Colicin import into Escherichia coli cells

Mechanisms of bacterial immunity, protection, and survival during interbacterial warfare

Most bacteria live in communities, often with closely related strains and species with whom they must compete for space and resources. Consequently, bacteria have acquired or evolved mechanisms to antagonize competitors through the production of antibacterial toxins. Similar to bacterial systems that combat phage infection and mechanisms to thwart antibiotics, bacteria have also acquired and evolved features to protect themselves from antibacterial toxins. Just as there is a large body of research identifying and characterizing antibacterial proteins and toxin delivery systems, studies of bacterial mechanisms to resist and survive assault from competitors' weapons have also expanded tremendously. Emerging data are beginning to reveal protective processes and mechanisms that are as diverse as the toxins themselves. Protection against antibacterial toxins can be acquired by horizontal gene transfer, receptor or target alteration, induction of protective functions, physical barriers, and other diverse processes. Here, we review recent studies in this rapidly expanding field.

Escherichia coli killing by epidemiologically successful sublineages of Shigella sonnei is mediated by colicins

BACKGROUND

Shigella sp. are enteric pathogens which causes >125 million cases of shigellosis annually. S. sonnei accounts for about a quarter of those cases and is increasingly prevalent in industrialising nations. Being an enteric pathogen, S. sonnei benefits from outcompeting gut commensals such as Escherichia coli to establish itself and cause disease. There are numerous mechanisms that bacterial pathogens use to outcompete its rivals including molecules called colicins. A Type 6 Secretion System (T6SS) was recently described as contributing to E. coli killing in S. sonnei.

METHODS

We used Bulk Phenotyping of Epidemiological Replicates (BPER) which combined bacterial Genome Wide Association Studies (bGWAS) and high throughput phenotyping on a collection of S. sonnei surveillance isolates to identify the genetic features associated with E. coli killing and explore their relationship with epidemiological behaviour. We further explored the presence of colicins and T6SS components in the isolates using genomics, laboratory experimentation, and proteomics.

FINDINGS

Our bGWAS analysis returned known and novel colicin and colicin related genes as significantly associated with E. coli killing. In silico analyses identified key colicin clusters responsible for the killing phenotype associated with epidemiologically successful sub-lineages. The killing phenotype was not associated with the presence of a T6SS. Laboratory analyses confirmed the presence of the key colicin clusters and that killing was contact-independent.

INTERPRETATION

Colicins are responsible for E. coli killing by S. sonnei, not a T6SS. This phenotype contributes to shaping the observed epidemiology of S. sonnei and may contribute to its increasing prevalence globally. BPER is an epidemiologically relevant approach to phenotypic testing that enables the rapid identification of genetic drivers of phenotypic changes, and assessment of their relevance to epidemiology in natural settings.

FUNDING

Biotechnology and Biological Sciences Research Council, Biotechnology and Biological Sciences Research Council Doctoral Training Partnership studentship, Wellcome Trust, Medical Research Council (UK), French National Research Agency.

A Novel Deoxyribonuclease Low-Molecular-Weight Bacteriocin, Carocin S4, from Pectobacterium carotovorum subsp. carotovorum

Pectobacterium carotovorum subsp. carotovorum (Pcc) is known to produce different types of bacteriocins, active protein substances that inhibit or kill related strains and are known to be induced by several factors. In this paper, we report the discovery, isolation, characterization, and functional analysis of Carocin S4, a novel low-molecular-weight bacteriocin (LMWB) from Pcc. A 2750 bp gene fragment was isolated from the chromosomal DNA of Pcc mutant strain rif-TO6, a rifampicin-resistant strain of TO6. The gene contains caroS4K and caroS4I within two open reading frames, which encode CaroS4K and CaroS4I, with molecular weights of about 90 kD and 10 kD, respectively. The unique characteristics of Carocin S4 were revealed after homology analysis with the previously discovered bacteriocins from Pcc. CaroS4K, which shares 23% and 85% homology with CaroS1K and CaroS3K, respectively, is also a deoxyribonuclease. However, unlike the two which can only hydrolyze genomic DNA, CaroS4K hydrolyzes both genomic and plasmid DNA. On the other hand, CaroS4K was found to be 90% homologous with CaroS2K but works differently in killing the target cell, as the latter is a ribonuclease. The optimal reaction temperature for CaroS4K to hydrolyze dsDNA is approximately 50 °C and requires the divalent metal ions Mg²⁺, Ca²⁺, and Zn²⁺ to catalyze its DNase activity. This study reveals another nuclease type of bacteriocin in Pcc, with CaroS4K and CaroS4I functioning as killer and immunity proteins, respectively.

Qualitative and Quantitative Methods to Measure Antibacterial Activity Resulting from Bacterial Competition

In the environment, bacteria compete for niche occupancy and resources; they have, therefore, evolved a broad variety of antibacterial weapons to destroy competitors. Current laboratory techniques to evaluate antibacterial activity are usually labor intensive, low throughput, costly, and time consuming. Typical assays rely on the outgrowth of colonies of prey cells on selective solid media after competition. Here, we present fast, inexpensive, and complementary optimized protocols to qualitatively and quantitively measure antibacterial activity. The first method is based on the degradation of a cell-impermeable chromogenic substrate of the β-galactosidase, a cytoplasmic enzyme released during lysis of the attacked reporter strain. The second method relies on the lag time required for the attacked cells to reach a defined optical density after the competition, which is directly dependent on the initial number of surviving cells. Key features First method utilizes the release of β-galactosidase as a proxy for bacterial lysis. Second method is based on the growth timing of surviving cells. Combination of two methods discriminates between cell death and lysis, cell death without lysis, or survival to quasi-lysis. Methods optimized to various bacterial species such as Escherichia coli, Pseudomonas aeruginosa, and Myxococcus xanthus. Graphical overview.

Revisiting the steps of Salmonella gut infection with a focus on antagonistic interbacterial interactions

A commensal microbial community is established in the mammalian gut during its development, and these organisms protect the host against pathogenic invaders. The hallmark of noninvasive Salmonella gut infection is the induction of inflammation via effector proteins secreted by the type III secretion system, which modulate host responses to create a new niche in which the pathogen can overcome the colonization resistance imposed by the microbiota. Several studies have shown that endogenous microbes are important to control Salmonella infection by competing for resources. However, there is limited information about antimicrobial mechanisms used by commensals and pathogens during these in vivo disputes for niche control. This review aims to revisit the steps that Salmonella needs to overcome during gut colonization-before and after the induction of inflammation-to achieve an effective infection. We focus on a series of reported and hypothetical antagonistic interbacterial interactions in which both contact-independent and contact-dependent mechanisms might define the outcome of the infection.

Bacteriocins to Thwart Bacterial Resistance in Gram Negative Bacteria

An overuse of antibiotics both in human and animal health and as growth promoters in farming practices has increased the prevalence of antibiotic resistance in bacteria. Antibiotic resistant and multi-resistant bacteria are now considered a major and increasing threat by national health agencies, making the need for novel strategies to fight bugs and super bugs a first priority. In particular, Gram-negative bacteria are responsible for a high proportion of nosocomial infections attributable for a large part to Enterobacteriaceae, such as pathogenic Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. To cope with their highly competitive environments, bacteria have evolved various adaptive strategies, among which the production of narrow spectrum antimicrobial peptides called bacteriocins and specifically microcins in Gram-negative bacteria. They are produced as precursor peptides that further undergo proteolytic cleavage and in many cases more or less complex posttranslational modifications, which contribute to improve their stability and efficiency. Many have a high stability in the gastrointestinal tract where they can target a single pathogen whilst only slightly perturbing the gut microbiota. Several microcins and antibiotics can bind to similar bacterial receptors and use similar pathways to cross the double-membrane of Gram-negative bacteria and reach their intracellular targets, which they also can share. Consequently, bacteria may use common mechanisms of resistance against microcins and antibiotics. This review describes both unmodified and modified microcins [lasso peptides, siderophore peptides, nucleotide peptides, linear azole(in)e-containing peptides], highlighting their potential as weapons to thwart bacterial resistance in Gram-negative pathogens and discusses the possibility of cross-resistance and co-resistance occurrence between antibiotics and microcins in Gram-negative bacteria.

Bifurcated binding of the OmpF receptor underpins import of the bacteriocin colicin N into Escherichia coli

Colicins are Escherichia coli-specific bacteriocins that translocate across the outer bacterial membrane by a poorly understood mechanism. Group A colicins typically parasitize the proton-motive force-linked Tol system in the inner membrane via porins after first binding an outer membrane protein receptor. Recent studies have suggested that the pore-forming group A colicin N (ColN) instead uses lipopolysaccharide as a receptor. Contrary to this prevailing view, using diffusion-precipitation assays, native state MS, isothermal titration calorimetry, single-channel conductance measurements in planar lipid bilayers, and in vivo fluorescence imaging, we demonstrate here that ColN uses OmpF both as its receptor and translocator. This dual function is achieved by ColN having multiple distinct OmpF-binding sites, one located within its central globular domain and another within its disordered N terminus. We observed that the ColN globular domain associates with the extracellular surface of OmpF and that lipopolysaccharide (LPS) enhances this binding. Approximately 90 amino acids of ColN then translocate through the porin, enabling the ColN N terminus to localize within the lumen of an OmpF subunit from the periplasmic side of the membrane, a binding mode reminiscent of that observed for the nuclease colicin E9. We conclude that bifurcated engagement of porins is intrinsic to the import mechanism of group A colicins.

The B12 receptor BtuB alters the membrane integrity of Caulobacter crescentus

Vitamin B12 is one of the most complex biomolecules in nature. Since few organisms can synthesize B12de novo, most bacteria utilize highly sensitive and specialized transporters to scavenge B12 and its precursors. In Gram-negative bacteria, BtuB is the outer membrane TonB-dependent receptor for B12. In the fresh water bacterium Caulobacter crescentus, btuB is among the most highly expressed genes. In this study, we characterized the function of BtuB in C. crescentus and unveiled a potential new function of this receptor involved in cellular fitness. Under standard minimal or rich growth conditions, we found that supplements of vitamin B12 to cultures of C. crescentus provided no significant advantage in growth rate. Using a B12 methionine auxotroph, we showed that BtuB in C. crescentus is capable of transporting B12 at low pico-molar range. A btuB knockout strain displayed higher sensitivity to detergents and to changes in osmotic pressure compared to the wild-type. Electron micrographs of this knockout strain revealed a morphology defect. The sensitivity observed in the btuB knockout strain was not due to changes in membrane permeability or altered S-layer levels. Our results demonstrate that btuB deletion mutants exhibit increased susceptibility to membrane stressors, suggesting a potential role of this receptor in membrane homeostasis. Because we only tested BtuB's function under laboratory conditions, we cannot eliminate the possibility that BtuB also plays a key role as a B12 scavenger in C. crescentus when growing in its highly variable and nutrient-limited natural environment.

Inflammation fuels colicin Ib-dependent competition of Salmonella serovar Typhimurium and E. coli in enterobacterial blooms

The host's immune system plays a key role in modulating growth of pathogens and the intestinal microbiota in the gut. In particular, inflammatory bowel disorders and pathogen infections induce shifts of the resident commensal microbiota which can result in overgrowth of Enterobacteriaceae (“inflammation-inflicted blooms”). Here, we investigated competition of the human pathogenic Salmonella enterica serovar Typhimurium strain SL1344 (S. Tm) and commensal E. coli in inflammation-inflicted blooms. S. Tm produces colicin Ib (ColIb), which is a narrow-spectrum protein toxin active against related Enterobacteriaceae. Production of ColIb conferred a competitive advantage to S. Tm over sensitive E. coli strains in the inflamed gut. In contrast, an avirulent S. Tm mutant strain defective in triggering gut inflammation did not benefit from ColIb. Expression of ColIb (cib) is regulated by iron limitation and the SOS response. CirA, the cognate outer membrane receptor of ColIb on colicin-sensitive E. coli, is induced upon iron limitation. We demonstrate that growth in inflammation-induced blooms favours expression of both S. Tm ColIb and the receptor CirA, thereby fuelling ColIb dependent competition of S. Tm and commensal E. coli in the gut. In conclusion, this study uncovers a so-far unappreciated role of inflammation-inflicted blooms as an environment favouring ColIb-dependent competition of pathogenic and commensal representatives of the Enterobacteriaceae family.

Colicins are bacterial protein toxins which show potent activity against sensitive strains in vitro. Ecological models suggest that colicins play a major role in modulating dynamics of bacterial populations in the gut. However, previous studies could not readily confirm these predictions by respective in vivo experiments. In animal models, colicin-producing strains only show a minor or even absent fitness benefit over sensitive competitors. Here, we propose that the gut environment plays a crucial role in generating conditions for bacterial competition by colicin Ib (ColIb). Gut inflammation favours overgrowth of Enterobacteriaceae (“inflammation-inflicted Enterobacterial blooms”). We show that a pathogenic Salmonella Typhimurium (S. Tm) strain benefits from ColIb production in competition against commensal E. coli upon growth in inflammation-inflicted blooms. In the absence of gut inflammation, ColIb production did not confer a competitive advantage to S. Tm. In the inflamed gut, the genes for ColIb production in S. Tm and its corresponding ColIb-surface receptor CirA in E. coli were markedly induced, as compared to the non-inflamed gut. Therefore, environmental conditions in inflammation-inflicted blooms favour colicin-dependent competition of Enterobacteriaceae by triggering ColIb production and susceptibility at the same time. Our findings reveal a role of colicins as important bacterial fitness factors in inflammation-induced blooms.

Antagonistic functions between the RNA chaperone Hfq and an sRNA regulate sensitivity to the antibiotic colicin

The RNA chaperone Hfq is a key regulator of the function of small RNAs (sRNAs). Hfq has been shown to facilitate sRNAs binding to target mRNAs and to directly regulate translation through the action of sRNAs. Here, we present evidence that Hfq acts as the repressor of cirA mRNA translation in the absence of sRNA. Hfq binding to cirA prevents translation initiation, which correlates with cirA mRNA instability. In contrast, RyhB pairing to cirA mRNA promotes changes in RNA structure that displace Hfq, thereby allowing efficient translation as well as mRNA stabilization. Because CirA is a receptor for the antibiotic colicin Ia, in addition to acting as an Fur (Ferric Uptake Regulator)-regulated siderophore transporter, translational activation of cirA mRNA by RyhB promotes colicin sensitivity under conditions of iron starvation. Altogether, these results indicate that Fur and RyhB modulate an unexpected feed-forward loop mechanism related to iron physiology and colicin sensitivity.

Molecular characterization of antibiotic resistant Salmonella Typhimurium and Salmonella Kentucky isolated from pre- and post-chill whole broilers carcasses

There is conflicting data regarding whether commercial chilling has any effect on persistence of Salmonella serovars, including antibiotic resistant variants, on chicken carcasses. A total of 309 Salmonella Typhimurium and Salmonella Kentucky isolates recovered from pre- and post-chill whole broiler carcasses were characterized for genetic relatedness using Pulsed Field Gel Electrophoresis (PFGE) and for the presence of virulence factors (invA, pagC, spvC) by PCR and for aerobactin and colicin production by bioassays. A subset of these isolates (n = 218) displaying resistance to either sulfisoxazole and/or ceftiofur [S. Typhimurium (n = 66) and S. Kentucky (n = 152)] were further tested for the presence of associated antibiotic resistance elements (class-I integrons and blaCMY genes) by PCR. All 145 ceftiofur resistant S. Kentucky and S. Typhimurium isolates possessed blaCMY genes. Class-I integrons were only detected in 6.1% (n = 4/66) of sulfisoxazole resistant S. Typhimurium isolates. The PFGE analysis revealed the presence of genetically diverse populations within the recovered isolates but clusters were generally concordant with serotypes and antimicrobial resistance profiles. At a 100% pattern similarity index, thirty-six percent of the undistinguishable S. Typhimurium and 22% of the undistinguishable S. Kentucky isolates were recovered from the same chilling step. All isolates possessed the invA and pagC genes, but only 1.4%possessed spvC. Irrespective of the chilling step, there was a significant difference (P < 0.05) in the production of aerobactin and colicin between S. Typhimurium and S. Kentucky isolates. Taken together, these results indicate that chilling impacted the recovery of particular Salmonella clonal groups but had no effect on the presence of class-I integrons, blaCMY genes, and tested virulence factors.

Translocation trumps receptor binding in colicin entry into Escherichia coli

Of the steps involved in the killing of Escherichia coli by colicins, binding to a specific outer-membrane receptor was the best understood and earliest characterized. Receptor binding was believed to be an indispensable step in colicin intoxication, coming before the less well-understood step of translocation across the outer membrane to present the killing domain to its target. In the process of identifying the translocator for colicin Ia, I created chimaeric colicins, as well as a deletion missing the entire receptor-binding domain of colicin Ia. The normal pathway for colicin Ia killing was shown to require two copies of Cir: one that serves as the primary receptor and a second copy that serves as translocator. The novel Ia colicins retain the ability to kill E. coli, even in the absence of receptor binding, as long as they can translocate via their Cir translocator. Experiments to determine whether colicin M uses a second copy of its receptor, FhuA, as its translocator were hampered by precipitation of colicin M chimaeras in inclusion bodies. Nevertheless, I show that receptor binding can be bypassed for killing, as long as a translocation pathway is maintained for colicin M. These experiments suggest that colicin M, unlike colicin Ia, may normally use a single copy of FhuA as both its receptor and its translocator. Colicin E1 can kill in the absence of receptor binding, using translocation through TolC.

Harmonic analysis of the fluorescence response of bimane adducts of colicin E1 at helices 6, 7, and 10

The pre-channel state of helices 6, 7, and 10 (Val(447)-Gly(475) and Ile(508)-Ile(522)) of colicin E1 was investigated by a site-directed fluorescence labeling technique. A total of 44 cysteine variants were purified and covalently labeled with monobromobimane fluorescent probe. A variety of fluorescence properties of the bimane fluorophore were measured for both the soluble and membrane-bound states of the channel peptide, including the fluorescence emission maximum, fluorescence anisotropy, and membrane bilayer penetration depth. Using site-directed fluorescence labeling combined with our novel helical periodicity analysis method, the data revealed that helices 6, 7, and 10 are separate amphipathic α-helices with a calculated periodicity of T = 3.34 ± 0.08 for helix 6, T = 3.56 ± 0.03 for helix 7, and T = 2.99 ± 0.12 for helix 10 in the soluble state. In the membrane-bound state, the helical periodicity was determined to be T = 3.00 ± 0.15 for helix 6, T = 3.68 ± 0.03 for helix 7, and T = 3.47 ± 0.04 for helix 10. Dual fluorescence quencher analysis showed that both helices 6 and 7 adopt a tilted topology that correlates well with the analysis based on the fluorescence anisotropy profile. These data provide further support for the umbrella model of the colicin E1 channel domain.

Repression of btuB gene transcription in Escherichia coli by the GadX protein

Background

BtuB (B  twelve uptake) is an outer membrane protein of Escherichia coli, it serves as a receptor for cobalamines uptake or bactericidal toxin entry. A decrease in the production of the BtuB protein would cause E. coli to become resistant to colicins. The production of BtuB has been shown to be regulated at the post-transcriptional level. The secondary structure switch of 5' untranslated region of butB and the intracellular concentration of adenosylcobalamin (Ado-Cbl) would affect the translation efficiency and RNA stability of btuB. The transcriptional regulation of btuB expression is still unclear.

Results

To determine whether the btuB gene is also transcriptionally controlled by trans-acting factors, a genomic library was screened for clones that enable E. coli to grow in the presence of colicin E7, and a plasmid carrying gadX and gadY genes was isolated. The lacZ reporter gene assay revealed that these two genes decreased the btuB promoter activity by approximately 50%, and the production of the BtuB protein was reduced by approximately 90% in the presence of a plasmid carrying both gadX and gadY genes in E. coli as determined by Western blotting. Results of electrophoretic mobility assay and DNase I footprinting indicated that the GadX protein binds to the 5' untranslated region of the btuB gene. Since gadX and gadY genes are more highly expressed under acidic conditions, the transcriptional level of btuB in cells cultured in pH 7.4 or pH 5.5 medium was examined by quantitative real-time PCR to investigate the effect of GadX. The results showed the transcription of gadX with 1.4-fold increase but the level of btuB was reduced to 57%.

Conclusions

Through biological and biochemical analysis, we have demonstrated the GadX can directly interact with btuB promoter and affect the expression of btuB. In conclusion, this study provides the first evidence that the expression of btuB gene is transcriptionally repressed by the acid responsive genes gadX and gadY.

Interaction of the colicin K bactericidal toxin with components of its import machinery in the periplasm of Escherichia coli

Colicins are bacterial antibiotic toxins produced by Escherichia coli cells and are active against E. coli and closely related strains. To penetrate the target cell, colicins bind to an outer membrane receptor at the cell surface and then translocate their N-terminal domain through the outer membrane and the periplasm. Once fully translocated, the N-terminal domain triggers entry of the catalytic C-terminal domain by an unknown process. Colicin K uses the Tsx nucleoside-specific receptor for binding at the cell surface, the OmpA protein for translocation through the outer membrane, and the TolABQR proteins for the transit through the periplasm. Here, we initiated studies to understand how the colicin K N-terminal domain (KT) interacts with the components of its transit machine in the periplasm. We first produced KT fused to a signal sequence for periplasm targeting. Upon production of KT in wild-type strains, cells became partly resistant to Tol-dependent colicins and sensitive to detergent, released periplasmic proteins, and outer membrane vesicles, suggesting that KT interacts with and titrates components of its import machine. Using a combination of in vivo coimmunoprecipitations and in vitro pulldown experiments, we demonstrated that KT interacts with the TolA, TolB, and TolR proteins. For the first time, we also identified an interaction between the TolQ protein and a colicin translocation domain.

Interaction of colicin E7 with the major coat protein (g8p) may confer limited protection on colicinogenic Escherichia coli against M13 bacteriophage infection

Colicin release provides producer strains with a competitive advantage under certain circumstances. We found that propagation of M13 bacteriophage in cells producing colicin E7 is impaired, without alteration in the efficiency of bacteriophage adsorption, as compared with non-producing cells. In contrast to the protective effect of the colicin against M13 bacteriophage infection, the endogenously expressed colicin does not confer limited protection against transfection with M13 bacteriophage DNA. Furthermore, it was found that the translocation-receptor-binding domain and toxicity domain of the colicin are able to interact with the M13 major coat protein, g8p, during bacteriophage infection. Based on these observations, we propose that interaction between colicin E7 and g8p during infection interferes with g8p depolymerizing into the cytoplasmic membrane during bacteriophage DNA penetration, thus resulting in the limited protection against M13 bacteriophage infection.

A predicted S-type pyocin shows a bactericidal activity against clinical Pseudomonas aeruginosa isolates through membrane damage

The nucleic acid sequence at the positions 1067817-1066321 of Pseudomonas aeruginosa PAO1 genome was predicted to encode a novel S-type pyocin, designated S5, based on the genome sequence. However, its antimicrobial spectrum, activity and mechanism have not been investigated. Herein, we report that pyocin S5 has an antimicrobial activity against seven clinical P. aeruginosa isolates (DWW3, InA, InB, In3, In4, In7, and In8). Among them, DWW3 is most sensitive with a minimum inhibitory concentration of 12.6 microg/ml and a killing percentage of 95.7 at 225 microg/ml. Further, we demonstrated that the antimicrobial mechanism of pyocin S5 is membrane damage, evidenced by the leakage of intracellular materials, the increase of membrane permeability, and cell surface disruption.

Health considerations regarding horizontal transfer of microbial transgenes present in genetically modified crops

The potential effects of horizontal gene transfer on human health are an important item in the safety assessment of genetically modified organisms. Horizontal gene transfer from genetically modified crops to gut microflora most likely occurs with transgenes of microbial origin. The characteristics of microbial transgenes other than antibiotic-resistance genes in market-approved genetically modified crops are reviewed. These characteristics include the microbial source, natural function, function in genetically modified crops, natural prevalence, geographical distribution, similarity to other microbial genes, known horizontal transfer activity, selective conditions and environments for horizontally transferred genes, and potential contribution to pathogenicity and virulence in humans and animals. The assessment of this set of data for each of the microbial genes reviewed does not give rise to health concerns. We recommend including the above-mentioned items into the premarket safety assessment of genetically modified crops carrying transgenes other than those reviewed in the present study.

The colicin Ia receptor, Cir, is also the translocator for colicin Ia

Colicin Ia, a channel-forming bactericidal protein, uses the outer membrane protein, Cir, as its primary receptor. To kill Escherichia coli, it must cross this membrane. The crystal structure of Ia receptor-binding domain bound to Cir, a 22-stranded plugged beta-barrel protein, suggests that the plug does not move. Therefore, another pathway is needed for the colicin to cross the outer membrane, but no 'second receptor' has ever been identified for TonB-dependent colicins, such as Ia. We show that if the receptor-binding domain of colicin Ia is replaced by that of colicin E3, this chimera effectively kills cells, provided they have the E3 receptor (BtuB), Cir, and TonB. This is consistent with wild-type Ia using one Cir as its primary receptor (BtuB in the chimera) and a second Cir as the translocation pathway for its N-terminal translocation (T) domain and its channel-forming C-terminal domain. Deletion of colicin Ia's receptor-binding domain results in a protein that kills E. coli, albeit less effectively, provided they have Cir and TonB. We show that purified T domain competes with Ia and protects E. coli from being killed by it. Thus, in addition to binding to colicin Ia's receptor-binding domain, Cir also binds weakly to its translocation domain.

A Novel extracytoplasmic function (ECF) sigma factor regulates virulence in Pseudomonas aeruginosa

Next to the two-component and quorum sensing systems, cell-surface signaling (CSS) has been recently identified as an important regulatory system in Pseudomonas aeruginosa. CSS systems sense signals from outside the cell and transmit them into the cytoplasm. They generally consist of a TonB-dependent outer membrane receptor, a sigma factor regulator (or anti-sigma factor) in the cytoplasmic membrane, and an extracytoplasmic function (ECF) sigma factor. Upon perception of the extracellular signal by the receptor the ECF sigma factor is activated and promotes the transcription of a specific set of gene(s). Although most P. aeruginosa CSS systems are involved in the regulation of iron uptake, we have identified a novel system involved in the regulation of virulence. This CSS system, which has been designated PUMA3, has a number of unusual characteristics. The most obvious difference is the receptor component which is considerably smaller than that of other CSS outer membrane receptors and lacks a β-barrel domain. Homology modeling of PA0674 shows that this receptor is predicted to be a bilobal protein, with an N-terminal domain that resembles the N-terminal periplasmic signaling domain of CSS receptors, and a C-terminal domain that resembles the periplasmic C-terminal domains of the TolA/TonB proteins. Furthermore, the sigma factor regulator both inhibits the function of the ECF sigma factor and is required for its activity. By microarray analysis we show that PUMA3 regulates the expression of a number of genes encoding potential virulence factors, including a two-partner secretion (TPS) system. Using zebrafish (Danio rerio) embryos as a host we have demonstrated that the P. aeruginosa PUMA3-induced strain is more virulent than the wild-type. PUMA3 represents the first CSS system dedicated to the transcriptional activation of virulence functions in a human pathogen.

Pseudomonas aeruginosa is a versatile pathogen; these bacteria are able to cause an infection in humans and other mammals, zebrafish, insects, nematodes and even plants. P. aeruginosa evolved an impressive amount of gene regulation systems to be able to express the right virulence genes under the right circumstances. The best studied examples of these are the two-component systems and the autoinducers. In addition, P. aeruginosa is also able to regulate virulence genes using the pyoverdine cell-surface signaling system (CSS). Genome analysis shows that there are multiple putative CSS systems in P. aeruginosa. In this paper we have studied a novel CSS system with a number of remarkable characteristics and show that this system is involved in the regulation of several putative virulence factors. Induction of this system leads to increased virulence in our zebrafish embryo infection model. Our study provides new insights into the regulation of virulence by P. aeruginosa.

Characterization of the orf1-tolQRA operon in Pseudomonas aeruginosa

The tol-pal genes play important roles in maintaining outer membrane integrity, transmembrane transportation, and cell division in Gram-negative bacteria. In Pseudomonas aeruginosa, an important human opportunistic pathogen, the tol-oprL genes are organized uniquely in three operons, orf1-tolQRA, tolB and oprL-orf2, and are regulated by iron availability. Similarity between TolQRA and the iron transport system ExbBD-TonB also exists in P. aeruginosa and they can replace each other imperfectly. It is of importance to investigate the regulation and functions of this membrane complex. In the present study, we characterized the promoters and expression profiles of the orf1-tolQRA operon and investigated the function of Orf1. Primer extension was carried out by using both isotope-labeled and florescence labeled primers and the expression profiles were determined using both lacZ and luxCDABE-based transcriptional fusions. Our results revealed two distinct promoters at the upstream region of tolQRA; the one located in front of orf1 was constitutive whereas the other within the orf1 coding region was iron regulated. Expression profiles indicate the tol genes were also downregulated by the quorum-sensing systems during the late stage of growth. Unlike tolQ and tolA, we were able to construct a viable orf1 knockout strain, and the mutant exhibited altered cell and colony morphology, providing first evidence that Orf1 plays a non-essential role in the Tol-OprL complex in P. aeruginosa.

Genome-wide screens: novel mechanisms in colicin import and cytotoxicity

Only two new genes (fkpA and lepB) have been identified to be required for colicin cytotoxicity in the last 25 years. Genome-wide screening using the 'Keio collection' to test sensitivity to colicins (col) A, B, D, E1, E2, E3, E7 and N from groups A and B, allowed identification of novel genes affecting cytotoxicity and provided new information on mechanisms of action. The requirement of lipopolysaccharide for colN cytotoxicity resides specifically in the lipopolysaccharide inner-core and first glucose. ColA cytotoxicity is dependent on gmhB and rffT genes, which function in the biosynthesis of lipopolysaccharide and enterobacterial common antigen. Of the tol genes that function in the cytoplasmic membrane translocon, colE1 requires tolA and tolR but not tolQ for activity. Peptidoglycan-associated lipoprotein, which interacts with the Tol network, is not required for cytotoxicity of group A colicins. Except for TolQRA, no cytoplasmic membrane protein is essential for cytotoxicity of group A colicins, implying that TolQRA provides the sole pathway for their insertion into/through the cytoplasmic membrane. The periplasmic protease that cleaves between the receptor and catalytic domains of colE7 was not identified, implying either that the responsible gene is essential for cell viability, or that more than one gene product has the necessary proteolysis function.

The role of SOS boxes in enteric bacteriocin regulation

Bacteriocins are a large and functionally diverse family of toxins found in all major lineages of Bacteria. Colicins, those bacteriocins produced by Escherichia coli, serve as a model system for investigations of bacteriocin structure-function relationships, genetic organization, and their ecological role and evolutionary history. Colicin expression is often dependent on host regulatory pathways (such as the SOS system), is usually confined to times of stress, and results in death of the producing cells. This study investigates the role of the SOS system in mediating this unique form of toxin expression. A comparison of all the sequenced enteric bacteriocin promoters reveals that over 75 % are regulated by dual, overlapping SOS boxes, which serve to bind two LexA repressor proteins. Furthermore, a highly conserved poly-A motif is present in both of the binding sites examined, indicating enhanced affinity of the LexA protein for the binding site. The use of gene expression analysis and deletion mutations further demonstrates that these unique LexA cooperative binding regions result in a fine tuning of bacteriocin production, limiting it to times of stress. These results suggest that the evolution of dual SOS boxes elegantly accomplishes the task of increasing the amount of toxin produced by a cell while decreasing the rate of uninduced production, effectively reducing the cost of colicin production. This hypothesis may explain why such a promoter motif is present at such high frequencies in natural populations of bacteriocin-producing enteric bacteria.

Colicin N binds to the periphery of its receptor and translocator, outer membrane protein F

Colicins kill Escherichia coli after translocation across the outer membrane. Colicin N displays an unusually simple translocation pathway, using the outer membrane protein F (OmpF) as both receptor and translocator. Studies of this binary complex may therefore reveal a significant component of the translocation pathway. Here we show that, in 2D crystals, colicin is found outside the porin trimer, suggesting that translocation may occur at the protein-lipid interface. The major lipid of the outer leaflet interface is lipopolysaccharide (LPS). It is further shown that colicin N binding displaces OmpF-bound LPS. The N-terminal helix of the pore-forming domain, which is not required for pore formation, rearranges and binds to OmpF. Colicin N also binds artificial OmpF dimers, indicating that trimeric symmetry plays no part in the interaction. The data indicate that colicin is closely associated with the OmpF-lipid interface, providing evidence that this peripheral pathway may play a role in colicin transmembrane transport.

Genes under positive selection in Escherichia coli

We used a comparative genomics approach to identify genes that are under positive selection in six strains of Escherichia coli and Shigella flexneri, including five strains that are human pathogens. We find that positive selection targets a wide range of different functions in the E. coli genome, including cell surface proteins such as beta barrel porins, presumably because of the involvement of these genes in evolutionary arms races with other bacteria, phages, and/or the host immune system. Structural mapping of positively selected sites on trans-membrane beta barrel porins reveals that the residues under positive selection occur almost exclusively in the extracellular region of the proteins that are enriched with sites known to be targets of phages, colicins, or the host immune system. More surprisingly, we also find a number of other categories of genes that show very strong evidence for positive selection, such as the enigmatic rhs elements and transposases. Based on structural evidence, we hypothesize that the selection acting on transposases is related to the genomic conflict between transposable elements and the host genome.

The trans-envelope Tol-Pal complex is part of the cell division machinery and required for proper outer-membrane invagination during cell constriction in E. coli

Fission of bacterial cells involves the co-ordinated invagination of the envelope layers. Invagination of the cytoplasmic membrane (IM) and peptidoglycan (PG) layer is likely driven by the septal ring organelle. Invagination of the outer membrane (OM) in Gram-negative species is thought to occur passively via its tethering to the underlying PG layer with generally distributed PG-binding OM (lipo)proteins. The Tol-Pal system is energized by proton motive force and is well conserved in Gram-negative bacteria. It consists of five proteins that can connect the OM to both the PG and IM layers via protein-PG and protein-protein interactions. Although the system is needed to maintain full OM integrity, and for class A colicins and filamentous phages to enter cells, its precise role has remained unclear. We show that all five components accumulate at constriction sites in Escherichia coli and that mutants lacking an intact system suffer delayed OM invagination and contain large OM blebs at constriction sites and cell poles. We propose that Tol-Pal constitutes a dynamic subcomplex of the division apparatus in Gram-negative bacteria that consumes energy to establish transient trans-envelope connections at/near the septal ring to draw the OM onto the invaginating PG and IM layers during constriction.

Structure of colicin I receptor bound to the R-domain of colicin Ia: implications for protein import

Colicin Ia is a 69 kDa protein that kills susceptible Escherichia coli cells by binding to a specific receptor in the outer membrane, colicin I receptor (70 kDa), and subsequently translocating its channel forming domain across the periplasmic space, where it inserts into the inner membrane and forms a voltage-dependent ion channel. We determined crystal structures of colicin I receptor alone and in complex with the receptor binding domain of colicin Ia. The receptor undergoes large and unusual conformational changes upon colicin binding, opening at the cell surface and positioning the receptor binding domain of colicin Ia directly above it. We modelled the interaction with full-length colicin Ia to show that the channel forming domain is initially positioned 150 A above the cell surface. Functional data using full-length colicin Ia show that colicin I receptor is necessary for cell surface binding, and suggest that the receptor participates in translocation of colicin Ia across the outer membrane.

Tilted, extended, and lying in wait: the membrane-bound topology of residues Lys-381-Ser-405 of the colicin E1 channel domain

The membrane-bound closed state (zero potential) of the helix 3 segment (Lys-381-Ser-405) of the colicin E1 channel domain was investigated by site-directed fluorescence labeling using a bimane probe tethered to a single cysteine residue of each mutant protein. A number of fluorescence properties of the tethered bimane probe were measured for the soluble channel mutant proteins as well as for the membrane-bound proteins. A new method called helical periodicity surface analysis was employed to fit the fluorescence data to a harmonic wave function using four different statistical methods. The fit of the various data sets to a harmonic wave function indicated that the periodicity of helix 3 in the membrane-bound state is typical for an amphipathic alpha helix (3.7-4.0 residues per turn and an angular frequency between 90 and 97 degrees). Notably, upon membrane binding, helix 3 elongates from 15 residues (soluble structure) to 20 residues by a three- and two-residue extension at the N- and C-termini of the helix, respectively. Dual quencher analysis also revealed that helix 3 is appressed to the surface of the membrane with its N-terminus more deeply buried within the interfacial region of the bilayer than its C-terminus. Finally, contrary to a previous report, our data show that helices 3 and 4 remain separate and independent helices upon membrane association in the absence of a membrane potential.

Molecular Mimicry Enables Competitive Recruitment by a Natively Disordered Protein

We report the crystal structure of the Escherichia coli TolB-Pal complex, a protein-protein complex involved in maintaining the integrity of the outer membrane (OM) in all Gram-negative bacteria that is parasitized by colicins (protein antibiotics) to expedite their entry into cells. Nuclease colicins competitively recruit TolB using their natively disordered regions (NDRs) to disrupt its complex with Pal, which is thought to trigger translocation of the toxin across a locally destabilized OM. The structure shows induced-fit binding of peptidoglycan-associated lipoprotein (Pal) to the beta-propeller domain of TolB causing the N-terminus of one of its alpha-helices to unwind and several residues to undergo substantial changes in conformation. The resulting interactions with TolB are known to be essential for the stability of the complex and the bacterial OM. Structural comparisons with a TolB-colicin NDR complex reveal that colicins bind at the Pal site, mimicking rearranged Pal residues while simultaneously appearing to block induced-fit changes in TolB. The study therefore explains how colicins recruit TolB in the bacterial periplasm and highlights a novel binding mechanism for a natively disordered protein.

High level expression of His-tagged colicin 5 in E. coli and characterization of its narrow-spectrum bactericidal activity and pore-forming action

Since antibiotics with a broad spectrum of activity would select for resistance among the normal flora, colicins having a narrow spectrum of activity can potentially be developed as novel antibiotics. Colicin-based bactericidal proteins with modified spectra of activity might also be developed by further gene fusion or gene modification. To achieve these goals, it is necessary to first build an efficient system to produce large amounts of colicin. In the presence of an immunity gene, we successfully constructed an expression vector pQE30-cfa-cfi producing high levels of His-tagged colicin 5 (60-80 mg/L). We found that the purified His-tagged colicin 5 possesses narrow-spectrum bactericidal activity against nonimmune Escherichia coli cells. It is highly toxic to sensitive E. coli cells at a low concentration of 0.01 microg/ml, while it is nontoxic to other tested gram-negative bacteria, gram-positive bacteria and yeast at a high concentration of 1000 microg/ml. His-tagged colicin 5 kills sensitive cells by permeabilizing their cell membranes. It is not hemolytic to rabbit erythrocytes and has no obvious cytotoxicity to other nucleated mammalian cells at a high concentration of 500 microg/ml. The His-tagged colicin 5 is similar to wild-type colicin 5 in spectrum and bactericidal activity against E. coli. It is a potential novel antibiotic particularly for treating human and animal infections caused by pathogenic E. coli. Besides producing high level of colicin 5, the highly efficient expression vector constructed here might also be a useful tool to develop colicin-based artificial bactericidal proteins.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Mutational Analyses Define Helix Organization and Key Residues of a Bacterial Membrane Energy-transducing Complex

In Gram-negative bacteria, many biological processes are coupled to inner membrane ion gradients. Ions transit at the interface of helices of integral membrane proteins, generating mechanical energy to drive energetic processes. To better understand how ions transit through these channels, we used a model system involved in two different processes, one of which depends on inner membrane energy. The Tol machinery of the Escherichia coli cell envelope is dedicated to maintaining outer membrane stability, a process driven by the proton-motive force. The Tol system is parasitized by bacterial toxins called colicins, which are imported through the outer membrane using an energy-independent process. Herein, we mutated TolQ and TolR transmembrane residues, and we analyzed the mutants for outer membrane stability, colicin import and protein complex formation. We identified residues involved in the assembly of the complex, and a new class of discriminative mutations that conferred outer membrane destabilization identical to a tol deletion mutant, but which remained fully sensitive to colicins. Further genetic approaches revealed transmembrane helix interactions and organization in the bilayer, and suggested that most of the discriminative residues are located in a putative aqueous ion channel. We discuss a model for the function of related bacterial molecular motors.

The TolC homologue of Brucella suis is involved in resistance to antimicrobial compounds and virulence

Brucella spp., like other pathogens, must cope with the environment of diverse host niches during the infection process. In doing this, pathogens evolved different type of transport systems to help them survive and disseminate within the host. Members of the TolC family have been shown to be involved in the export of chemically diverse molecules ranging from large protein toxins to small toxic compounds. The role of proteins from the TolC family in Brucella and other alpha-2-proteobacteria has been explored little. The gene encoding the unique member of the TolC family from Brucella suis (BepC) was cloned and expressed in an Escherichia coli mutant disrupted in the gene encoding TolC, which has the peculiarity of being involved in diverse transport functions. BepC fully complemented the resistance to drugs such as chloramphenicol and acriflavine but was incapable of restoring hemolysin secretion in the tolC mutant of E. coli. An insertional mutation in the bepC gene strongly affected the resistance phenotype of B. suis to bile salts and toxic chemicals such as ethidium bromide and rhodamine and significantly decreased the resistance to antibiotics such as erythromycin, ampicillin, tetracycline, and norfloxacin. Moreover, the B. suis bepC mutant was attenuated in the mouse model of infection. Taken together, these results suggest that BepC-dependent efflux processes of toxic compounds contribute to B. suis survival inside the host.

Competitive recruitment of the periplasmic translocation portal TolB by a natively disordered domain of colicin E9

The natively disordered N-terminal 83-aa translocation (T) domain of E group nuclease colicins recruits OmpF to a colicin-receptor complex in the outer membrane (OM) as well as TolB in the periplasm of Escherichia coli, the latter triggering translocation of the toxin across the OM. We have identified the 16-residue TolB binding epitope in the natively disordered T-domain of the nuclease colicin E9 (ColE9) and solved the crystal structure of the complex. ColE9 folds into a distorted hairpin within a canyon of the six-bladed beta-propeller of TolB, using two tryptophans to bolt the toxin to the canyon floor and numerous intramolecular hydrogen bonds to stabilize the bound conformation. This mode of binding enables colicin side chains to hydrogen-bond TolB residues in and around the channel that runs through the beta-propeller and that constitutes the binding site of peptidoglycan-associated lipoprotein (Pal). Pal is a globular binding partner of TolB, and their association is known to be important for OM integrity. The structure is therefore consistent with translocation models wherein the colicin disrupts the TolB-Pal complex causing local instability of the OM as a prelude to toxin import. Intriguingly, Ca(2+) ions, which bind within the beta-propeller channel and switch the surface electrostatics from negative to positive, are needed for the negatively charged T-domain to bind TolB with an affinity equivalent to that of Pal and competitively displace it. Our study demonstrates that natively disordered proteins can compete with globular proteins for binding to folded scaffolds but that this can require cofactors such as metal ions to offset unfavorable interactions.

Colicin M Exerts Its Bacteriolytic Effect via Enzymatic Degradation of Undecaprenyl Phosphate-linked Peptidoglycan Precursors

Colicin M was earlier demonstrated to provoke Escherichia coli cell lysis via inhibition of cell wall peptidoglycan (murein) biosynthesis. As the formation of the O-antigen moiety of lipopolysaccharides was concomitantly blocked, it was hypothesized that the metabolism of undecaprenyl phosphate, an essential carrier lipid shared by these two pathways, should be the target of this colicin. However, the exact target and mechanism of action of colicin M was unknown. Colicin M was now purified to near homogeneity, and its effects on cell wall peptidoglycan metabolism reinvestigated. It is demonstrated that colicin M exhibits both in vitro and in vivo enzymatic properties of degradation of lipid I and lipid II peptidoglycan intermediates. Free undecaprenol and either 1-pyrophospho-MurNAc-pentapeptide or 1-pyrophospho-MurNAc-(pentapeptide)-Glc-NAc were identified as the lipid I and lipid II degradation products, respectively, showing that the cleavage occurred between the lipid moiety and the pyrophosphoryl group. This is the first time such an activity is described. Neither undecaprenyl pyrophosphate nor the peptidoglycan nucleotide precursors were substrates of colicin M, indicating that both undecaprenyl and sugar moieties were essential for activity. The bacteriolytic effect of colicin M therefore appears to be the consequence of an arrest of peptidoglycan polymerization steps provoked by enzymatic degradation of the undecaprenyl phosphate-linked peptidoglycan precursors.

Toward elucidating the membrane topology of helix two of the colicin E1 channel domain

The membrane-bound closed state of the colicin E1 channel domain was investigated by site-directed fluorescence labeling using a bimane fluorophore attached to each single cysteine residue within helix 2 of each mutant protein. The fluorescence properties of the bimane fluorophore were measured for the membrane-associated form of the closed channel and included fluorescence emission maximum, fluorescence anisotropy, apparent polarity, surface accessibility, and membrane bilayer penetration depth. The fluorescence data show that helix 2 is an amphipathic alpha-helix that is situated parallel to the membrane surface, but it is less deeply embedded within the bilayer interfacial region than is helix 1 in the closed channel. A least squares fit of the various data sets to a harmonic wave function indicated that the periodicity and angular frequency for helix 2 in the membrane-bound state are typical for an amphipathic alpha-helix (3.8 +/- 0.1 residues per turn and 94 +/- 4 degrees, respectively) that is located at an interfacial region of a membrane bilayer. Dual quencher analysis also revealed that helix 2 is peripherally membrane associated, with one face of the helix dipping into the interfacial region of the lipid bilayer and the other face projecting outwardly into the aqueous solvent. Finally, our data show that helices 1 and 2 remain independent helices upon membrane association with a short connector link (Tyr(363)-Gly(364)) and that short amphipathic alpha-helices participate in the formation of a lipid-dependent, toroidal pore for this colicin.

An E. coli lon mutant conferring partial resistance to colicin may reveal a novel role in regulating proteins involved in the translocation of colicin

Initially, we found that a lon mutant confers partial resistance against colicin. The results of Western blotting detected a decrease in the protein expression levels of BtuB and OmpF involved in colicin translocation in the lon mutant. Moreover, 2-D gel analysis revealed that the expression level of some scavenger proteins marks the lon mutant as being in a situation similar to oxidative stress. OxyRS and SoxRS are the two major response regulators for oxidative stress. Our RT-PCR analysis revealed an elevation of expression of the oxyS gene in the lon mutant. An immunoblot assay further confirmed that overexpression of oxyS RNA can negatively control on the expression of BtuB protein. Probably the BtuB is negatively regulated by a global regulator, oxyS, induced during oxidative stress.

Candida albicans cell wall ssa proteins bind and facilitate import of salivary histatin 5 required for toxicity

Fungicidal activity of Hst 5 is initiated by binding to cell surface proteins on Candida albicans, followed by intracellular transport to cytoplasmic effectors leading to cell death. As we identified heat shock 70 proteins (Ssa1p and/or Ssa2p) from C. albicans lysates that bind Hst 5, direct interactions between purified recombinant Ssa proteins and Hst 5 were tested by pull-down and yeast two-hybrid assays. Pulldown of both native complexes and those stabilized by cross-linking demonstrated higher affinity of Hst 5 for Ssa2p than for Ssa1p, in agreement with higher levels of interactions between Ssa2p and Hst 5 measured by yeast two-hybrid analyses. C. albicans ssa1Delta and ssa2Delta mutants were constructed to examine Hst 5 binding, translocation, and candidacidal activities. Both ssa1Delta and ssa2Delta mutants were indistinguishable from wild-type cells in growth and hyphal formation. However, C. albicans ssa2Delta mutants were highly resistant to the candidacidal activity of Hst 5, although the ssa1Delta mutant did not have any significant reduction in killing by Hst 5. Total cellular binding of 125I-Hst 5 in the ssa2Delta mutant was reduced to one-third that of wild-type cells, in contrast to the ssa1Delta mutant whose total cellular binding of Hst 5 was similar to the wild-type strain. Intracellular transport of Hst 5 was significantly impaired in the ssa2Delta mutant strain, but only mildly so in the ssa1Delta mutant. Thus, C. albicans Ssa2p facilitates fungicidal activity of Hst 5 in binding and intracellular translocation, whereas Ssa1p appears to have a lesser functional role in Hst 5 toxicity.