Resolution of glpA and glpT loci into separate operons in Escherichia coli K-12 strains

Multiple regulatory elements for the glpA operon encoding anaerobic glycerol-3-phosphate dehydrogenase and the glpD operon encoding aerobic glycerol-3-phosphate dehydrogenase in Escherichia coli: further characterization of respiratory control

In Escherichia coli, sn-glycerol-3-phosphate can be oxidized by two different flavo-dehydrogenases, an anaerobic enzyme encoded by the glpACB operon and an aerobic enzyme encoded by the glpD operon. These two operons belong to the glp regulon specifying the utilization of glycerol, sn-glycerol-3-phosphate, and glycerophosphodiesters. In glpR mutant cells grown under conditions of low catabolite repression, the glpA operon is best expressed anaerobically with fumarate as the exogenous electron acceptor, whereas the glpD operon is best expressed aerobically. Increased anaerobic expression of glpA is dependent on the fnr product, a pleiotropic activator of genes involved in anaerobic respiration. In this study we found that the expression of a glpA1(Oxr) (oxygen-resistant) mutant operon, selected for increased aerobic expression, became less dependent on the FNR protein but more dependent on the cyclic AMP-catabolite gene activator protein complex mediating catabolite repression. Despite the increased aerobic expression of glpA1(Oxr), a twofold aerobic repressibility persisted. Moreover, anaerobic repression by nitrate respiration remained normal. Thus, there seems to exist a redox control apart from the FNR-mediated one. We also showed that the anaerobic repression of the glpD operon was fully relieved by mutations in either arcA (encoding a presumptive DNA recognition protein) or arcB (encoding a presumptive redox sensor protein). The arc system is known to mediate pleiotropic control of genes of aerobic function.

Divergent transcription of the sn-glycerol-3-phosphate active transport (glpT) and anaerobic sn-glycerol-3-phosphate dehydrogenase (glpA glpC glpB) genes of Escherichia coli K-12

The glpTQ operon and the glpA and glpB genes are located adjacent to one another near min 49 of the linkage map of Escherichia coli K-12. The positions and directions of transcription of the glpA and glpB genes with respect to the glpTQ operon were determined in the present work. Strains harboring Mu d1(Ap lac) fusions in either glpA or glpB were converted to the respective lambda p1(209) lysogens. Induction of these lysogens with mitomycin C resulted in production of Lac+ phage progeny which carried adjacent chromosomal DNA. Genetic crosses with a collection of glpT mutant strains were performed with several such phage lines. A fine-structure deletion map of the glpT gene was thus constructed. All phages used for this mapping carried DNA starting with the promoter-proximal end of glpT. This indicated that the glpTQ operon and the glpA and glpB genes are transcribed divergently. Additional evidence supporting this conclusion was obtained by physical mapping of restriction endonuclease cleavage sites in plasmids carrying these genes and in plasmids carrying glpA-lacZ or glpB-lacZ fusions. A new designation (glpC) for the gene encoding the 41,000-Mr subunit of the anaerobic sn-glycerol-3-phosphate dehydrogenase was proposed to distinguish it from the glpA gene, which encodes the 62,000-Mr subunit of the dehydrogenase, and the glpB gene, which encodes a membrane anchor subunit of the dehydrogenase. These three genes were present in an operon transcribed in the order glpA glpC glpB in the clockwise direction on the linkage map of E. coli.

Chromosomal mapping of mutations affecting glycerol and glucose catabolism in Pseudomonas aeruginosa PAO

Mutations causing deficiencies in the inducible, membrane-associated sn-glycerol-3-phosphate dehydrogenase (glpD) and in inducible glucose transport (glcT) were mapped on the Pseudomonas aeruginosa PAO1 chromosome by using the generalized transducing phages F116L and G101. These mutations, in separate catabolic regulatory units, were cotransducible with a previously described cluster of carbohydrate catabolic gene loci (zwf-1 eda-9001 edd-1) that maps at ca. 50 to 53 min on the chromosome. Mutant strain PFB362 (glcT1) did not transport glucose and did not produce a functional, periplasmic, glucose-binding protein that is required for glucose transport. This mutation was cotransducible with zwf-1 (70%), nalA (29%), and phe-2 (19%) but not with glpD1 or leu-10. The glpD1 mutation in strain PRP408 was cotransducible with zwf-1 (5%), eda-9001 (4%), and edd-1 (1%) and also with ami-151 (17%) and phe-2 (33%). These results expand the number of known carbohydrate catabolism genes that are clustered in the 50- to 55-min region of the PAO1 chromosome and allow us to propose the following relative gene order: ami-151 glpD1 phe-2 nalA zwf-1 eda-9001 edd-1 glcT1 leu-10. Three independently obtained nal determinants for high-level resistance to nalidixic acid, which were employed in these studies, exhibited similar cotransduction frequencies with several flanking marker mutations.

Characterization of a glpK transducing phage

A specialized glpK transducing phage, lambda glpK100, has been isolated and characterized with respect to DNA structure. The glpK component of the glpKF operon has been localized within a 2.0 kilobase pair (kbp) region of the approximately 8.24 kbp bacterial DNA insert, and the positions of BamHI, EcoRI and HindIII restriction sites within this DNA have been identified.

Identification of the glpT-encoded sn-glycerol-3-phosphate permease of Escherichia coli, an oligomeric integral membrane protein

A collection of hybrid plasmids carrying either the wild-type or mutated glpT gene was generated in vitro and used to characterize the glpT-dependent active transport system for sn-glycerol-3-phosphate in Escherichia coli K-12. Restriction endonuclease analysis and recloning of DNA fragments localized glpT to a 3-kilobase pair PstI-HpaI segment of DNA. Comparison of DNA carrying glpT-lacZ fusions with DNA carrying intact glpT allowed determination of the direction of transcription. Through characterization of the proteins synthesized by strains harboring hybrid plasmids carrying amber, missense, or deletion mutations in glpT, it was shown that glpT is a promoter-proximal gene in an operon consisting of at least two genes. The gene product of glpT, the sn-glycerol-3-phosphate permease, was found associated with the inner membrane. It could be solubilized by treatment with sodium dodecyl sulfate at 50 degrees C. Its molecular weight, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was dependent upon sample treatment before electrophoresis. The apparent molecular weight was 44,000 when membrane fractions were heated to 50 degrees C; subsequent treatment at 95 degrees C modified the protein such that it migrated faster (apparent molecular weight = 33,000). Several missense mutations in glpT were negatively dominant over wild-type glpT, indicating that the active form of the permease is multimeric. A gene (named glpQ) promoter distal to glpT codes for a periplasmic protein. This protein had previously been named GLPT protein to indicate its relationship to the glpT gene. The present report demonstrates that it is not the gene product of glpT and is not required for active transport of sn-glycerol-3-phosphate.

Only one gene is required for the glpT-dependent transport of sn-glycerol-3-phosphate in Escherichia coli

Deletion and point mutants defective in the glpT-dependent sn-glycerol-3-phosphate transport system were isolated and located on the Escherichia coli chromosome. They mapped in glpT in the clockwise order gyrA, glpA, glpT at around 48 min on the Escherichia coli linkage map. The mutations within glpT were ordered by deletion mapping, three factor crosses, and by crosses involving lambda transducing bacteriophages carrying glpT-lac operon fusions. Results obtained using these fusion phages indicated that glpT is transcribed in the counterclockwise direction on the E. coli linkage map. Complementation analysis using these mutants revealed only one complementation group. Thus, one gene is necessary and sufficient for the proton motive force-dependent sn-glycerol-3-phosphate transport system.

Use of Escherichia coli operon-fusion strains for the study of glycerol 3-phosphate transport activity

Strains of Escherichia coli K-12 deleted in the native lac operon and bearing both a wild-type glpT operon encoding for sn-glycerol 3-phosphate (G3P) transport and a hybrid operon in which glpT operator and promoter regions are fused to the lacZ gene were constructed. In strains with such a hybrid operon, beta-galactosidase and beta-galactoside permease become inducible by G3P. In these mutants the function and maturation of the glpT-coded proteins should be distinguishable from the level of gene expression, since the beta-galactosidase activity can serve as an index of the latter. With the aid of such mutants, it was shown that: (i) the expressions of the two neighboring operons, glpT and glpA (encoding anaerobic G3P dehydrogenase), are not coordinate; (ii) upon induction, the appearance of the cytoplasmic beta-galactosidase activity preceded that of methyl-beta-D-thiogalactoside transport activity (requiring only a cytoplasmic membrane protein) by about 4 min and that of G3P transport activity (requiring both a cytoplasmic membrane protein and a periplasmic protein) by about 9 min; and (iii) when cells grown at several temperatures from 24 to 42 degrees C were measured for G3P transport activity at 30 degrees C, the activity increased with the growth temperature, indicating that, within the range studied, the rate of transport increases with the fluidity of membrane phospholipids.