Exogenous induction of the Escherichia coli hexose phosphate transport system defined by uhp-lac operon fusions

Influence of grape consumption on the human microbiome

Over the years, a substantial body of information has accumulated suggesting dietary consumption of grapes may have a positive influence on human health. Here, we investigate the potential of grapes to modulate the human microbiome. Microbiome composition as well as urinary and plasma metabolites were sequentially assessed in 29 healthy free-living male (age 24-55 years) and female subjects (age 29-53 years) following two-weeks of a restricted diet (Day 15), two-weeks of a restricted diet with grape consumption (equivalent to three servings per day) (Day 30), and four-weeks of restricted diet without grape consumption (Day 60). Based on alpha-diversity indices, grape consumption did not alter the overall composition of the microbial community, other than with the female subset based on the Chao index. Similarly, based on beta-diversity analyses, the diversity of species was not significantly altered at the three time points of the study. However, following 2 weeks of grape consumption, taxonomic abundance was altered (e.g., decreased Holdemania spp. and increased Streptococcus thermophiles), as were various enzyme levels and KEGG pathways. Further, taxonomic, enzyme and pathway shifts were observed 30 days following the termination of grape consumption, some of which returned to baseline and some of which suggest a delayed effect of grape consumption. Metabolomic analyses supported the functional significance of these alterations wherein, for example, 2'-deoxyribonic acid, glutaconic acid, and 3-hydroxyphenylacetic acid were elevated following grape consumption and returned to baseline following the washout period. Inter-individual variation was observed and exemplified by analysis of a subgroup of the study population showing unique patterns of taxonomic distribution over the study period. The biological ramifications of these dynamics remain to be defined. However, while it seems clear that grape consumption does not perturb the eubiotic state of the microbiome with normal, healthy human subjects, it is likely that shifts in the intricate interactive networks that result from grape consumption have physiological significance of relevance to grape action.

Cross Talk Inhibition Nullified by a Receiver Domain Missense Substitution

In two-component signal transduction, a sensor protein transmitter module controls cognate receiver domain phosphorylation. Most receiver domain sequences contain a small residue (Gly or Ala) at position T + 1 just distal to the essential Thr or Ser residue that forms part of the active site. However, some members of the NarL receiver subfamily have a large hydrophobic residue at position T + 1. Our laboratory previously isolated a NarL mutant in which the T + 1 residue Val-88 was replaced with an orthodox small Ala. This NarL V88A mutant confers a striking phenotype in which high-level target operon expression is both signal (nitrate) and sensor (NarX and NarQ) independent. This suggests that the NarL V88A protein is phosphorylated by cross talk from noncognate sources. Although cross talk was enhanced in ackA null strains that accumulate acetyl phosphate, it persisted in pta ackA double null strains that cannot synthesize this compound and was observed also in narL(+) strains. This indicates that acetate metabolism has complex roles in mediating NarL cross talk. Contrariwise, cross talk was sharply diminished in an arcB barA double null strain, suggesting that the encoded sensors contribute substantially to NarL V88A cross talk. Separately, the V88A substitution altered the in vitro rates of NarL autodephosphorylation and transmitter-stimulated dephosphorylation and decreased affinity for the cognate sensor, NarX. Together, these experiments show that the residue at position T + 1 can strongly influence two distinct aspects of receiver domain function, the autodephosphorylation rate and cross talk inhibition.

IMPORTANCE

Many bacterial species contain a dozen or more discrete sensor-response regulator two-component systems that convert a specific input into a distinct output pattern. Cross talk, the unwanted transfer of signals between circuits, occurs when a response regulator is phosphorylated inappropriately from a noncognate source. Cross talk is inhibited in part by the high interaction specificity between cognate sensor-response regulator pairs. This study shows that a relatively subtle missense change from Val to Ala nullifies cross talk inhibition, enabling at least two noncognate sensors to enforce an inappropriate output independently of the relevant input.

The involvement of transport proteins in transcriptional and metabolic regulation

Transport proteins have sometimes gained secondary regulatory functions that influence gene expression and metabolism. These functions allow communication with the external world via mechanistically distinctive signal transduction pathways. In this brief review we focus on three transport systems in Escherichia coli that control and coordinate carbon, exogenous hexose-phosphate and phosphorous metabolism. The transport proteins that play central roles in these processes are: first, the phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS), second, the glucose-6-phosphate receptor, UhpC, and third, the phosphate-specific transporter, PstSABC, respectively. While the PTS participates in multiple complex regulatory processes, three of which are discussed here, UhpC and the Pst transporters exemplify differing strategies.

Identification of GutQ from Escherichia coli as a D-arabinose 5-phosphate isomerase

The glucitol operon (gutAEBDMRQ) of Escherichia coli encodes a phosphoenolpyruvate:sugar phosphotransferase system that metabolizes the hexitol D-glucitol (sorbitol). The functions for all but the last gene, gutQ, have been previously assigned. The high sequence similarity between GutQ and KdsD, a D-arabinose 5-phosphate isomerase (API) from the 3-deoxy-D-manno-octulosonate (KDO)-lipopolysaccharide (LPS) biosynthetic pathway, suggested a putative activity, but its role within the context of the gut operon remained unclear. Accordingly, the enzyme was cloned, overexpressed, and characterized. Recombinant GutQ was shown to indeed be a second copy of API from the E. coli K-12 genome with biochemical properties similar to those of KdsD, catalyzing the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P). Genomic disruptions of each API gene were constructed in E. coli K-12. TCM11[(deltakdsD)] was capable of sustaining essential LPS synthesis at wild-type levels, indicating that GutQ functions as an API inside the cell. The gut operon remained inducible in TCM7[(deltagutQ)], suggesting that GutQ is not directly involved in d-glucitol catabolism. The conditional mutant TCM15[(deltagutQdeltakdsD)] was dependent on exogenous A5P both for LPS synthesis/growth and for upregulation of the gut operon. The phenotype was suppressed by complementation in trans with a plasmid encoding a functional copy of GutQ or by increasing the amount of A5P in the medium. As there is no obvious obligatory role for GutQ in the metabolism of d-glucitol and there is no readily apparent link between D-glucitol metabolism and LPS biosynthesis, it is suggested that A5P is not only a building block for KDO biosynthesis but also may be a regulatory molecule involved in expression of the gut operon.

Cooperativity in signal transfer through the Uhp system of Escherichia coli

The UhpABC regulatory system in enterobacteria controls the expression of the hexose phosphate transporter UhpT. Signaling is initiated through sensing of extracellular glucose 6-phosphate by membrane-bound UhpC, which in turn modulates the histidine-protein kinase UhpB. Together with the cytoplasmic response regulator UhpA, they constitute a typical two-component regulatory system based on His-to-Asp phosphoryl transfer. Activated (i.e., phosphorylated) UhpA binds to the promoter region of uhpT, resulting in initiation of transcription. We have investigated the contribution of transmembrane signaling (through UhpBC) and intracellular activation (through UhpA) to the overall Uhp response (UhpT expression) in vivo. UhpA activation could be made independent of transmembrane signaling when (Delta)uhpBC cells were grown on pyruvate. Inorganic phosphate interfered with glucose 6-phosphate-dependent, UhpBC-mediated, as well as pyruvate-mediated activation of UhpA. The relationship between the concentration of inducer (glucose 6-phosphate) and the Uhp induction rate was nonhyperbolic, indicating positive cooperativity. The degree of cooperativity was affected by the carbon or energy source available to the cells for growth. As pyruvate-mediated activation of UhpA in (Delta)uhpBC cells could result in considerably stronger UhpT expression than glucose 6-phosphate-dependent activation through UhpBC, the observed positive cooperativity for the overall pathway in wild-type cells may reflect the previously described cooperative binding of UhpA to the uhpT promoter (J. L. Dahl et al., J. Biol. Chem. 272:1910-1919, 1997).

Identification of the periplasmic cobalamin-binding protein BtuF of Escherichia coli

Cells of Escherichia coli take up vitamin B(12) (cyano-cobalamin [CN-Cbl]) and iron chelates by use of sequential active transport processes. Transport of CN-Cbl across the outer membrane and its accumulation in the periplasm is mediated by the TonB-dependent transporter BtuB. Transport across the cytoplasmic membrane (CM) requires the BtuC and BtuD proteins, which are most related in sequence to the transmembrane and ATP-binding cassette proteins of periplasmic permeases for iron-siderophore transport. Unlike the genetic organization of most periplasmic permeases, a candidate gene for a periplasmic Cbl-binding protein is not linked to the btuCED operon. The open reading frame termed yadT in the E. coli genomic sequence is related in sequence to the periplasmic binding proteins for iron-siderophore complexes and was previously implicated in CN-Cbl uptake in Salmonella. The E. coli yadT product, renamed BtuF, is shown here to participate in CN-Cbl uptake. BtuF protein, expressed with a C-terminal His(6) tag, was shown to be translocated to the periplasm concomitant with removal of a signal sequence. CN-Cbl-binding assays using radiolabeled substrate or isothermal titration calorimetry showed that purified BtuF binds CN-Cbl with a binding constant of around 15 nM. A null mutation in btuF, but not in the flanking genes pfs and yadS, strongly decreased CN-Cbl utilization and transport into the cytoplasm. The growth response to CN-Cbl of the btuF mutant was much stronger than the slight impairment previously described for btuC, btuD, or btuF mutants. Hence, null mutations in btuC and btuD were constructed and revealed that the btuC mutant had a strong impairment similar to that of the btuF mutant, whereas the btuD defect was less pronounced. All mutants with defective transport across the CM gave rise to frequent suppressor variants which were able to respond at lower levels of CN-Cbl but were still defective in transport across the CM. These results finally establish the identity of the periplasmic binding protein for Cbl uptake, which is one of few cases where the components of a periplasmic permease are genetically separated.

Investigation of in vivo cross-talk between key two-component systems of Escherichia coli

Intracellular signal transfer in bacteria is dominated by phosphoryl transfer between conserved transmitter and receiver domains in regulatory proteins of so-called two-component systems. Escherichia coli contains 30 such systems, which allow it to modulate gene expression, enzyme activity and the direction of flagellar rotation. The authors have investigated whether, and to what extent, these separate systems form (an) interacting network(s) in vivo, focussing on interactions between four major systems, involved in the responses to the availability of phosphorylated sugars (Uhp), phosphate (Pho), nitrogen (Ntr) and oxygen (Arc). Significant cross-talk was not detectable in wild-type cells. Decreasing expression levels of succinate dehydrogenase (reporting Arc activation), upon activation of the Pho system, appeared to be independent of signalling through PhoR. Cross-talk towards NtrC did occur, however, in a ntrB deletion strain, upon joint activation of Pho, Ntr and Uhp. UhpT expression was demonstrated when cells were grown on pyruvate, through non-cognate phosphorylation of UhpA by acetyl phosphate.

Glucose-6-phosphate-dependent phosphoryl flow through the Uhp two-component regulatory system

Expression of the UhpT sugar-phosphate transporter in Escherichia coli is regulated at the transcriptional level via the UhpABC signalling cascade. Sensing of extracellular glucose 6-phosphate (G6P), by membrane-bound UhpC, modulates a second membrane-bound protein, UhpB, resulting in autophosphorylation of a conserved histidine residue in the cytoplasmic (transmitter) domain of the latter. Subsequently, this phosphoryl group is transferred to a conserved aspartate residue in the response-regulator UhpA, which then initiates uhpT transcription, via binding to the uhpT promoter region. This study demonstrates the hypothesized transmembrane signal transfer in an ISO membrane set-up, i.e. in a suspension of UhpBC-enriched membrane vesicles, UhpB autophosphorylation is stimulated, in the presence of [gamma-(32)P]ATP, upon intra-vesicular sensing of G6P by UhpC. Subsequently, upon addition of UhpA, very rapid and transient UhpA phosphorylation takes place. When P approximately UhpA is added to G6P-induced UhpBC-enriched membrane vesicles, rapid UhpA dephosphorylation occurs. So, in the G6P-activated state, UhpB phosphatase activity dominates over kinase activity, even in the presence of saturating amounts of G6P. This may imply that maximal in vivo P approximately UhpA levels are low and/or that, to keep sufficient P approximately UhpA accumulated to induce uhpT transcription, the uhpT promoter DNA itself is involved in stabilization/sequestration of P approximately UhpA.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Genetic analysis of metabolic crosstalk and its impact on thiamine synthesis in Salmonella typhimurium

The first five steps in de novo purine biosynthesis are involved in the formation of the 4-amino-5-hydroxymethyl-2-methyl pyrimidine (HMP) moiety of thiamine. We show here that the first enzyme in de novo purine biosynthesis, PurF, is required for thiamine synthesis during aerobic growth on some but not other carbon sources. We show that PurF-independent thiamine synthesis depends on the recently described alternative pyrimidine biosynthetic (APB) pathway. Null mutations in zwf (encoding glucose-6-dehydrogenase), gnd (encoding gluconate-6-P dehydrogenase), purE (encoding aminoimidazole ribonucleotide carboxylase), and purR (encoding a regulator of gene expression) were found to affect the function of the APB pathway. A model is presented to account for the involvement of these gene products in thiamine biosynthesis via the APB pathway. Results presented herein demonstrate that function of the APB pathway can be prevented either by blocking intermediate formation or by diverting intermediate(s) from the pathway. Strong genetic evidence supports the conclusion that aminoimidazole ribotide (AIR) is an intermediate in the APB pathway.

Regulation of Escherichia coli adenylate cyclase activity during hexose phosphate transport

In Escherichia coli, cAMP levels vary with the carbon source used in the culture medium. These levels are dependent on the cellular concentration of phosphorylated EnzymeIIAglc, a component of the glucose-phosphotransferase system, which activates adenylate cyclase (AC). When cells are grown on glucose 6-phosphate (Glc6P), the cAMP level is particularly low. In this study, we investigated the mechanism leading to the low cAMP level when Glc6P is used as the carbon source, i.e. the mechanism preventing the activation of AC by phosphorylated EnzymeIIAglc. Glc6P is transported via the Uhp system which is inducible by extracellular Glc6P. The Uhp system comprises a permease UhpT and three proteins UhpA, UhpB and UhpC which are necessary for uhpT gene transcription. Controlled expression of UhpT in the absence of the regulatory proteins (UhpA, UhpB and UhpC) allowed us to demonstrate that (i) the Uhp regulatory proteins do not prevent the activation of AC by direct interaction with EnzymeIIAglc and (ii) an increase in the amount of UhpT synthesized (corresponding to an increase in the amount of Glc6P transported) correlates with a decrease in the cAMP level. We present data indicating that Glc6P per se or its degradation is unlikely to be responsible for the low cAMP level. It is concluded that the level of cAMP in the cell is determined by the flux of Glc6P through UhpT.

Involvement of the oxidative pentose phosphate pathway in thiamine biosynthesis in Salmonella typhimurium

purF mutants of Salmonella typhimurium are known to require a source of both purine and thiamine; however, exogenous pantothenate may be substituted for the thiamine requirement. We show here that the effect of pantothenate is prevented by blocks in the oxidative pentose phosphate pathway, gnd (encoding gluconate 6-phosphate [6-P] dehydrogenase) or zwf (encoding glucose 6-P dehydrogenase). We further show that the defects caused by these mutations can be overcome by increasing ribose 5-P, suggesting that ribose 5-P may play a role in the ability of pantothenate to substitute for thiamine.

The family of organo-phosphate transport proteins includes a transmembrane regulatory protein

This review article briefly summarizes aspects of our current understanding of the Uhp sugar phosphate transport system in enteric bacteria, particularly the mode of genetic regulation of its synthesis. This regulation occurs by a process that involves an example of the very widespread and ever-growing group of so-called two-component bacterial regulatory systems, a mechanism of response to environmental signals that employs phosphate transfer reactions between constituent proteins. Of emphasis here is the unusual involvement in transmembrane signaling of the UhpC protein which is related in sequence and structure to some transport proteins, including the very protein whose synthesis it helps regulate.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Interplay between the membrane-associated UhpB and UhpC regulatory proteins

Expression of the Escherichia coli uhpT gene, encoding the sugar phosphate transport protein, is induced by extracellular glucose-6-phosphate and requires the function of the uhpABC regulatory genes. The UhpA and UhpB proteins are related to the response-regulator and sensor-kinase proteins of two-component regulatory systems, whereas the UhpC protein is related to UhpT and homologous transport proteins. To investigate the role of segments of the membrane-associated UhpB and UhpC regulatory proteins, a series of mutations were constructed in vitro by insertion of a 12- or 24-bp oligonucleotide linker at 44 sites within the uhpABCT locus. The effect of these mutations on regulation of a uhpT-lacZ transcriptional reporter was assayed with the mutated uhp alleles in single copy on the chromosome. All but one of the insertions in uhpA or uhpT were inactive for transcription activation or transport, respectively. In contrast, about half of the insertions in uhpB and uhpC retained Uhp expression, and insertions at four sites in uhpB and at one site in uhpC conferred high-level constitutive expression. The constitutive mutants in UhpB resulted from insertions in the nonpolar amino-terminal half of the protein, and all insertions in that half of UhpB affected Uhp expression in some manner, which suggests that the transmembrane segments of UhpB might negatively regulate the kinase activity of the carboxyl portion. The constitutive behavior of all but one of these uhpB alleles was dependent on the presence of active forms of both UhpA and UhpC, which suggests that UhpB and UhpC act jointly as a complex in the signaling process.

Promoter elements required for positive control of transcription of the Escherichia coli uhpT gene

The uhpABCT locus of Escherichia coli encodes the transport system which allows the cell to accumulate a variety of sugar phosphates in unaltered form. The expression of uhpT, the gene encoding the transport protein, is regulated by the uhpABC gene products. The UhpA protein is required for expression; its deduced amino acid sequence shows that it belongs to a subfamily of bacterial transcription regulators including NarL, DegU, and FixJ. Members of this subfamily have an amino-terminal phosphorylation domain characteristic of so-called two-component regulators, such as OmpR, CheY, PhoB, and NtrC, and a carboxyl-terminal domain conserved among many transcriptional activators, including LuxR and MalT. The major sequence elements in the uhpT promoter that are needed for uhpT expression were investigated. Northern (RNA) hybridization analysis showed that the uhpT transcript was only present in cells induced for UhpT transport activity. The start site of transcription was identified by primer extension. Comparison of the regions upstream of the uhpT transcription start site in E. coli and Salmonella typhimurium suggested the presence of four sequence elements that might be involved in promoter function: a typical -10 region, a short inverted repeat centered at -32, a long inverted repeat centered at -64, and a cyclic AMP receptor protein-binding sequence centered at -103. Deletion and linker substitution mutations in the promoter demonstrated that the presence of the cyclic AMP receptor protein-binding site resulted in about an eightfold increase in promoter activity and that the -64, -32, and -10 elements were essential for promoter function. In vivo titration of transcriptional activator UhpA by the intact or mutant promoters on multicopy plasmids identified the -64 element as the UhpA-binding site. The two halves of the -64 inverted repeat did not contribute equally to promoter function and did not have to be intact for UhpA titration. The sequence recognized by UhpA is predicted to be 5' -GGCAAAACNNNGAAA.

Structure and function of the uhp genes for the sugar phosphate transport system in Escherichia coli and Salmonella typhimurium

Expression of the Escherichia coli sugar phosphate transport system, encoded by the uhpT gene, is regulated by external glucose 6-phosphate through the action of three linked regulatory genes, uhpABC. The nucleotide sequence of the uhp region cloned from Salmonella typhimurium was determined. The deduced Uhp polypeptide sequences from the two organisms are highly related. Comparison with the corrected sequence from E. coli revealed that the four uhp genes are closely spaced, with minimal intergenic distances, and that uhpC is nearly identical in length to uhpT, both of which have substantial sequence relatedness along their entire lengths. To facilitate analysis of uhp gene function, we isolated insertions of a kanamycin resistance (Km) cassette throughout the uhp region. In-frame deletions that removed almost the entire coding region of individual or multiple uhp genes were generated by use of restriction sites at the ends of the Km cassette. The phenotypes of the Km insertions and the in-frame deletions confirmed that all three regulatory genes are required for Uhp function. Whereas the deletion of uhpA completely abolished the expression of a uhpT-lacZ reporter gene, the deletion of uhpB or uhpC resulted in a partially elevated basal level of expression that was not further inducible. These results indicated that UhpB and perhaps UhpC play both positive and negative roles in the control of uhpT transcription. Translational fusions of the uhpBCT genes to topological reporter gene phoA were generated by making use of restriction sites provided by the Km cassette or with transposon TnphoA. The alkaline phosphatase activities of the resultant hybrid proteins were consistent with models predicting that UhpC and UhpT have identical transmembrane topologies, with 10 to 12 transmembrane segments, and that UhpB has 4 to 8 amino-terminal transmembrane segments that anchor the polar carboxyl-terminal half of the protein to the cytoplasmic side of the inner membrane.

Induction of the nag regulon of Escherichia coli by N-acetylglucosamine and glucosamine: role of the cyclic AMP-catabolite activator protein complex in expression of the regulon

The divergent nag regulon located at 15.5 min on the Escherichia coli map encodes genes necessary for growth on N-acetylglucosamine and glucosamine. Full induction of the regulon requires both the presence of N-acetylglucosamine and a functional cyclic AMP (cAMP)-catabolite activator protein (CAP) complex. Glucosamine produces a lower level of induction of the regulon. A nearly symmetric consensus CAP-binding site is located in the intergenic region between nagE (encoding EIINag) and nagB (encoding glucosamine-6-phosphate deaminase). Expression of both nagE and nagB genes is stimulated by cAMP-CAP, but the effect is more pronounced for nagE. In fact, very little expression of nagE is observed in the absence of cAMP-CAP, whereas 50% maximum expression of nagB is observed with N-acetylglucosamine in the absence of cAMP-CAP. Two mRNA 5' ends separated by about 100 nucleotides were located before nagB, and both seem to be similarly subject to N-acetylglucosamine induction and cAMP-CAP stimulation. To induce the regulon, N-acetylglucosamine or glucosamine must enter the cell, but the particular transport mechanism used is not important.

Induction of levansucrase in Bacillus subtilis: an antitermination mechanism negatively controlled by the phosphotransferase system

The target of the induction by sucrose of the levansucrase gene is a transcription terminator (sacRt) located upstream from the coding sequence, sacB. The two-gene locus sacX-sacY (formerly sacS) and the ptsI gene were previously shown to be involved in this induction. ptsI encodes enzyme I of the phosphoenolpyruvate-dependent phosphotransferase system. SacX is strongly homologous to sucrose-specific phosphotransferase system-dependent permeases. SacY is a positive regulator of sacB. Here we show that SacY is probably an antiterminator interacting directly with sacRt, since in Escherichia coli the presence of the sacY gene stimulates the expression of a reporter gene fused downstream from sacRt. Missense mutations affecting sacY were sequenced, and the sacB regulation was studied in isogenic strains carrying these mutations or in vitro-generated mutations affecting sacX, sacY, or ptsI. The phenotype of double mutants suggests a model in which SacX might be a sucrose sensor that would be phosphorylated by the phosphotransferase system and, in this state, could inhibit the SacY antiterminator. Exogenous sucrose, or a mutation inactivating the phosphotransferase system, would dephosphorylate SacX and allow antitermination at sacRt.

Mapping of the Escherichia coli acid glucose-1-phosphatase gene agp and analysis of its expression in vivo by use of an agp-phoA protein fusion

The agp gene of Escherichia coli encodes an acid glucose-1-phosphatase, one of the numerous phosphatases optimally active between pH 4 and 6 found in the periplasmic space of this bacterium. An agp-phoA protein fusion linked to a gene conferring kanamycin resistance was inserted into the chromosome in place of agp by homologous recombination and was mapped to minute 22.6. Because the activity of glucose-1-phosphatase cannot be measured accurately in whole cells, the alkaline phosphatase activity of the agp-phoA hybrid protein was used to monitor the expression of the chromosomal agp gene. The expression of agp was subject to catabolite repression but was unaffected by the concentration of inorganic phosphate in the growth medium. The product of the agp gene was required for growth on glucose-1-phosphate as the sole carbon source, a function for which alkaline phosphatase or other acid phosphatases cannot substitute.

Role of uhp genes in expression of the Escherichia coli sugar-phosphate transport system

The uhpABCT locus of Escherichia coli is responsible for expression of the sugar-phosphate transport system and its induction by external glucose 6-phosphate. Expression of uhpT-lacZ fusions depended on the function of uhpA, uhpB, and uhpC but not of uhpT. A plasmid carrying only uhpT conferred transport activity in a host strain deleted for the uhp region. Thus, uhpT encodes the polypeptide required for transport function, and the other three uhp genes regulate uhpT transcription. The presence of uhpA at elevated copy number resulted in a substantial increase in uhpT expression. This elevated expression was only about 50% of the level seen in induced haploid cells, and no further increase occurred after addition of inducer. Activation by multicopy uhpA was not affected by the status of uhpC but was decreased in the absence of uhpB, suggesting a role for UhpB in directly activating UhpA. Transcription of uhpA, monitored by expression of a uhpA-lacZ fusion, was not affected by either inducer or the presence of the wild-type uhpA allele. The presence of multiple copies of the uhpT promoter region reduced uhpT expression in strains with uhpA in single copy number but not in those with multiple copies, consistent with competition for the activator. Amino acid sequence comparisons showed that UhpA was homologous to a family of bacterial regulatory proteins, some of which act as transcriptional activators (OmpR, PhoB, NtrC, and DctD). The C-terminal portion of UhpB displayed matches to the corresponding portions of another family of proteins (EnvZ, PhoMR, NtrB, and DctB) that participate in regulation of gene expression in response to environmental factors.

Nucleotide sequence of the uhp region of Escherichia coli

The Escherichia coli uhp region encodes the transport system that mediates the uptake of a number of sugar phosphates as well as the regulatory components that are responsible for induction of this transport system by external glucose 6-phosphate. Four uhp genes have been identified by analysis of the complementation behavior and polypeptide coding capacity of plasmids carrying subcloned regions or transposon insertions. The nucleotide sequence of a 6.5-kilobase segment that contains the 3' end of the ilvBN operon and the entire uhp region was determined. Four open reading frames were identified in the locations expected for the various uhp genes; all were oriented in the same direction, counterclockwise relative to the genetic map. The properties of the polypeptides predicted from the nucleotide sequence were consistent with their observed features. The 196-amino-acid UhpA polypeptide has the composition characteristic of a soluble protein and bears homology to the DNA-binding regions of many regulatory activators and repressors. The 518-amino-acid UhpB and the 199-amino-acid UhpC regulatory proteins contain substantial segments of hydrophobic character. Similarly, the 463-amino-acid UhpT transporter is a hydrophobic protein with numerous potential transmembrane segments. The UhpC regulatory protein has substantial sequence homology to part of UhpT, suggesting that this regulatory protein might have evolved by duplication of the gene for the transporter and that its role in transmembrane signaling may involve sugar-phosphate-binding sites and transmembrane orientations similar to those of the transport protein.

Identification of uhp polypeptides and evidence for their role in exogenous induction of the sugar phosphate transport system of Escherichia coli K-12

Cells of Escherichia coli possess a transport system that catalyzes the accumulation, in unaltered form, of a variety of sugar phosphates. Induction of the transport activity occurs in response to external glucose 6-phosphate and does not require detectable entry of this inducer. To define the genes that encode the Uhp transport system and those that mediate its exogenous induction, transposon insertions were isolated and mapped within a 6.5-kilobase HindIII-BamHI fragment that carries the entire uhp region. The transposon insertions were transferred by homologous recombination onto the chromosome to test their effect on Uhp expression when all genes were present in single copy number. The complementation behavior of plasmids carrying the insertions or subcloned fragments of the region was compared with their polypeptide coding capacity in maxicells. These studies defined three uhp regulatory genes (uhpABC), all of which are necessary for expression of the uhpT gene, which encodes the transporter. The products of uhpB and uhpC are not required when uhpA is present on a multicopy plasmid. The four genes, uhpA, uhpB, uhpC, and uhpT, are transcribed in the same direction, and their products have apparent molecular weights of 25,000, 48,000, 20,000, and 38,000, respectively. The UhpB and UhpT polypeptides are associated with the membrane fraction. These results led to a model of regulation in which the UhpB and UhpC regulatory proteins prevent the ability of UhpA to activate transcription of the uhpT gene under noninducing conditions.

Inactivation of the ampD gene causes semiconstitutive overproduction of the inducible Citrobacter freundii beta-lactamase

In Citrobacter freundii and Enterobacter cloacae, synthesis of AmpC beta-lactamase is inducible by the addition of beta-lactams to the growth medium. Spontaneous mutants that constitutively overproduce the enzyme occur at a high frequency. When the C. freundii ampC beta-lactamase gene is cloned into Escherichia coli together with the regulatory gene ampR, beta-lactamase expression from the clone is inducible. Spontaneous cefotaxime-resistant mutants were selected from an E. coli strain carrying the cloned C. freundii ampC and ampR genes on a plasmid. Virtually all isolates had chromosomal mutations leading to semiconstitutive overproduction of beta-lactamase. The mutation ampD2 in one such mutant was caused by an IS1 insertion into the hitherto unknown ampD gene, located between nadC and aroP at minute 2.4 on the E. coli chromosome. The wild-type ampD allele cloned on a plasmid could fully trans-complement beta-lactamase-overproducing mutants of both E. coli and C. freundii, restoring the wild-type phenotype of highly inducible enzyme synthesis. This indicates that these E. coli and C. freundii mutants have their lesions in ampD. We hypothesize that induction of beta-lactamase synthesis is caused by blocking of the AmpD function by the beta-lactam inducer and that this leads directly or indirectly to an AmpR-mediated stimulation of ampC expression.

Genetic mapping and transcriptional orientation of the fimD gene

Expression of type 1 fimbriae in Escherichia coli exhibits phase variation whereby individual cells can alternate between states of organelle expression and nonexpression. Strains carrying fim-lac operon fusions in which lac operon expression is under the control of a fim promoter undergo Lac+ in equilibrium Lac- phase variation. We have determined the genetic map location and direction of transcription of a fim-lac operon fusion which was obtained by insertion of lac into a locus we have named fimD. We found the gene order to be as follows: valS fimD uxuA serB. The direction of transcription of fimD was found to be clockwise on the E. coli chromosome.

Genetic control of the hexose phosphate transport system of Escherichia coli: mapping of deletion and insertion mutations in the uhp region

The Escherichia coli transport system responsible for the accumulation of a number of sugar phosphates is encoded by the uhp region and is induced by external, but not intracellular, glucose 6-phosphate. To delineate the genetic organization of the uhp region, a total of 225 independent point, deletion, and transposon Tn10 insertion mutations were collected. Mutations conferring the Uhp-phenotype were obtained on the basis of their resistance to fosfomycin and their inability to use sugar phosphates as carbon source. Deletions of uhp sequences were obtained as a consequence of imprecise excision of Tn10 insertions located on either side of uhp. Conjugal crosses between these deletions and the point of insertion mutations allowed determination of the relative order of the uhp alleles and of the deletion endpoints. Specialized lambda transducing phages carrying a uhpT-lac operon fusion and various amounts of adjacent uhp material were isolated and used as genetic donors. Results from these crosses corroborated those obtained in the conjugal crosses. The locations of the mutant alleles were compared with the regulatory properties of Uhp+ revertants of these alleles. This comparison suggested the existence of at least three genes in which mutation yields the Uhp-phenotype. Mapping experiments were consistent with the gene order pyrE-gltS-uhpTRA-ilvB, where uhpT encodes the transport system and uhpR and uhpA are regulatory genes whose products are necessary for proper uhp regulation.

Molecular cloning of the uhp region and evidence for a positive activator for expression of the hexose phosphate transport system of Escherichia coli

The uhp locus of Escherichia coli contains genes for the sugar phosphate transport system (uhpT) and the regulatory system which allows its induction by external glucose 6-phosphate (uhpRA). The uhp region was cloned onto high-copy-number plasmids, both from Uhp(+) plasmids of the Clarke-Carbon collection and from genetically characterized specialized transducing phages carrying uhpT-lac operon fusions. Two Clarke-Carbon plasmids and their Uhp(+) subclones in pBR322 shared restriction sites defining the uhp region, but exhibited different regulation of Uhp expression and dependence on chromosomal uhp genotype. Plasmid pLC17-47 and derivatives conferred constitutive glucose 6-phosphate uptake activity in all strains, even those with complete deletions of uhp. These plasmids also rendered constitutive the expression of a chromosomal uhpT-lac operon fusion. Plasmid pLC40-33 conferred inducible Uhp expression, which required the presence of the uhpA(+) gene on the chromosome. The induced transport levels in all strains carrying these plasmids were not appreciably amplified over haploid levels. Similar behavior was seen with the cloned operon fusions. A fusion-bearing plasmid that carried an intact regulatory system (uhpR(+)A(+)) exhibited trans-dominant constitutive expression of beta-galactosidase, regardless of the chromosomal uhp genotype. In contrast, the cloned fusion carrying only uhpR(+) gave glucose 6-phosphate-inducible production of beta-galactosidase that was dependent on the presence of chromosomal uhpA(+). Expression of both fusions in the haploid state was inducible. From these results, it was concluded that the uhpA product is necessary for uhpT transcription and that elevated dosage of uhpA results in at least partially constitutive expression of uhpT. A tentative model for uhp regulation is presented.

Regulation of adenylate cyclase synthesis in Escherichia coli: studies with cya-lac operon and protein fusion strains

We have isolated cya-lac operon and protein fusions in Escherichia coli K-12, and we used these to study the regulation of cya, the structural gene for adenylate cyclase. Data obtained from these fusion strains suggest that neither cyclic AMP (cAMP) nor the cAMP receptor protein plays a major role in transcriptional or translational regulation of cya expression. Modulation of intracellular cAMP concentrations elicited only weak repression of cya-lac fusion activity under conditions of high intracellular cAMP, relative to fusion activity under conditions of low intracellular cAMP. The functional cAMP receptor protein was required for this effect. Incorporation of delta crp into cya-lac fusion strains did not affect fusion expression in glucose-grown cells as compared with similarly cultured isogenic crp+ strains. Furthermore, 20 independently obtained mutants derived from a cya-lacZ protein fusion strain exhibiting a weak Lac+ phenotype were isolated, and it was determined that the mutants had beta-galactosidase activities ranging from 2- to 77-fold greater than those of the parental strain. None of the mutations responsible for this increase in fusion activity map in the crp locus. We used these mutants to aid in the identification of a 160,000-dalton cya-lacZ hybrid protein. Finally, chromosome mobilization experiments, using cya-lac fusion strains, allowed us to infer a clockwise direction of transcription for the cya gene relative to the standard E. coli genetic map.