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

Glycerol-3-phosphate transporter of Escherichia coli: structure, function and regulation

Glycerol-3-phosphate (G3P) plays a major role in glycolysis and phospholipid biosynthesis in the cell. Escherichia coli uses a secondary membrane transporter protein, GlpT, to uptake G3P into the cytoplasm. The crystal structure of the protein was recently determined to 3.3 A resolution. The protein consists of an N- and a C-terminal domain, each formed by a compact bundle of six transmembrane alpha-helices. The substrate-translocation pore is found at the domain interface and faces the cytoplasm. At the closed end of the pore is the substrate binding site, which is formed by two arginine residues. In combination with biochemical data, the crystal structure suggests a single binding site, alternating access mechanism for substrate translocation, namely, the substrate bound at the N- and C-terminal domain interface is transported across the membrane via a rocker-switch type of movement of the domains. Furthermore, GlpT may serve as a structural and mechanistic paradigm for other secondary active membrane transporters.

phnE and glpT genes enhance utilization of organophosphates in Escherichia coli K-12

Wild-type Escherichia coli K-12 strain JA221 grows poorly on low concentrations (< or = 1 mM) of diisopropyl fluorophosphate and its hydrolysis product, diisopropyl phosphate (DIPP), as sole phosphorus sources. Spontaneous organophosphate utilization (OPU) mutants were isolated that efficiently utilized these alternate sources of phosphate. A genomic library was constructed from one such OPU mutant, and two genes were isolated that conferred the OPU phenotype to strain JA221 upon transformation. These genes were identified as phnE and glpT. The original OPU mutation represented phnE gene activation and corresponded to the same 8-bp unit deletion from the cryptic wild-type E. coli K-12 phnE gene that has been shown previously to result in phnE activation. In comparison, sequence analysis revealed that the observed OPU phenotype conferred by the glpT gene was not the result of a mutation. PCR clones of glpT from both the mutant and the wild type were found to confer the OPU phenotype to JA221 when they were present on the high-copy-number pUC19 plasmid but not when they were present on the low-copy-number pWSK29 plasmid. This suggests that the OPU phenotype associated with the glpT gene is the result of amplification and overproduction of the glpT gene product. Both the active phnE and multicopy glpT genes facilitated effective metabolism of low concentrations of DIPP, whereas only the active phnE gene could confer the ability to break down a chromogenic substrate, 5-bromo-4-chloro-3-indoxyl phosphate-p-toluidine (X-Pi). This result indicates that in E. coli, X-Pi is transported exclusively by the Phn system, whereas DIPP (or its metabolite) may be transported by both Phn and Glp systems.

Utilization of a mini-Dlac transposable element to create an alpha-complementation and regulated expression system for cloning in Pseudomonas aeruginosa

A lac-based alpha-complementation and expression system was developed for use in molecular cloning in Pseudomonas aeruginosa. A bacteriophage D3112-based mini-Dlac transposable element, containing the lacIq-regulated lacZ delta M15 gene next to a selectable marker, was constructed. Mixed D3112 lysates were used to transduce P. aeruginosa PAO1, and derivatives containing randomly inserted chromosomal copies of the mini-Dlac element were obtained. Transformation of the PAO1::mini-Dlac transductants with the broad-host-range vector, pUCP19, led to the formation of blue colonies on indicator medium in the presence of inducer. In contrast, transformants harboring the pUCP19 derivative pCDO, containing the catechol-2,3-dioxygenase (C23O)-encoding xylE gene under lac promoter control, were white on the same medium. Expression of xylE was tightly controlled by single-copy mini-Dlac-encoded lac repressor and in induced cultures was increased more than 100-fold over that observed in uninduced cultures. The usefulness of the system for molecular cloning in P. aeruginosa was demonstrated by ligating size-fractionated PAO1 chromosomal fragments into pUCP19, followed by transformation of the newly isolated PAO1::mini-Dlac host. All randomly chosen white colonies contained recombinant plasmids, with inserts of the correct size range, while blue colonies contained pUCP19 alone. The functionality of the system was also shown in another frequently studied strain, PA103.

The pho regulon-dependent Ugp uptake system for glycerol-3-phosphate in Escherichia coli is trans inhibited by Pi

sn-Glycerol-3-phosphate (G3P) or glyceryl phosphoryl phosphodiesters, the substrates of the phoB-dependent Ugp transport system, when transported exclusively through this system, can serve as a sole source of phosphate but not as a sole source of carbon (H. Schweizer, M. Argast, and W. Boos, J. Bacteriol. 150:1154-1163, 1982). In order to explain this phenomenon, we tested two possibilities: repression of the pho regulon by Ugp-mediated transport and feedback inhibition by internal G3P or its degradation product Pi. Using an ugp-lacZ fusion, we found that the expression of ugp does not decline upon exposure to G3P, in contrast to the repressing effect of transport of Pi via the Pst system. This indicated that the Ugp system becomes inhibited after the uptake and metabolism of G3P. Using 32P-labeled G3P, we observed that little Pi is released by cells taking up G3P via the Ugp system but large amounts of Pi are released when the cells are taking up G3P via the GlpT system. Using a glpD mutant that could not oxidize G3P but which could still phosphorylate exogenous glycerol to G3P after GlpF-mediated transport of glycerol, we could not find trans inhibition of Ugp-mediated uptake of exogenous 14C-G3P. However, when allowing uptake of Pi via Pst, we observed a time-dependent inhibition of 14C-G3P taken up by the Ugp transport system. Inhibition was half maximal after 2 min and could be elicited by Pi concentrations below 0.5 mM. Cells had to be starved for Pi in order to observe this inhibition. We conclude that the activity of the Ugp transport system is controlled by the level of internal phosphate.

Two plasmids, X1918 and Z1918, for easy recovery of the xylE and lacZ reporter genes

Two plasmids, X1918 and Z1918, were constructed which contain the promoter-less xylE and lacZ reporter genes flanked symmetrically by the multiple cloning site (MCS) from pUC19. These cassettes can easily be derived from the multicopy plasmid, pUC1918.

Mutations in phoB, the positive gene activator of the pho regulon in Escherichia coli, affect the carbohydrate phenotype on MacConkey indicator plates

Mutants defective in phoB, the positive gene activator of the Escherichia coli pho regulon, exhibit aberrant behaviour on MacConkey indicator plates. They appear pale in the presence of a fermentable carbon source such as trehalose, maltose or glucose. The addition of at least 5 mM phosphate corrects this defect. Colonies of phoB+ strains turn red on MacConkey indicator plates and derepress the pho regulon when the cells are able to ferment the carbon source. In contrast, the inability to ferment the carbon source maintains the pho regulon in the repressed state.

Effect of glpT and glpD mutations on expression of the phoA gene in Escherichia coli

In vivo 31P nuclear magnetic resonance analysis of Escherichia coli cells showed that the intracellular concentration of P(i) remained constant in wild-type and in a glpT mutant strain whether the cells were grown on excess (2 mM) P(i) or sn-glycerol-3-phosphate as a phosphate source. The function of the phoA promoter (measured by beta-galactosidase activity in a phoA-lacZ fusion strain) was repressed when glpT+ cells were utilizing sn-glycerol-3-phosphate as the sole source of phosphate. These cells were devoid of alkaline phosphatase activity. However, the phoA promoter was fully active in a glpT mutant. These results indicated that the repression of the enzyme synthesis was not due to a variation in the level of cytoplasmic P(i) but was due to the P(i) excreted into the periplasm and/or to the medium.

Carbon-starvation induction of the ugp operon, encoding the binding protein-dependent sn-glycerol-3-phosphate transport system in Escherichia coli

The gene products of the ugp operon of Escherichia coli are responsible for the uptake of sn-glycerol-3-phosphate and certain glycerophosphodiesters. The regulation of ugp is mainly phoBR-dependent. Significant expression, however, can be observed even in the presence of high concentrations of phosphate, a condition which normally completely represses pho expression. Pho-independent ugp expression was found to be derepressed during the late logarithmic growth phase due to carbon starvation. Among different carbon sources tested, glucose caused the most complete repression. Addition of cAMP prevented glucose repression, indicating that a cAMP-CRP control mechanism may be directly or indirectly involved in the carbon-starvation response. This conclusion is supported by the fact that pho-independent ugp expression correlated with the presence of the cya and crp gene products.

Identification and DNA sequence of tdcR, a positive regulatory gene of the tdc operon of Escherichia coli

Efficient in vivo expression of the biodegradative threonine dehydratase (tdc) operon of Escherichia coli is dependent on a regulatory gene, tdcR. The tdcR gene is located 198 base pairs upstream of the tdc operon and is transcribed divergently from this operon. The nucleotide sequence of tdcR and two unrelated reading frames has been determined. The deduced amino acid sequence of TdcR indicates that it is a polypeptide of Mr 12,000 with 99 amino acid residues and contains a potential helix-turn-helix DNA binding motif. Deletion analysis and minicell expression of the tdcR gene suggest that TdcR may serve as a trans-acting positive activator for the tdc operon.

Convenient transfer of lacZ-gene fusions to phage M13 by in vivo recombination and their use for nucleotide sequencing

We describe a method that facilitates the sequencing of lacZ fusion joints based on in vivo subcloning onto phage M13. The method is useful for lacZ fusions that are isolated with the transposable lambda placMu phage into plasmids carrying the pBR322 bla gene. In vivo cloning of lacZ fusions is accomplished by recombination with two M13 phages carrying 5' or 3' segments of the bla gene, adjacent but differing in orientation to lacZ'. The presented method allows rapid sequencing of many fusion joints without subcloning in vitro.

The transmembrane topology of the sn-glycerol-3-phosphate permease of Escherichia coli analysed by phoA and lacZ protein fusions

The Escherichia coli glpT gene encodes a transport protein that mediates uptake of sn-glycerol-3-phosphate. This permease is a member of a class of bacterial organophosphate permeases which transport substrates by antiport with inorganic phosphate. The glpT gene product, probably an oligomer of a single polypeptide chain, is thought to span the cytoplasmic membrane several times, as predicted by the hydropathic profile. Protein fusions, in which varying lengths of the amino-terminal end of the permease is attached to alkaline phosphatase (phoA) and to beta-galactosidase (lacZ) were constructed. On the assumption that phoA fusions only exhibit high enzymatic activity when fused to extra-cytoplasmic regions of the target protein, whereas lacZ fusions will only be active when the beta-galactosidase portion is attached to cytoplasmic domains of the target protein, the activities of the fusions were used to test a two-dimensional model for the permease. The model proposes that GlpT contains 12 transmembrane segments divided by a larger cytoplasmic region. Despite some limitation caused by hot-spot sites of transpositions, the TnphoA approach was consistent with the model. In contrast, we feel that the enzymatic activity of lacZ fusions is only a limited parameter for studying the topology of a complex membrane protein.

Characteristics of a ugp-encoded and phoB-dependent glycerophosphoryl diester phosphodiesterase which is physically dependent on the ugp transport system of Escherichia coli

The ugp-encoded transport system of Escherichia coli accumulates sn-glycerol-3-phosphate with high affinity; it is binding protein mediated and part of the pho regulon. Here, we report that glycerophosphoryl diesters (deacylated phospholipids) are also high-affinity substrates for the ugp-encoded system. The diesters are not taken up in an unaltered form but are hydrolyzed during transport to sn-glycerol-3-phosphate plus the corresponding alcohols. The enzyme responsible for this reaction is not essential for the translocation of sn-glycerol-3-phosphate or for the glycerophosphoryl diesters but can only hydrolyze diesters that are in the process of being transported. Diesters in the periplasm or in the cytoplasm were not recognized, and no enzymatic activity could be detected in cellular extracts. The enzyme is encoded by the last gene in the ugp operon, termed ugpQ. The product of the ugpQ gene, expressed in minicells, has an apparent molecular weight of 17,500. We present evidence that only one major phoB-dependent promoter controls all ugp genes.

Characterization of an iron sensitive Mud1 mutant in E. coli lacking the ribonucleotide reductase subunit B2

The mutant, generated by a Mud1 insertion, formed long non-viable filaments in the presence of iron and air. Under anaerobic conditions normal growth in the presence of iron was observed. The mutation was mapped by P1 transductions at 48 min on the genetic map of Escherichia coli. By Southern blotting the insertion point was determined to be in nrdB, the structural gene for the ribonucleotide reductase subunit B2. The mutation could be complemented by the cloned nrdB gene. Up to now it was assumed that E. coli possesses only one enzyme for the synthesis of deoxyribonucleotides and only conditional lethal (temperature sensitive) mutants were isolated in nrdB. The insertion of Mud1 in nrdB should lead to a complete loss of the essential B2 subunit. Since the strain was able to grow under anaerobic conditions on minimal medium lacking deoxyribonucleotides and additional pathway for the synthesis of deoxyribonucleotides is postulated.

Nucleotide sequence and transcriptional startpoint of the glpT gene of Escherichia coli: extensive sequence homology of the glycerol-3-phosphate transport protein with components of the hexose-6-phosphate transport system

The nucleotide sequences of the glpT gene of Escherichia coli and its regulatory region have been elucidated and the primary structure of the glycerol-3-phosphate transport protein deduced. Extensive amino acid sequence homology was found with two other cytoplasmic membrane proteins: the functionally related hexose-6-phosphate transport protein, and the UHPC protein involved in regulating hexose-6-phosphate uptake. Although no significant amino acid sequence homology was found with other transport proteins, such as the arabinose, citrate, glucose, melibiose, lactose or xylose transporters, all of these proteins share a common secondary structure arrangement with the GLP T protein as they apparently contain twelve membrane-spanning alpha-helical segments. The promoter for glpT was located by transcript mapping and shown to overlap a site to which catabolite activator protein binds in vitro. These findings indicate how catabolite repression may be mediated but do not explain its physiological significance in glycerol metabolism.

Cloning and characterization of the aerobic sn-glycerol-3-phosphate dehydrogenase structural gene glpD of Escherichia coli K-12

The glpD gene encoding aerobic sn-glycerol-3-phosphate dehydrogenase of Escherichia coli K-12 was cloned into pACYC177 from a lambda glpD transducing phage. The recombinant plasmid, designated pSH55, carried a 7.4-kilobase-pair HindIII fragment containing the glpD and glpR genes. The glpD gene was subcloned into pACYC177 on a 4.4-kilobase-pair BamHI-HindIII fragment. Expression of the cloned glpD gene was regulated in the manner previously described for the chromosomal glpD gene. The position of glpD on this plasmid was determined by Tn1000 insertional inactivation experiments. The glpD gene product, a polypeptide of Mr 55,000, was detected in a maxicell system. Truncated polypeptides replaced the 55,000-molecular-weight polypeptide when plasmid derivatives harboring Tn1000 insertions that inactivate glpD were used as templates. The sizes of these polypeptides confirmed the previously determined direction of transcription and allowed estimation of the translation start site. Determination of the apparent Mr of a hybrid protein encoded by a glpD'-'lacZ fusion provided additional evidence for the position of the glpD control region. The amino-terminal 30 to 60 amino acids of this hybrid protein (provided by glpD) were sufficient for efficient membrane localization of glpD'-'lacZ-encoded beta-galactosidase activity. The glpD3 mutation was mapped within the glpD gene, providing additional evidence that glpD is the structural gene for aerobic sn-glycerol-3-phosphate dehydrogenase.

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.

Genetic and morphological characterization of ftsB and nrdB mutants of Escherichia coli

The ftsB gene of Escherichia coli is believed to be involved in cell division. In this report, we show that plasmids containing the nrdB gene could complement the ftsB mutation, suggesting that ftsB is an allele of nrdB. We compared changes in the cell shape of isogenic nrdA, nrdB, ftsB, and pbpB strains at permissive and restrictive temperatures. Although in rich medium all strains produced filaments at the restrictive temperature, in minimal medium only a 50 to 100% increase in mean cell mass occurred in the nrdA, nrdB, and ftsB strains. The typical pbpB cell division mutant also formed long filaments at low growth rates. Visualization of nucleoid structure by fluorescence microscopy demonstrated that nucleoid segregation was affected by nrdA, nrdB, and ftsB mutations at the restrictive temperature. Measurements of beta-galactosidase activity in lambda p(sfiA::lac) lysogenic nrdA, nrdB, and ftsB mutants in rich medium at the restrictive temperature showed that filamentation in the nrdA mutant was caused by sfiA (sulA) induction, while filamentation in nrdB and ftsB mutants was sfiA independent, suggesting an SOS-independent inhibition of cell division.

Ca2+-induced permeabilization of the Escherichia coli outer membrane: comparison of transformation and reconstitution of binding-protein-dependent transport

Ca2+ treatment renders the outer membrane of Escherichia coli reversibly permeable for macromolecules. We investigated whether Ca2+-induced uptake of exogenous protein into the periplasm occurs by mechanisms similar to Ca2+-induced uptake of DNA into the cytoplasm during transformation. Protein import through the outer membrane was monitored by measuring reconstitution of maltose transport after the addition of shock fluid containing maltose-binding protein. DNA import through the outer and inner membrane was measured by determining the efficiency of transformation with plasmid DNA. Both processes were stimulated by increasing Ca2+ concentrations up to 400 mM. Plasmolysis was essential for a high efficiency; reconstitution and transformation could be stimulated 5- and 40-fold, respectively, by a high concentration of sucrose (400 mM) in cells incubated with a suboptimal Ca2+ concentration (50 mM). The same divalent cations that promote import of DNA (Ca2+, Ba2+, Sr2+, Mg2+, and Ni2+) also induced import of protein. Ca2+ alone was found to be inefficient in promoting reconstitution; successive treatment with phosphate and Ca2+ ions was essential. Transformation also was observed in the absence of phosphate, but could be stimulated by pretreatment with phosphate. The optimal phosphate concentrations were 100 mM and 1 to 10 mM for reconstitution and transformation, respectively. Heat shock, in which the cells are rapidly transferred from 0 to 42 degrees C, affected the two processes differently. Incubation of cells at 0 degrees C in Ca2+ alone allows rapid entry of protein, but not of DNA. Transformation was observed only when exogenous DNA was still present during the heat shock. Shock fluid containing maltose-binding protein inhibited transformation (with 6 microgram of DNA per ml, half-maximal inhibition occurred at around 300 microgram of shock fluid per ml). DNA inhibited reconstitution (with 5 microgram of shock fluid per ml, half-maximal inhibition occurred at around 3 mg of DNA per ml).

Regulation of ugp, the sn-glycerol-3-phosphate transport system of Escherichia coli K-12 that is part of the pho regulon

The expression of the ugp-dependent sn-glycerol-3-phosphate transport system that is part of the pho regulon was studied in mutants of Escherichia coli K-12 containing regulatory mutations of the pho regulon. The phoR and phoST gene products exerted a negative control on the expression of ugp. Induction of the system was positively controlled by the phoB, phoM, and phoR gene products. Using a ugp-lacZ operon fusion, we showed that the ugp and phoA genes were coordinately derepressed and repressed.

Defective secretion of maltose- and ribose-binding proteins caused by a truncated periplasmic protein in Escherichia coli

The secretion in Escherichia coli of a C-terminally truncated periplasmic enzyme from Salmonella typhimurium, the glpQ-encoded glycerolphosphate phosphodiesterase, was studied. Plasmid pRH100, carrying the truncated glpQ gene, directs the synthesis of a 30,000-molecular-weight (30 K) protein that is processed to a mature 27.5 K protein. (The mature wild-type protein is a 38 K protein.) The truncated protein is not released into the periplasm but remains membrane associated, although it becomes protease sensitive after conversion of cells to spheroplasts. The presence of pRH100 strongly reduces the amount of some other proteins in the periplasm, including the maltose- and ribose-binding proteins. The reduction does not occur at the level of transcription or early translation, as shown by lacZ fusions to the gene coding for the structural gene of the maltose-binding protein. Outer membrane proteins are not affected. A hydroxylamine-induced mutation in the sequence of glpQ corresponding to the mature polypeptide overcomes the inhibitory effect of pRH100. The mutated gene no longer directs the synthesis of the 30/27.5 K protein but directs that of a new 19 K protein which is not membrane bound. We propose that sorting signals in the mature GIpQ protein are necessary for effective translocation to the periplasm and that the C-terminal third of the protein is essential for release into the periplasm.

Repressor for the sn-glycerol-3-phosphate regulon of Escherichia coli K-12: cloning of the glpR gene and identification of its product

The glpR gene encoding the repressor for the glp regulon of Escherichia coli was cloned from a library of HindIII DNA fragments established in bacteriophage lambda. Phages harboring glpR were isolated by selection for sn-glycerol-3-phosphate dehydrogenase function encoded by glpD, which is adjacent to glpR on the E. coli linkage map. Restriction endonuclease analysis and recloning of DNA fragments localized glpR to a 3-kilobase-pair EcoRI-SalI segment of DNA. Strains exhibiting constitutive expression of the glp operons were strongly repressed after introduction of multicopy plasmids containing the glpR gene. Analysis of proteins labeled in minicells harboring either glpR+ recombinant plasmids or a glpR::Tn5 derivative showed that the glpR gene product is a protein with an apparent molecular weight of 33,000.

sn-Glycerol-3-phosphate auxotrophy of plsB strains of Escherichia coli: evidence that a second mutation, plsX, is required

sn-Glycerol-3-phosphate auxotrophs defective in phospholipid synthesis contain a Km-defective sn-glycerol-3-phosphate acyltransferase. Detailed genetic analysis revealed that two mutations were required for the auxotrophic phenotype. One mutation, in the previously described plsB locus (sn-glycerol-3-phosphate acyltransferase structural gene), mapped near min 92 on the Escherichia coli linkage map. Isolation of Tn10 insertions cotransducible with the auxotrophy in phage P1 crosses revealed that a second mutation was required with plsB26 to confer the sn-glycerol-3-phosphate auxotrophic phenotype. This second locus, plsX, mapped between pyrC and purB near min 24 on the E. coli linkage map. Tn10 insertions near plsX allowed detailed mapping of the genetic loci in this region. A clockwise gene order putA pyrC flbA flaL flaT plsX fabD ptsG thiK purB was inferred from results of two- and three-factor crosses. Strains harboring the four possible configurations of the mutant and wild-type plsB and plsX loci were constructed. Isogenic plsB+ plsX+, plsB+ plsX50, and plsB26 plsX+ strains grew equally well on glucose minimal medium without sn-glycerol-3-phosphate. In addition, plsX or plsX+ had no apparent effect on sn-glycerol-3-phosphate acyltransferase activity measured in membrane preparations. The molecular basis for the plsX requirement for conferral of sn-glycerol-3-phosphate auxotrophy in these strains remains to be established.

Mapping of markers in the gyrA-his region of Escherichia coli

Discrepancies between our mapping data concerning the cdd and gat marker of Escherichia coli and data obtained by Josephsen et al. (1983) as well as with the current linkage map of Escherichia coli (Bachmann 1983) led us to reinvestigate the mapping of markers in the gyrA-his region by P1 transduction. In addition, we isolated Hfr strains by integrating the temperature-sensitive F'ts114lac+ episome via lac homology of cir'lacZ and mgl'lacZ fusions into the chromosome. From the results of the P1 transductions and using these Hfr strains as donors in crosses it became clear that the transcription of cir and mgl is the same and counterclockwise on the chromosome, with cdd and gat as the counterclockwise markers to mgl and cir. We conclude that our previously published sequence of markers was incorrect and is in fact inverted. The present data indicate the following sequence of markers in clockwise order: his gat cdd mgl cir fpk gyrA.

Characterization of the ugp region containing the genes for the phoB dependent sn-glycerol-3-phosphate transport system of Escherichia coli

The ugp structural genes, coding for the pho regulon dependent sn-glycerol-3-phosphate transport system, were cloned in pBR322 and characterized. The expression of the cloned ugp system was phoB dependent. Cells containing the ugp plasmid overproduced the G3P binding protein upon phosphate starvation. Tn5 mutagenesis of the cloned DNA revealed that the ugp genes are organized in two separate operons which comprise at least four genes: ugpB and ugpD constitute one operon, ugpA and ugpC constitute the other. The structural gene for the G3P binding protein (G3PBP) is ugpB. The ugpC gene product was also synthesized in minicells as a polypeptide, with an apparent molecular weight of 40,000. No gene products could be assigned to the ugpA and ugpD genes. Hybridization experiments allowed the physical characterization of 20 kb of DNA adjacent to the ugp genes on the E. coli chromosome including the liv genes.

Use of phi(glp-lac) in studies of respiratory regulation of the Escherichia coli anaerobic sn-glycerol-3-phosphate dehydrogenase genes (glpAB)

Expression of the glpA operon encoding the extrinsic membrane anaerobic sn-glycerol-3-phosphate dehydrogenase complex of Escherichia coli K-12 was studied in five strains carrying independent glpA-lac operon fusions. The location of the fusions was confirmed by transduction. Two of the strains produced an enzymatically active anaerobic sn-glycerol-3-phosphate dehydrogenase that accumulated in the cytoplasmic fraction of the cells. This suggests the loss of a specific membrane anchor subunit encoded by a distal gene, glpB, which was disrupted by the insertion. beta-Galactosidase in all five strains carrying phi(glpA-lac) was highly inducible by glycerol only anaerobically. A mutation in fnr, a pleiotropic activator gene, prevented full induction of the phi(glpA-lac), demonstrating that the Fnr protein is a positive regulator of the primary dehydrogenase as well as of the terminal reductases of anaerobic respiratory chains. Low concentrations of the respiratory poison KCN had a permissive effect on aerobic expression of phi(glpA-lac). Aerobic expression of the hybrid operon was also enhanced in isogenic derivatives of the fusion strains deficient in protoporphyrin biosynthesis (hemA). Thus, heme proteins may play a role in mediating aerobic repression of the anaerobic respiratory chain.

sn-Glycerol-3-phosphate transport in Salmonella typhimurium

Salmonella typhimurium contains a transport system for sn-glycerol-3-phosphate that is inducible by growth on glycerol and sn-glycerol-3-phosphate. In fully induced cells, the system exhibited an apparent Km of 50 microM and a Vmax of 2.2 nmol/min . 10(8) cells. The corresponding system in Escherichia coli exhibits, under comparable conditions, a Km of 14 microM and a Vmax of 2.2 nmol/min . 10(8) cells. Transport-defective mutants were isolated by selecting for resistance against the antibiotic fosfomycin. They mapped in glpT at 47 min in the S. typhimurium linkage map, 37% cotransducible with gyrA. In addition to the glpT-dependent system, S. typhimurium LT2 contains, like E. coli, a second, ugp-dependent transport system for sn-glycerol-3-phosphate that was derepressed by phosphate starvation. A S. typhimurium DNA bank containing EcoRI restriction fragments in phage lambda gt7 was used to clone the glpT gene in E. coli. Lysogens that were fully active in the transport of sn-glycerol-3-phosphate with a Km of 33 microM and a Vmax of 2.0 nmol/min . 10(8) cells were isolated in a delta glpT mutant of E. coli. The EcoRI fragment harboring glpT was 3.5 kilobases long and carried only part of glpQ, a gene distal to glpT but on the same operon. The fragment was subcloned in multicopy plasmid pACYC184. Strains carrying this hybrid plasmid produced large amounts of cytoplasmic membrane protein with an apparent molecular weight of 33,000, which was identified as the sn-glycerol-3-phosphate permease. Its properties were similar to the corresponding E. coli permease. The presence of the multicopy glpT hybrid plasmid had a strong influence on the synthesis or assembly of other cell envelope proteins of E. coli. For instance, the periplasmic ribose-binding protein was nearly absent. On the other hand, the quantity of an unidentified E. coli outer membrane protein usually present only in small amounts increased.

cir, a gene conferring resistance to colicin I maps between mgl and fpk on the Escherichia coli chromosome

With the help of the tetracycline resistance transposon Tn10 in and around the mgl genes the gene cir was mapped. cir is 80% cotransducible with mgl by P1 transduction. The sequence of the surrounding markers in clockwise order was established as: cdd fpk cir mgl gyrA. The direction of transcription in cir was determined as clockwise on the Escherichia coli chromosome. The gene product of cir, an outer membrane receptor for colicin I, is not part of the mgl operon. It is not regulated by D-fucose, the inducer of the mgl system and mutants defective in cir are unimpaired in the uptake of substrates of the mgl-dependent transport system.

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.