Amplification of the lactose carrier protein in Escherichia coli using a plasmid vector

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.

Preparation, characterization, and properties of monoclonal antibodies against the lac carrier protein from Escherichia coli

Monoclonal antibodies directed against the lac carrier protein purified from the membrane of Escherichia coli were prepared by somatic cell fusion of mouse myeloma cells with splenocytes from an immunized mouse. Several clones produce antibodies that react with the purified protein as demonstrated by solid-phase radioimmunoassay and by immunoblotting experiments; culture supernatants from the clones inhibit active transport of lactose in isolated membrane vesicles. Five stable clones were selected for expansion, formal cloning, and production of ascites fluid, and the antibodies secreted in vivo by each clone also were found to inhibit lactose transport. Antibody from hybridoma 4B1, an IgG2a immunoglobulin, inhibits active transport of lactose in proteoliposomes reconstituted with purified lac carrier and in right-side-out membrane vesicles. In contrast, the antibody has no effect on the generation of the proton electrochemical gradient by membrane vesicles nor does it alter the ability of vesicles containing the lac carrier to bind p-nitrophenyl-alpha-D-galactopyranoside. In order to achieve 50% inhibition of transport activity, a 2- to 3-fold molar excess of antibody to lac carrier is required, regardless of the amount of lac carrier in the membrane. Thus, the concentration of antibody required for a given degree of inhibition is proportional to the amount of lac carrier in the membrane. Finally, antibody-induced inhibition occurs within seconds, an observation suggesting that the epitope is accessible on the surface of the membrane.

Lactose carrier protein of Escherichia coli. Reconstitution of galactoside binding and countertransport

A procedure for the reconstitution of the lactose carrier protein, a galactoside:proton symporter in Escherichia coli, is described. Starting from cytoplasmic membranes derived from carrier-overproducing strains, essentially all proteins including 89% of the carrier are solubilized by a mixture of dodecyl/tetradecyl polyoxyethylene (n = 9.5) ether and dodecyl O-beta-D-maltoside. In the micellar state the carrier binds substrates with reduced affinity. Addition of E. coli phospholipids and removal of detergents by a hydrophobic column yields small vesicles (50-60-nm diameter). In these vesicles, about 70% of the carrier is recovered and reconstituted carrier is identical to native carrier in terms of substrate binding. After fusion of the small vesicles into larger vesicles (1-5 micrometers), rapid countertransport of galactosides is demonstrated. Attempts to show active galactoside transport by the imposition of artificial electrical potential or pH gradients were unsuccessful, most likely because the reconstituted vesicles are in fact highly permeable to protons.

Isolation of Plasmid DNA from Butyrivibrio fibrisolvens

A procedure based on successive precipitation of cell lysates with sodium dodecyl sulfate-NaCl and polyethylene glycol 6000 was developed which allows the isolation of plasmid DNA from Butyrivibrio fibrisolvens. A survey of B. fibrisolvens strains isolated from the bovine rumen showed that plasmids are a common feature of this species.

Enhanced expression of cro-beta-galactosidase fusion proteins under the control of the PR promoter of bacteriophage lambda

Hybrid plasmids carrying cro-lacZ gene fusions have been constructed by joining DNA segments carrying the PR promoter and the start of the cro gene of bacteriophage lambda to the lacZ gene fragment carried by plasmid pLG400 . Plasmids in which the translational reading frames of the cro and lacZ genes are joined in-register (type I) direct the synthesis of elevated levels of cro-beta-galactosidase fusion protein amounting to 30% of the total cellular protein, while plasmids in which the genes are fused out-of-register (type II) produce a low level of beta-galactosidase protein. Sequence rearrangements downstream of the cro initiator AUG were found to influence the efficiency of translation, and have been correlated with alterations in the RNA secondary structure of the ribosome-binding site. Plasmids which direct the synthesis of high levels of beta-galactosidase are conditionally lethal and can only be propagated when the PR promoter is repressed. Deletion of sequences downstream of the lacZ gene restored viability, indicating that this region of the plasmid encodes a function which inhibits the growth of the cells. The different applications of these plasmids for expression of cloned genes are discussed.

Mutations in the lacY gene of Escherichia coli define functional organization of lactose permease

Mutations in the lacY gene of Escherichia coli have been used to analyze the functional organization of lactose permease. Deletions suggest that the NH2 terminus of lactose permease is not essential and can be replaced by residues of the cytoplasmic enzyme beta-galactosidase. Negative dominant mutations in the lacY gene can be explained by the assumption that membrane-associated lactose permease is active as a dimer or oligomer. The map positions of these mutations and other point mutations that lower or alter the sugar specificity define regions of lactose permease involved in sugar or proton binding and transport.

Molecular cloning of the tolC locus of Escherichia coli K-12 with the use of transposon Tn10

We have cloned the tolC gene of E. coli K-12 into pSF2124 by using transposon Tn10 as the marker to first isolate the relevant DNA fragment. The gene is on a 10.5 kb EcoRI fragment, and Tn5 insertion mutagenesis locates the gene near one end of this EcoRI fragment. An EcoRI-PstI fragment has been subcloned into pBR322 to facilitate further analysis of the gene.

Identification of the AraE transport protein of Escherichia coli

1. Two arabinose-inducible proteins are detected in membrane preparations from strains of Escherichia coli containing arabinose-H+ (or fucose-H+) transport activity; one protein has an apparent subunit relative molecular mass (Mr) of 36 000-37 000 and the other has Mr 27 000. 2. An araE deletion mutant was isolated and characterized; it has lost arabinose-H+ symport activity and the arabinose-inducible protein of Mr 36 000, but not the protein of Mr 27 000. 3. An araE+ specialized transducing phage was characterized and used to re-introduce the araE+ gene into the deletion strain, a procedure that restores both arabinose-H+ symport activity and the protein of Mr 36,000. 4. N-Ethylmaleimide inhibits arabinose transport and partially inhibits arabinose-H+ symport activity. 5. N-Ethylmaleimide modifies an arabinose-inducible protein of Mr 36 000-38 000, and arabinose protects the protein against the reagent. 6. These observations identify an arabinose-transport protein of Escherichia coli as the product of the araE+ gene. 7. The protein was recognized as a single spot staining with Coomassie Blue after two-dimensional gel electrophoresis.

Substrate specificity and transport properties of the glycerol facilitator of Escherichia coli

The specificity of the glycerol facilitator (glpF) of Escherichia coli was studied with an osmotic method. This transport system allowed the entry of polyols (glycerol and erythritol), pentitols, and hexitols. The analogous sugars were not transported. However, urea, glycine, and DL-glyceraldehyde could use this pathway to enter the cell. The glpF protein allowed the rapid efflux of preequilibrated xylitol. Glycerol surprisingly did not inhibit the uptake of xylitol, and xylitol only slightly reduced the uptake of glycerol. The observation and the insensitivity of the xylitol transport to low temperature suggest that the facilitator behaves as a membrane channel.

Lactose carrier protein of Escherichia coli. Structure and expression of plasmids carrying the Y gene of the lac operon

The previously described hybrid plasmid pC7 which carries lacI+O+delta(Z)Y+A+ on a 12.3 X 10(6)-Mr DNA fragment [Teather et al. (1978) Mol. Gen. Genet. 159, 239-248] was partially digested with the restriction endonuclease EcoRI under conditions reducing the recognition sequence to d(A-A-T-T) and ligated to the vector pB322. lac Y-carrying inserts of various sized (Mr 1.5-4.7 X 10(6)) were obtained. Hybrid plasmid pTE18 (2300-base-pair insert) carries part of the I (repressor) gene, the promotor-operator region, part of the Z (beta-galactosidase) gene, the Y (lactose carrier) gene and part of the A (transacetylase) gene. Upon induction of pTE18-harbouring strains the Y-gene product is expressed at a nearly constant rate for several generations and accumulates to a level of 12-16% of the total cytoplasmic membrane protein. Integration into the membrane leads to active carrier as judged by binding and transport measurements.

Evidence for phase separation in the membrane of an osmotically stabilized fatty acid auxotroph of E. coli and its biological significance

1. An unsaturated fatty acid auxotroph of Escherichia coli accumulated a high content of saturated fatty acids in its membrane when it was cultured under osmotically stabilized conditions. The physicochemical properties of the phospholipid extracts and of the membrane fraction from the cells were investigated by means of proton magnetic resonance, infrared spectroscopy and differential scanning calorimetry. 2. Physicochemical studies indicate that the phospholipid bilayers in the membranes exhibit at least two phase transitions, a minor one at approx. 19 degrees C and a major one at approx. 43 degrees C. Between the two temperatures, gel and liquid crystalline domains co-exist. Moreover, even in the gel state, phospholipids seem to segregated into domains containing different proportions of unsaturated fatty acids. 3. The Arrhenius plot of beta-galactoside transport rates is biphasic. The inflection point is at 22 degrees C. This means that the appearance of the fluid region in the bilayer at approx. 19 degrees C is important in the activation of membrane transport.

Sequence of the lactose permease gene

The nucleotide sequence of the lacY gene coding for lactose permease (M protein) in Escherichia coli has been determined. The sequence includes the intergenic regions between the lacZ (beta-galactosidase) and lacY genes as well as the region between the lacY and lacA (transacetylase) genes. Lactose permease is predicted to consist of 417 residues (71% nonpolar), resulting in a protein with a molecular weight of 46,504. The reading frame was confirmed by the sequence of a nonsense mutation changing codon 33 from UGG to UAG.

In vitro and in vivo products of E. coli lactose permease gene are identical

The lacY gene product synthesised in vitro is identical to lactose permease isolated from cytoplasmic membranes as determined by apparent molecular weight and N-terminal amino acid sequence. The amino acid composition of the in vivo product agrees well with that predicted from the DNA sequence. The data assign the translational start on the DNA sequence and demonstrate that this protein is processed only by deformylation but not by proteolytic cleavage at the N-terminus.

Transport of alpha-p-nitrophenylgalactoside by the lactose carrier of Escherichia coli

alpha-p-Nitrophenylgalactoside was found to be accumulated by the lactose transport-system of Escherichia coli. This fact may help to resolve the differences in the reported number of sugar binding sites of the lactose transport protein in nonenergized and energized membrane vesicles.