An in vivo pathway for disulfide bond isomerization in Escherichia coli

Mutations in dsbA and dsbB, but not dsbC, lead to an enhanced sensitivity of Escherichia coli to Hg2+ and Cd2+

The Dsb proteins are involved in disulfide bond formation, reduction and isomerisation in a number of Gram-negative bacteria. Mutations in dsbA or dsbB, but not dsbC, increase the proportion of proteins with free thiols in the periplasm compared to wild-type. We investigated the effects of mutations in these genes on the bacterial resistance to mercuric and cadmium salts. Mutations in genes involved primarily in disulfide formation (dsbA and dsbB) generally enhanced the sensitivity to Hg2+ and Cd2+ while a mutation of the dsbC gene (primarily an isomerase of disulfide bonds) had no effect. Mutations of the dsb genes had no effect on the expression of the mercury-resistance determinants of the transposon Tn501.

In vivo and in vitro function of the Escherichia coli periplasmic cysteine oxidoreductase DsbG

We have characterized in vivo and in vitro the recently identified DsbG from Escherichia coli. In addition to sharing sequence homology with the thiol disulfide exchange protein DsbC, DsbG likewise was shown to form a stable periplasmic dimer, and it displays an equilibrium constant with glutathione comparable with DsbA and DsbC. DsbG was found to be expressed at approximately 25% the level of DsbC. In contrast to earlier results (Andersen, C. L., Matthey-Dupraz, A., Missiakas, D., and Raina, S. (1997) Mol. Microbiol. 26, 121-132), we showed that dsbG is not essential for growth and that dsbG null mutants display no defect in folding of multiple disulfide-containing heterologous proteins. Overexpression of DsbG, however, was able to restore the ability of dsbC mutants to express heterologous multidisulfide proteins, namely bovine pancreatic trypsin inhibitor, a protein with three disulfides, and to a lesser extent, mouse urokinase (12 disulfides). As in DsbC, the putative active site thiols in DsbG are completely reduced in vivo in a dsbD-dependent fashion, as would be expected if DsbG is acting as a disulfide isomerase or reductase. However, the latter is not likely because DsbG could not catalyze insulin reduction in vitro. Overall, our results indicate that DsbG functions primarily as a periplasmic disulfide isomerase with a narrower substrate specificity than DsbC.

Respiratory chain strongly oxidizes the CXXC motif of DsbB in the Escherichia coli disulfide bond formation pathway

Escherichia coli DsbB has four essential cysteine residues, among which Cys41 and Cys44 form a CXXC redox active site motif and the Cys104-Cys130 disulfide bond oxidizes the active site cysteines of DsbA, the disulfide bond formation factor in the periplasm. Functional respiratory chain is required for the cell to keep DsbA oxidized. In this study, we characterized the roles of essential cysteines of DsbB in the coupling with the respiratory chain. Cys104 was found to form the inactive complex with DsbA under respiration-defective conditions. While DsbB, under normal aerobic conditions, is in the oxidized state, having two intramolecular disulfide bonds, oxidation of Cys104 and Cys130 requires the presence of Cys41-Cys44. Remarkably, the Cys41-Cys44 disulfide bond is refractory to reduction by a high concentration of dithiothreitol, unless the membrane is solubilized with a detergent. This reductant resistance requires both the respiratory function and oxygen, since Cys41-Cys44 became sensitive to the reducing agent when membrane was prepared from quinone- or heme-depleted cells or when a membrane sample was deaerated. Thus, the Cys41-Val-Leu-Cys44 motif of DsbB is kept both strongly oxidized and strongly oxidizing when DsbB is integrated into the membrane with the normal set of respiratory components.

The genetics of disulfide bond metabolism

Disulfide bonds are required for the stability and function of a large number of proteins. Genetic analysis in combination with biochemical studies have elucidated the main catalysts involved in facilitating these processes in the cell. All enzymes involved in thiol-disulfide metabolism have a conserved active site that consists of two cysteine residues, separated by two intervening amino acids, the Cys-Xaa-Xaa-Cys motif. While these enzymes are capable of catalyzing both disulfide bond formation and reduction, they have evolved to perform one or the other reaction more efficiently. In the cytoplasm, multiple pathways are involved in the reduction of disulfide bonds that occur as part of the catalytic cycle of a variety of metabolic enzymes. In the bacterial periplasm, a system for the efficient introduction as well as isomerization of disulfide bonds is in place. In eukaryotes, disulfide bonds are introduced into proteins in the endoplasmic reticulum. Genetic studies have recently begun to reveal new features of this process. While the enzyme mechanisms of thiol-disulfide oxidoreductases have been the subject of much scrutiny, questions remain regarding where and when they act in vivo, their specificities, and the maintenance of the redox environment that determines their function.

Expression of active human tissue-type plasminogen activator in Escherichia coli

The formation of native disulfide bonds in complex eukaryotic proteins expressed in Escherichia coli is extremely inefficient. Tissue plasminogen activator (tPA) is a very important thrombolytic agent with 17 disulfides, and despite numerous attempts, its expression in an active form in bacteria has not been reported. To achieve the production of active tPA in E. coli, we have investigated the effect of cooverexpressing native (DsbA and DsbC) or heterologous (rat and yeast protein disulfide isomerases) cysteine oxidoreductases in the bacterial periplasm. Coexpression of DsbC, an enzyme which catalyzes disulfide bond isomerization in the periplasm, was found to dramatically increase the formation of active tPA both in shake flasks and in fermentors. The active protein was purified with an overall yield of 25% by using three affinity steps with, in sequence, lysine-Sepharose, immobilized Erythrina caffra inhibitor, and Zn-Sepharose resins. After purification, approximately 180 microgram of tPA with a specific activity nearly identical to that of the authentic protein can be obtained per liter of culture in a high-cell-density fermentation. Thus, heterologous proteins as complex as tPA may be produced in an active form in bacteria in amounts suitable for structure-function studies. In addition, these results suggest the feasibility of commercial production of extremely complex proteins in E. coli without the need for in vitro refolding.

Disulfide bond formation in the Escherichia coli cytoplasm: an in vivo role reversal for the thioredoxins

Cytoplasmic proteins do not generally contain structural disulfide bonds, although certain cytoplasmic enzymes form such bonds as part of their catalytic cycles. The disulfide bonds in these latter enzymes are reduced in Escherichia coli by two systems; the thioredoxin pathway and the glutathione/glutaredoxin pathway. However, structural disulfide bonds can form in proteins in the cytoplasm when the gene (trxB) for the enzyme thioredoxin reductase is inactivated by mutation. This disulfide bond formation can be detected by assessing the state of the normally periplasmic enzyme alkaline phosphatase (AP) when it is localized to the cytoplasm. Here we show that the formation of disulfide bonds in cytoplasmic AP in the trxB mutant is dependent on the presence of two thioredoxins in the cell, thioredoxins 1 and 2, the products of the genes trxA and trxC, respectively. Our evidence supports a model in which the oxidized forms of these thioredoxins directly catalyze disulfide bond formation in cytoplasmic AP, a reversal of their normal role. In addition, we show that the recently discovered thioredoxin 2 can perform many of the roles of thioredoxin 1 in vivo, and thus is able to reduce certain essential cytoplasmic enzymes. Our results suggest that the three most effective cytoplasmic disulfide-reducing proteins are thioredoxin 1, thioredoxin 2 and glutaredoxin 1; expression of any one of these is sufficient to support aerobic growth. Our results help to explain how the reducing environment in the cytoplasm is maintained so that disulfide bonds do not normally occur.

The reductive enzyme thioredoxin 1 acts as an oxidant when it is exported to the Escherichia coli periplasm

Thioredoxin 1 is a major thiol-disulfide oxidoreductase in the cytoplasm of Escherichia coli. One of its functions is presumed to be the reduction of the disulfide bond in the active site of the essential enzyme ribonucleotide reductase. Thioredoxin 1 is kept in a reduced state by thioredoxin reductase. In a thioredoxin reductase null mutant however, most of thioredoxin 1 is in the oxidized form; recent reports have suggested that this oxidized form might promote disulfide bond formation in vivo. In the Escherichia coli periplasm, the protein disulfide isomerase DsbC is maintained in the reduced and active state by the membrane protein DsbD. In a dsbD null mutant, DsbC accumulates in the oxidized form. This oxidized form is then able to promote disulfide bond formation. In both these cases, the inversion of the function of these thiol oxidoreductases appears to be due to an altered redox balance of the environment in which they find themselves. Here, we show that thioredoxin 1 attached to the alkaline phosphatase signal sequence can be exported into the E. coli periplasm. In this new environment for thioredoxin 1, we show that thioredoxin 1 can promote disulfide bond formation and, therefore, partially complement a dsbA strain defective for disulfide bond formation. Thus, we provide evidence that by changing the location of thioredoxin 1 from cytoplasm to periplasm, we change its function from a reductant to an oxidant. We conclude that the in vivo redox function of thioredoxin 1 depends on the redox environment in which it is localized.

Inactivation of DsbA, but not DsbC and DsbD, affects the intracellular survival and virulence of Shigella flexneri

In this study, three mutants, dsbA::kan, dsbC-kan, and dsbD-kan, of Shigella flexneri serotype 5 were constructed and characterized to investigate the role of the periplasmic thiol:disulfide oxidoreductases in pathogenicity. In gentamicin protection assays and the Serény test, the dsbA mutant showed reduced virulence while the dsbC and dsbD mutants were similar to the wild type. That inactivation of dsbA was responsible for the reduced virulence was verified by complementation with the cloned wild-type gene in in vitro and in vivo assays. Despite the changed virulence behavior, the dsbA mutant could penetrate HeLa cells 15 min postinfection, consistent with the fact that it actively secretes Ipa proteins upon Congo red induction. Furthermore, the dsbA mutant was able to produce actin comets and protrusions, indicating its capacity for intra- and intercellular spread. However, a kinetic analysis of intracellular growth showed that the dsbA mutant barely grew in HeLa cells during a 4-h infection whereas the wild type had a doubling time of 41 min. Electron microscopy analysis revealed that dsbA mutant bacteria were trapped in protrusion-derived vacuoles surrounded by double membranes, resembling an icsB mutant reported previously. Moreover, the trapped bacteria appeared to be lysed simultaneously with the double membranes, resulting in characteristic empty vacuoles in the host cell cytosol. Thus, the attenuation mechanism for dsbA mutant appears to be more complicated than was previously suggested.

A new heat-shock gene, ppiD, encodes a peptidyl-prolyl isomerase required for folding of outer membrane proteins in Escherichia coli

We have identified a new folding catalyst, PpiD, in the periplasm of Escherichia coli. The gene encoding PpiD was isolated as a multicopy suppressor of surA, a mutation which severely impairs the folding of outer membrane proteins (OMPs). The ppiD gene was also identified based on its ability to be transcribed by the two-component system CpxR-CpxA. PpiD was purified to homogeneity and shown to have peptidyl-prolyl isomerase (PPIase) activity in vitro. The protein is anchored to the inner membrane via a single transmembrane segment, and its catalytic domain faces the periplasm. In addition, we have identified by site-directed mutagenesis some of the residues essential for its PPIase activity. A null mutation in ppiD leads to an overall reduction in the level and folding of OMPs and to the induction of the periplasmic stress response. The combination of ppiD and surA null mutations is lethal. This is the first time two periplasmic folding catalysts have been shown to be essential. Another unique aspect of PpiD is that its gene is regulated by both the Cpx two-component system and the sigma32 heat shock factor, known to regulate the expression of cytoplasmic chaperones.

Molecular mechanisms of cytochrome c biogenesis: three distinct systems

The past 10 years have heralded remarkable progress in the understanding of the biogenesis of c-type cytochromes. The hallmark of c-type cytochrome synthesis is the covalent ligation of haem vinyl groups to two cysteinyl residues of the apocytochrome (at a Cys-Xxx-Yyy-Cys-His signature motif). From genetic, genomic and biochemical studies, it is clear that three distinct systems have evolved in nature to assemble this ancient protein. In this review, common principles of assembly for all systems and the molecular mechanisms predicted for each system are summarized. Prokaryotes, plant mitochondria and chloroplasts use either system I or II, which are each predicted to use dedicated mechanisms for haem delivery, apocytochrome ushering and thioreduction. Accessory proteins of systems I and II co-ordinate the positioning of these two substrates at the membrane surface for covalent ligation. The third system has evolved specifically in mitochondria of fungi, invertebrates and vertebrates. For system III, a pivotal role is played by an enzyme called cytochrome c haem lyase (CCHL) in the mitochondrial intermembrane space.

The functional properties of DsbG, a thiol-disulfide oxidoreductase from the periplasm of Escherichia coli

Genetic studies have recently identified DsbG, a new member of the dsb group of redox proteins, which catalyze protein disulfide bond formation in the periplasm of Escherichia coli. We now demonstrate that DsbG functions primarily as an oxidant during protein disulfide bond formation, which is consistent with the low stability of its active site disulfide bond. There are indications, however, that the substrate range of DsbG may be narrower than the other periplasmic oxidative enzymes, DsbA and DsbC. Our observations further elaborate the pathway of disulfide bond formation in E. coli.

The active-site cysteines of the periplasmic thioredoxin-like protein CcmG of Escherichia coli are important but not essential for cytochrome c maturation in vivo

A new member of the family of periplasmic protein thiol:disulfide oxidoreductases, CcmG (also called DsbE), was characterized with regard to its role in cytochrome c maturation in Escherichia coli. The CcmG protein was shown to be membrane bound, facing the periplasm with its C-terminal, hydrophilic domain. A chromosomal, nonpolar in-frame deletion in ccmG resulted in the complete absence of all c-type cytochromes. Replacement of either one or both of the two cysteine residues of the predicted active site in CcmG (WCPTC) led to low but detectable levels of Bradyrhizobium japonicum holocytochrome c550 expressed in E. coli. This defect, but not that of the ccmG null mutant, could be complemented by adding low-molecular-weight thiol compounds to growing cells, which is in agreement with a reducing function for CcmG.

Overexpression of Escherichia coli oxidoreductases increases recombinant insulin-like growth factor-I accumulation

Transient overexpression of either DsbA or DsbC can double the yield of periplasmic insulin-like growth factor (IGF)-I in Escherichia coli to 8.5 g/liter. Strikingly, most of the overexpressed DsbA or DsbC is found in the reduced form, implying that enhanced disulfide isomerization is responsible for the substantial increase in IGF-I yield. All of the accumulated IGF-I has had the signal sequence removed, underscoring the secretion capacity of this organism as well as its utility for efficient production of polypeptide with the correct amino terminus. The overexpressed IGF-I constitutes approximately 30% of the total cell protein. Overproduction of active site mutants of DsbA instead of the wild-type gene do not produce this increase in yield. With wild-type levels of DsbA and DsbC, most of the secreted IGF-I is found in disulfide-linked aggregates, although 10% is soluble and about 5% is correctly folded. Contrary to expectations, overexpression of the disulfide oxidoreductases decreased the soluble fraction. Because the aggregated protein can be efficiently solubilized and refolded, the increased yield is a significant benefit for the production of IGF-I.

Contrasting routes of c-type cytochrome assembly in mitochondria, chloroplasts and bacteria

The biogenesis of bacterial c-type cytochromes generally involves many gene products--some of which may also have roles in other processes--and their interaction with the disulphide-bond-forming system of the bacterial periplasm. However, in some bacteria a simpler process appears to operate that might be related to the formation of c-type cytochromes in thylakoids of photosynthetic cells. The corresponding process in fungal mitochondria is distinct.

Expression of eukaryotic proteins in soluble form in Escherichia coli

At the optimum temperature for its growth (37 degrees C), Escherichia coli tends to accumulate heterologous proteins in insoluble form. Fusion protein technology has been used to increase the solubility of overexpressed proteins in this organism, but with variable degrees of success. Fusion to a mutant form of DsbA (DsbAmut) confers higher levels of solubility to heterologous proteins in a reproducible way, even when E. coli is grown at 37 degrees C. We have shown this to be true with a diverse sample of eukaryotic proteins: IGF-I, IGFBP-3, 3C proteinase, TGF beta-2, sTGF beta-RII, BDNF, GDNF, mEGFBP, leptin, and GFP. In addition, we have investigated the effects of charge average and proline content on the solubility of DsbAmut fusions. Coexpression of a protein prolyl isomerase [cyclophilin (L-)] and modification of selected asparagine residues to aspartic acid appear to have beneficial effects on the accumulation of soluble heterologous proteins.

Reduction of the periplasmic disulfide bond isomerase, DsbC, occurs by passage of electrons from cytoplasmic thioredoxin

The Escherichia coli periplasmic protein DsbC is active both in vivo and in vitro as a protein disulfide isomerase. For DsbC to attack incorrectly formed disulfide bonds in substrate proteins, its two active-site cysteines should be in the reduced form. Here we present evidence that, in wild-type cells, these two cysteines are reduced. Further, we show that a pathway involving the cytoplasmic proteins thioredoxin reductase and thioredoxin and the cytoplasmic membrane protein DsbD is responsible for the reduction of these cysteines. Thus, reducing potential is passed from cytoplasmic electron donors through the cytoplasmic membrane to DsbC. This pathway does not appear to utilize the cytoplasmic glutathione-glutaredoxin pathway. The redox state of the active-site cysteines of DsbC correlates quite closely with its ability to assist in the folding of proteins with multiple disulfide bonds. Analysis of the activity of mutant forms of DsbC in which either or both of these cysteines have been altered further supports the role of DsbC as a disulfide bond isomerase.

Making and breaking disulfide bonds

It is now well established that protein folding requires the assistance of folding helpers in vivo. The formation or isomerization of disulfide bonds in proteins is a slow process requiring catalysis. In nascent polypeptide chains the cysteine residues are in the thiol form. The formation of the disulfide bonds usually occurs simultaneously with the folding of the polypeptide, which means in the endoplasmic reticulum of eukaryotes or in the periplasm of Gram-negative bacteria. In prokaryotes, the existence of redox proteins involved in the formation of disulfide bonds containing proteins has recently been revealed in the periplasm. The discovery of these redox proteins through various genetic approaches will be summarized, as well as the most recent insights regarding their biochemical and biological activities.

Respiratory chain is required to maintain oxidized states of the DsbA-DsbB disulfide bond formation system in aerobically growing Escherichia coli cells

DsbA, the disulfide bond catalyst of Escherichia coli, is a periplasmic protein having a thioredoxin-like Cys-30-Xaa-Xaa-Cys-33 motif. The Cys-30-Cys-33 disulfide is donated to a pair of cysteines on the target proteins. Although DsbA, having high oxidizing potential, is prone to reduction, it is maintained essentially all oxidized in vivo. DsbB, an integral membrane protein having two pairs of essential cysteines, reoxidizes DsbA that has been reduced upon functioning. It is not known, however, what might provide the overall oxidizing power to the DsbA-DsbB disulfide bond formation system. We now report that E. coli mutants defective in the hemA gene or in the ubiA-menA genes markedly accumulate the reduced form of DsbA during growth under the conditions of protoheme deprivation as well as ubiquinone/menaquinone deprivation. Disulfide bond formation of beta-lactamase was impaired under these conditions. Intracellular state of DsbB was found to be affected by deprivation of quinones, such that it accumulates first as a reduced form and then as a form of a disulfide-linked complex with DsbA. This is followed by reduction of the bulk of DsbA molecules. These results suggest that the respiratory electron transfer chain participates in the oxidation of DsbA, by acting primarily on DsbB. It is remarkable that a cellular catalyst of protein folding is connected to the respiratory chain.

Disruption of the Pseudomonas aeruginosa dipZ gene, encoding a putative protein-disulfide reductase, leads to partial pleiotropic deficiency in c-type cytochrome biogenesis

The Pseudomonas aeruginosa dipZ gene has been cloned and sequenced. Whereas disruption of Escherichia coli dipZ (dsbD), the hydrophilic C-terminal domain of which has been deduced to be periplasmic and to function as a protein-disulfide reductase, leads to the absence of c-type cytochromes, disruption of P. aeruginosa dipZ attenuated, but did not abolish, holo-c-type cytochrome biosynthesis. Comparison of the P. aeruginosa DipZ sequence with three other DipZ sequences indicated that there are not only two conserved cysteine residues in the C-terminal hydrophilic domain, but also two more in the central highly hydrophobic domain. The latter would be located toward the centre of two of the eight membrane-spanning alpha-helices predicted to compose the hydrophobic central domain of DipZ. Both these cysteine residues, plus other transmembrane helix residues, notably prolines and glycines, are also conserved in a group of membrane proteins, related to Bacillus subtilis CcdA, which lack the N- and C-terminal hydrophilic domains of the DipZ proteins. It is proposed that DipZ of P. aeruginosa and other organisms transfers reducing power from the cytoplasm to the periplasm through reduction and reoxidation of an intramembrane disulfide bond, or other mechanism involving these cysteine residues, and that this function can also be performed by B. subtilis CcdA and other CcdA-like proteins. The failure of dipZ disruption to abolish c-type cytochrome synthesis in P. aeruginosa suggests that, in contrast to the situation in E. coli, the absence of DipZ can be compensated for by one or more other proteins, for example a CcdA-like protein acting in tandem with one or more thioredoxin-like proteins.

A thioreduction pathway tethered to the membrane for periplasmic cytochromes c biogenesis; in vitro and in vivo studies

The c-type cytochromes are distinguished from other heme proteins by the covalent ligation of two heme vinyl groups to two cysteine residues on the apoprotein (at a CXXCH domain). The present study was undertaken to elucidate the roles and topological locations of two of the proteins necessary for cytochrome c biogenesis, the HelX and Ccl2 proteins in the Gram-negative bacteria Rhodobacter capsulatus. From their primary sequence, each of these proteins has a CXXC motif that could be involved in the reduction of the cysteine residues of the apocytochromes c, a prerequisite for covalent ligation to the heme. Results of site-directed mutagenesis of HelX and Ccl2 demonstrate that each cysteine residue is required for the in vivo function of the protein. We demonstrate that the native HelX in R. capsulatus is tethered to the cytoplasmic membrane via its uncleaved signal sequence. Ccl2 is tethered by a single transmembrane domain present in the C terminus with the N-terminal two-thirds of the protein in the periplasm. Thus, both CXXC motifs are exposed to the periplasm. The complete HelX protein and the soluble N-terminal portion of Ccl2 (called Ccl2*) were overproduced and purified from periplasmic fractions. The Ccl2* signal sequence is efficiently processed. In vitro studies with these purified proteins indicate that although neither can reduce insulin, HelX can reduce the Ccl2 cysteine residues and the Ccl2 cysteine residues are oxidized by an apocytochrome c peptide containing the CXXCH domain. Revertants of an helX deletion mutant were isolated that regain the ability to make c-type cytochromes (and thus grow photosynthetically); some of these suppressor strains are enhanced for photosynthetic growth by the addition of thio-reducing agents. In contrast, revertants of a ccl2 deletion strain could not be isolated under any condition. These results suggest that the HelX and Ccl2 proteins form a thioreduction pathway (HelX-->Ccl2-->apocytochrome c) whereby Ccl2 function may be highly specific for apocytochromes c while HelX may act as a more general reductant of proteins with vicinal cysteines.

Biogenesis of respiratory cytochromes in bacteria

Biogenesis of respiratory cytochromes is defined as consisting of the posttranslational processes that are necessary to assemble apoprotein, heme, and sometimes additional cofactors into mature enzyme complexes with electron transfer functions. Different biochemical reactions take place during maturation: (i) targeting of the apoprotein to or through the cytoplasmic membrane to its subcellular destination; (ii) proteolytic processing of precursor forms; (iii) assembly of subunits in the membrane and oligomerization; (iv) translocation and/or modification of heme and covalent or noncovalent binding to the protein moiety; (v) transport, processing, and incorporation of other cofactors; and (vi) folding and stabilization of the protein. These steps are discussed for the maturation of different oxidoreductase complexes, and they are arranged in a linear pathway to best account for experimental findings from studies concerning cytochrome biogenesis. The example of the best-studied case, i.e., maturation of cytochrome c, appears to consist of a pathway that requires at least nine specific genes and more general cellular functions such as protein secretion or the control of the redox state in the periplasm. Covalent attachment of heme appears to be enzyme catalyzed and takes place in the periplasm after translocation of the precursor through the membrane. The genetic characterization and the putative biochemical functions of cytochrome c-specific maturation proteins suggest that they may be organized in a membrane-bound maturase complex. Formation of the multisubunit cytochrome bc, complex and several terminal oxidases of the bo3, bd, aa3, and cbb3 types is discussed in detail, and models for linear maturation pathways are proposed wherever possible.

A Salmonella typhimurium genetic locus which confers copper tolerance on copper-sensitive mutants of Escherichia coli

Three distinct clones from a Salmonella typhimurium genomic library were identified which suppressed the copper-sensitive (Cu(s)) phenotype of cutF mutants of Escherichia coli. One of these clones, pCUTFS2, also increased the copper tolerance of cutA, -C, and -E mutants, as well as that of a lipoprotein diacylglyceryl transferase (lgt) mutant of E. coli. Characterization of pCUTFS2 revealed that the genes responsible for suppression of copper sensitivity (scs) reside on a 4.36-kb DNA fragment located near 25.4 min on the S. typhimurium genome. Sequence analysis of this fragment revealed four open reading frames (ORF120, ORF627, ORF207, and ORF168) that were organized into two operons. One operon consisted of a single gene, scsA (ORF120), whereas the other operon contained the genes scsB (ORF627), scsC (ORF207), and scsD (ORF168). Comparison of the deduced amino acid sequences of the predicted gene products showed that ScsB, ScsC, and ScsD have significant homology to thiol-disulfide interchange proteins (CutA2, DipZ, CycZ, and DsbD) from E. coli and Haemophilus influenzae, to an outer membrane protein (Com1) from Coxiella burnetii, and to thioredoxin and thioredoxin-like proteins, respectively. The two operons were subcloned on compatible plasmids, and complementation analyses indicated that all four proteins are required for the increased copper tolerance of E. coli mutants. In addition, the scs locus also restored lipoprotein modification in lgt mutants of E. coli. Sequence analyses of the S. typhimurium scs genes and adjacent DNAs revealed that the scs locus is flanked by genes with high homology to the cbpA (predicted curved DNA-binding protein) and agp (acid glucose phosphatase) genes of E. coli located at 22.90 min (1,062.07 kb) and 22.95 min (1,064.8 kb) of the E. coli chromosome, respectively. However, examination of the E. coli chromosome revealed that these genes are absent at this locus and no evidence has thus been obtained for the occurrence of the scs locus elsewhere on the genome.

The role of the thioredoxin and glutaredoxin pathways in reducing protein disulfide bonds in the Escherichia coli cytoplasm

In Escherichia coli, two pathways use NADPH to reduce disulfide bonds that form in some cytoplasmic enzymes during catalysis: the thioredoxin system, which consists of thioredoxin reductase and thioredoxin, and the glutaredoxin system, composed of glutathione reductase, glutathione, and three glutaredoxins. These systems may also reduce disulfide bonds which form spontaneously in cytoplasmic proteins when E. coli is grown aerobically. We have investigated the role of both systems in determining the thiol-disulfide balance in the cytoplasm by determining the ability of protein disulfide bonds to form in mutants missing components of these systems. We find that both the thioredoxin and glutaredoxin systems contribute to reducing disulfide bonds in cytoplasmic proteins. In addition, these systems can partially substitute for each other in vivo since double mutants missing parts of both systems generally allow substantially more disulfide bond formation than mutants missing components of just one system. Some of these double mutants were found to require the addition of a disulfide reductant to the medium to grow well aerobically. Thus, E. coli requires either a functional thioredoxin or glutaredoxin system to reduce disulfide bonds which appear after each catalytic cycle in the essential enzyme ribonucleotide reductase and perhaps to reduce non-native disulfide bonds in cytoplasmic proteins. Our results suggest the existence of a novel thioredoxin in E. coli.

Regulation of Escherichia coli cell envelope proteins involved in protein folding and degradation by the Cpx two-component system

We show that the two-component signal transduction system of Escherichia coli, CpxA-CpxR, controls the expression of genes encoding cell envelope proteins involved in protein folding and degradation. These findings are based on three lines of evidence. First, activation of the Cpx pathway induces 5- to 10-fold the synthesis of DsbA, required for disulfide bond formation, and DegP, a major periplasmic protease. Second, using electrophoretic mobility shift and DNase I protection assays, we have shown that phosphorylated CpxR binds to elements upstream of the transcription start sites of dsbA, degP, and ppiA (rotA), the latter coding for a peptidyl-prolyl cis/trans isomerase. Third, we have demonstrated increased in vivo transcription of all three genes, dsbA, degP, and ppiA, when the Cpx pathway is activated. We have identified a putative CpxR consensus binding site that is found upstream of a number of other E. coli genes. These findings suggest a potentially extensive Cpx regulon including genes transcribed by sigma70 and sigma(E), which encode factors involved in protein folding as well as other cellular functions.

Differential in vivo roles played by DsbA and DsbC in the formation of protein disulfide bonds

Several Escherichia coli proteins participate in protein disulfide bond formation. Among them, DsbA is the primary factor that oxidizes target cysteines. Biochemical evidence indicates that DsbC has disulfide isomerization activity. To study intracellular functions of DsbA and DsbC, we used an alkaline phosphatase mutant, PhoA[SCCC], with the most amino-terminal cysteine replaced by serine. It was found that the remaining 3 cysteines in PhoA[SCCC] form a disulfide bond of incorrect as well as correct combinations. An aberrant disulfide bond was preferentially formed in wild-type cells, which was converted slowly to the normal disulfide bond. This conversion did not occur in the dsbC-disrupted cells. Overproduction of DsbC stimulated the formation of the correct disulfide bond. In contrast, the inefficiently formed disulfide bonds in the dsbA-disrupted cells, and the more efficiently formed disulfide bonds in the same strain in the presence of oxidized glutathione were mostly in the correct form. These results suggest that the DsbA-catalyzed reaction can be too rapid for some proteins. DsbA may simply oxidize available pairs of cysteines, which happen to be in an incorrect combination in the case of PhoA[SCCC]. In contrast, DsbC stimulates the formation of correct disulfide bonds and corrects previously introduced aberrant ones. Thus, DsbC acts to isomerize disulfide bonds in vivo.

Insertional inactivation of dsbA produces sensitivity to cadmium and zinc in Escherichia coli

In a search for genes that produce hypersensitivity to cadmium salts in Escherichia coli, random transposon mutagenesis with TnphoA was used. One of the mutant strains obtained was sensitive to Cd2+ and Zn2+. Sequence analysis showed that the TnphoA insertion was located in the dsbA gene coding for a periplasmic protein required for disulfide bond formation.