Genetic control of inducer exclusion by Escherichia coli

Reverse genetics of Escherichia coli glycerol kinase allosteric regulation and glucose control of glycerol utilization in vivo

Reverse genetics is used to evaluate the roles in vivo of allosteric regulation of Escherichia coli glycerol kinase by the glucose-specific phosphocarrier of the phosphoenolpyruvate:glycose phosphotransferase system, IIA(Glc) (formerly known as III(glc)), and by fructose 1,6-bisphosphate. Roles have been postulated for these allosteric effectors in glucose control of both glycerol utilization and expression of the glpK gene. Genetics methods based on homologous recombination are used to place glpK alleles with known specific mutations into the chromosomal context of the glpK gene in three different genetic backgrounds. The alleles encode glycerol kinases with normal catalytic properties and specific alterations of allosteric regulatory properties, as determined by in vitro characterization of the purified enzymes. The E. coli strains with these alleles display the glycerol kinase regulatory phenotypes that are expected on the basis of the in vitro characterizations. Strains with different glpR alleles are used to assess the relationships between allosteric regulation of glycerol kinase and specific repression in glucose control of the expression of the glpK gene. Results of these studies show that glucose control of glycerol utilization and glycerol kinase expression is not affected by the loss of IIA(Glc) inhibition of glycerol kinase. In contrast, fructose 1,6-bisphosphate inhibition of glycerol kinase is the dominant allosteric control mechanism, and glucose is unable to control glycerol utilization in its absence. Specific repression is not required for glucose control of glycerol utilization, and the relative roles of various mechanisms for glucose control (catabolite repression, specific repression, and inducer exclusion) are different for glycerol utilization than for lactose utilization.

A global repressor (Mlc) is involved in glucose induction of the ptsG gene encoding major glucose transporter in Escherichia coli

Glucose stimulates the expression of ptsG encoding the major glucose transporter in Escherichia coli. We isolated Tn 10 insertion mutations that confer constitutive expression of ptsG. The mutated gene was identified as mlc, encoding a protein that is known to be a repressor for transcription of several genes involved in carbohydrate utilization. Expression of ptsG was eliminated in a mlc crp double-negative mutant. The Mlc protein was overproduced and purified. In vitro transcription studies demonstrated that transcription of ptsG is stimulated by CRP-cAMP and repressed by Mlc. The action of Mlc is dominant over that of CRP-cAMP. DNase I footprinting experiments revealed that CRP-cAMP binds at two sites centred at -40.5 and -95.5 and that Mlc binds at two regions centred around -8 and -175. The binding of CRP-cAMP stimulated the binding of RNA polymerase to the promoter while Mlc inhibited the binding of RNA polymerase but not the binding of CRP-cAMP. Gel-mobility shift assay indicated that glucose does not affect the Mlc binding to the ptsG promoter. Our results suggest that Mlc is responsible for the repression of ptsG transcription and that glucose modulates the Mlc activity by unknown mechanism.

Location of galP , a gene which specifies galactose permease activity, on the Escherichia coli linkage map

A mutation which affects the activity of galactose permease in Escherichia coli K12 has been mapped. The mutant allele, designated galP , is cotransducible with the genetic markers for fructose 1, 6-bisphosphate aldolase ( fda ) and 3-phosphoglycerate dehydrogenase ( serA ), and lies on the opposite side of fda from serA at about minute 62 on the E. coli linkage map.

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.

Repression of inducible enzyme synthesis in a mutant of Escherichia coli K 12 deleted for the ptsH gene

The genome of lambda phage with thermosensitive repressor was inserted into the pts region of the Escherichia coli chromosome. This lysogenic culture possessed the PTS1 phenotype at 30 degrees C. A mutant strain with a deletion covering the ptsH gene was isolated after a prophage curing procedure. The deletion nature of the pts mutation was confirmed in genetical and biochemical experiments. The deletion covered a small fragment of the bacterial genome not extending in the ptsI and lig genes. The isolated deltaptsH mutant possessed all characteristics of known pts mutants: pleiotropical disturbances of transport and utilization of a number of carbohydrates, repression of the enzyme inducible synthesis, and resistance to glucose catabolite repression. From these and other data we can conclude that the phosphorylated form of the heat-stable protein HPr is involved (directly or indirectly) in activation of the DNA transcription process.

Catabolite repression in Escherichia coli K12 mutants defective in glucose transport

The phenomenon of glucose catabolite repression was studied in Escherichia coli mutants unable to transport this carbohydrate. The pts I,H mutant P34 was much less sensitive to permanent and transient repressive effect of glucose on beta-galactosidase synthesis than parental type. The 1103 mutant with lack of enzyme 1 of the phosphoenolpyruvate-dependent phosphotransferase system (ptsI) behaves as well as P34 mutant after addition of glucose to casamino acids mineral medium. But in minimal medium with succinate as the sole source of carbon cells of the 1103 mutant (in accordance with the data of Perlman and Pastan, 1969) show hightened sensibility to transient glucose repression. The effect of hypersensibility disappears when the lacI mutation rendering the beta-galactosidase synthesis to costitutivity is introduced in 1103 mutant. It is shown that the hightened sensibility of beta-galactosidase synthesis to glucose transient repression in 1103 mutant is not an effect of the pts mutation and most probably is due to "inducer exclusion" of the lac operon. It is also shown that if one introduces the P34 mutation in strain devoided of one of the enzymes II for glucose (gptA) (and due to this resistant to glucose catabolite repression) then the level of resistance in double mutant does not increase in spite of considerable supression of 14C glucose accumulation. It is discussed the role of separate components of Escherichia coli K12 glucose transport system in realization of the phenomenon of catabolite repression.