia coli the properties of pseudo-HPr suppressor mutations. These mutations suppressed the defects in a ptsH mutant which lacks HPr, one of the enzymes of the phosphoenolpyruvate: carbohydrate phosphotransferase system. The suppressor mutation was mapped in S. typhimurium at 3 min, closely linked to leu. The corresponding chromosomal fragment of 1.7 kb from S. typhimurium and E. coli (extending clockwise from ilvH) was cloned. In a maxicell system a protein with an approximate molecular weight of 36,000 was synthesized. Pseudo-HPr suppressor mutations (fruR) and a deletion extending clockwise from leu resulted in the constitutive expression of the fru operon containing the genes for IIFru (fruA), IIIFru (fruB), fructose 1-phosphate kinase (fruK) and pseudo-HPr (fruF). fruR probably codes for a repressor of the fru operon. Tn10 mutagenesis revealed the following order of genes in the fru operon: fruB-(fruK, fruF)-fruA. Pseudo-HPr activity could replace HPr in PEP-dependent phosphorylation of PTS carbohydrates. IIIFru could be phosphorylated both via HPr and pseudo-HPr, since mutants lacking pseudo-HPr activity were still able to phosphorylate fructose in the presence of added HPr. Both the pseudo-HPr suppressor mutations at 3 min and the deletion extending from leu had an additional phenotype. Introduction of these mutations or deletions was always accompanied by disappearance of PEP synthase activity. Complementation of such a mutant with the cloned fragments reversed both phenotypes at the same time. Possibly, the fruR gene product acts as an activator of the gene coding for PEP synthase.

Positive selection for loss of tetracycline resistance

A simple technique has been devised that allows direct plate selection of tetracycline-sensitive clones from a predominantly tetracycline-resistant population. The technique is especially useful in genetic methodologies based on the use of tetracycline resistance transposons, such as Tn10. Potential uses of the method include selection of deletion mutants, fine-structure mapping, generalized mapping, construction of multiply marked strains, elimination of tetracycline resistance transposons and plasmids and cloning. The technique is based on our finding that tetracycline-resistant cells are hypersensitive to lipophilic chelating agents, such as fusaric acid. This finding supports the contention that certain metal ions critically facilitate tetracycline uptake and leads us to suggest possible molecular mechanisms for tetracycline resistance.