Translational control of mRNA processing in the F1845 fimbrial operon of Escherichia coli

Differential Afa/Dr Fimbriae Expression in the Multidrug-Resistant Escherichia coli ST131 Clone

Many antibiotic resistant uropathogenic Escherichia coli (UPEC) strains belong to clones defined by their multilocus sequence type (ST), with ST131 being the most dominant. Although we have a good understanding of resistance development to fluoroquinolones and third-generation cephalosporins by ST131, our understanding of the virulence repertoire that has contributed to its global dissemination is limited. Here we show that the genes encoding Afa/Dr fimbriae, a group of adhesins strongly associated with UPEC that cause gestational pyelonephritis and recurrent cystitis, are found in approximately one third of all ST131 strains. Sequence comparison of the AfaE adhesin protein revealed a unique allelic variant carried by 82.9% of afa-positive ST131 strains. We identify the afa regulatory region as a hotspot for the integration of insertion sequence (IS) elements, all but one of which alter afa transcription. Close investigation demonstrated that the integration of an IS1 element in the afa regulatory region leads to increased expression of Afa/Dr fimbriae, promoting enhanced adhesion to kidney epithelial cells and suggesting a mechanism for altered virulence. Finally, we provide evidence for a more widespread impact of IS1 on ST131 genome evolution, suggesting that IS dynamics contribute to strain level microevolution that impacts ST131 fitness. IMPORTANCE E. coli ST131 is the most common antibiotic resistant UPEC clone associated with human urinary tract and bloodstream infections. Understanding the features of ST131 that have driven its global dissemination remains a critical priority if we are to counter its increasing antibiotic resistance. Here, we utilized a large collection of ST131 isolates to investigate the prevalence, regulation, and function of Afa/Dr fimbriae, a well-characterized UPEC colonization and virulence factor. We show that the afa genes are found frequently in ST131 and demonstrate how the integration of IS elements in the afa regulatory region modulates Afa expression, presenting an example of altered virulence capacity. We also exploit a curated set of ST131 genomes to map the integration of the antibiotic resistance-associated IS1 element in the ST131 pangenome, providing evidence for its widespread impact on ST131 genome evolution.

Identification of genes required for the fitness of Rhodococcus equi during the infection of mice via signature-tagged transposon mutagenesis

Rhodococcus equi is a Gram-positive facultative intracellular bacterium that causes pyogranulomatous pneumonia in foals and immunocompromised people. In the present study, signature-tagged transposon mutagenesis was applied for the negative selection of R. equi mutants that cannot survive in vivo. Twenty-five distinguishable plasmid-transposon (plasposon) vectors by polymerase chain reaction (PCR), each containing a unique oligonucleotide tag, were constructed and used to select the transposon mutants that have in vivo fitness defects using a mouse systemic infection model. Of the 4,560 transposon mutants, 102 mutants were isolated via a real-time PCR-based screening as the mutants were unable to survive in the mouse model. Finally, 50 single transposon insertion sites were determined via the self-cloning strategy. The insertion of the transposon was seen on the virulence plasmid in 15 of the 50 mutants, whereas the remaining 35 mutants had the insertion of transposon on the chromosome. The chromosomal mutants contained transposon insertions in genes involved in cellular metabolism, DNA repair and recombination, gene regulation, non-ribosomal peptide synthesis, and unknown functions. Additionally, seven of the chromosomal mutants showed a reduced ability to multiply in the macrophages in vitro. In this study, we have identified several biosynthetic pathways as fitness factors associated with the growth within macrophages and survival in mice.

Pathogenesis of human diffusely adhering Escherichia coli expressing Afa/Dr adhesins (Afa/Dr DAEC): current insights and future challenges

The pathogenicity and clinical pertinence of diffusely adhering Escherichia coli expressing the Afa/Dr adhesins (Afa/Dr DAEC) in urinary tract infections (UTIs) and pregnancy complications are well established. In contrast, the implication of intestinal Afa/Dr DAEC in diarrhea is still under debate. These strains are age dependently involved in diarrhea in children, are apparently not involved in diarrhea in adults, and can also be asymptomatic intestinal microbiota strains in children and adult. This comprehensive review analyzes the epidemiology and diagnosis and highlights recent progress which has improved the understanding of Afa/Dr DAEC pathogenesis. Here, I summarize the roles of Afa/Dr DAEC virulence factors, including Afa/Dr adhesins, flagella, Sat toxin, and pks island products, in the development of specific mechanisms of pathogenicity. In intestinal epithelial polarized cells, the Afa/Dr adhesins trigger cell membrane receptor clustering and activation of the linked cell signaling pathways, promote structural and functional cell lesions and injuries in intestinal barrier, induce proinflammatory responses, create angiogenesis, instigate epithelial-mesenchymal transition-like events, and lead to pks-dependent DNA damage. UTI-associated Afa/Dr DAEC strains, following adhesin-membrane receptor cell interactions and activation of associated lipid raft-dependent cell signaling pathways, internalize in a microtubule-dependent manner within urinary tract epithelial cells, develop a particular intracellular lifestyle, and trigger a toxin-dependent cell detachment. In response to Afa/Dr DAEC infection, the host epithelial cells generate antibacterial defense responses. Finally, I discuss a hypothetical role of intestinal Afa/Dr DAEC strains that can act as "silent pathogens" with the capacity to emerge as "pathobionts" for the development of inflammatory bowel disease and intestinal carcinogenesis.

The tmRNA ribosome-rescue system

The bacterial tmRNA quality control system monitors protein synthesis and recycles stalled translation complexes in a process termed "ribosome rescue." During rescue, tmRNA acts first as a transfer RNA to bind stalled ribosomes, then as a messenger RNA to add the ssrA peptide tag to the C-terminus of the nascent polypeptide chain. The ssrA peptide targets tagged peptides for proteolysis, ensuring rapid degradation of potentially deleterious truncated polypeptides. Ribosome rescue also facilitates turnover of the damaged messages responsible for translational arrest. Thus, tmRNA increases the fidelity of gene expression by promoting the synthesis of full-length proteins. In addition to serving as a global quality control system, tmRNA also plays important roles in bacterial development, pathogenesis, and environmental stress responses. This review focuses on the mechanism of tmRNA-mediated ribosome rescue and the role of tmRNA in bacterial physiology.

Killer and protective ribosomes

In prokaryotes, translation influences mRNA decay. The breakdown of most Escherichia coli mRNAs is initiated by RNase E, a 5'-dependent endonuclease. Some mRNAs are protected by ribosomes even if these are located far upstream of cleavage sites ("protection at a distance"), whereas others require direct shielding of these sites. I argue that these situations reflect different modes of interaction of RNase E with mRNAs. Protection at a distance is most impressive in Bacilli, where ribosomes can protect kilobases of unstable downstream sequences. I propose that this protection reflects the role in mRNA decay of RNase J1, a 5'-->3' exonuclease with no E. coli equivalent. Finally, recent years have shown that besides their protective role, ribosomes can also cleave their mRNA under circumstances that cause ribosome stalling. The endonuclease associated with this "killing" activity, which has a eukaryotic counterpart ("no-go decay"), is not characterized; it may be borne by the distressed ribosome itself.

Post-transcriptional processing of the LEE4 operon in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) employs a type III secretion system (T3SS) to export translocator and effector proteins required for mucosal colonization. The T3SS is encoded in a pathogenicity island called the locus of enterocyte effacement (LEE) that is organized in five major operons, LEE1 to LEE5. LEE4 encodes a regulator of secretion (SepL), translocators (EspA, D and B), two chaperones (CesD2 and L0017), a T3SS component (EscF) and an effector protein (EspF). It was originally proposed that the esp transcript is transcribed from a promoter located at the end of sepL but other authors suggested that this transcript is the result of a post-transcriptional processing event. In this study, we established that the espADB mRNA is generated by post-transcriptional processing at the end of the sepL coding sequence. RNase E is the endonuclease involved in the cleavage, but the interaction of this enzyme with other proteins through its C-terminal half is dispensable. A putative transcription termination event in the cesD2 coding region would generate the 3' end of the transcript. Similar to what has been described for other processed transcripts, the cleavage of LEE4 seems a mechanism to differentially regulate SepL and Esp protein production.

Escherichia coli DraE adhesin-associated bacterial internalization by epithelial cells is promoted independently by decay-accelerating factor and carcinoembryonic antigen-related cell adhesion molecule binding and does not require the DraD invasin

The Dr family of Escherichia coli adhesins are virulence factors associated with diarrhea and urinary tract infections. Dr fimbriae are comprised of two subunits. DraE/AfaE represents the major structural, antigenic, and adhesive subunit, which recognizes decay-accelerating factor (DAF) and carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) CEA, CEACAM1, CEACAM3, and CEACAM6 as binding receptors. The DraD/AfaD subunit caps fimbriae and has been implicated in the entry of Dr-fimbriated E. coli into host cells. In this study, we demonstrate that DAF or CEACAM receptors independently promote DraE-mediated internalization of E. coli by CHO cell transfectants expressing these receptors. We also found that DraE-positive recombinant bacteria adhere to and are internalized by primary human bladder epithelial cells which express DAF and CEACAMs. DraE-mediated bacterial internalization by bladder cells was inhibited by agents which disrupt lipid rafts, microtubules, and phosphatidylinositol 3-kinase (PI3K) activity. Immunofluorescence confocal microscopic examination of epithelial cells detected considerable recruitment of caveolin, beta(1) integrin, phosphorylated ezrin, phosphorylated PI3K, and tubulin, but not F-actin, by cell-associated bacteria. Finally, we demonstrate that the DraD subunit, previously implicated as an "invasin," is not required for beta(1) integrin recruitment or bacterial internalization.

The utility of affinity-tags for detection of a streptococcal protein from a variety of streptococcal species

There is no systematic examination of affinity tag utility in Gram-positive bacteria, which limits the investigation of protein function in this important group of bacteria as specific antibodies for many of native proteins are generally not available. In this study, we utilized an E. coli-streptococcal shuttle vector pVT1666 and constructed two sets of expression plasmids pVPT-CTag and pVPT-NTag, with each set containing five affinity tags (GST, GFP, HSV, T7 and Nano) that can be fused to either the C- or N-terminus of a target protein. A putative glycosyltransferase (Gtf2) essential for Fap1 glycosylation was used to demonstrate the utility of the cassettes in detection of Gtf2 fusion proteins, and the biological relevance of the proteins in our working strain Streptococcus parasanguinis. GFP and T7 tags were readily expressed in S. parasanguinis as either an N- or C-terminal fusion to Gtf2. Only the C- terminal fusion of GST and HSV were able to be identified in S. parasanguinis. The Nano tag was not detected in either E. coli or S. parasanguinis. Genetic complementation experiments indicated that all the tagged Gtf2 fusion proteins could restore the Gtf2 function in the null mutant except for the Nano-tagged Gtf2 at its N-terminal fusion. Using a T7-tagged Gtf2 fusion construct, we demonstrated that the fusion cassette is also useful in detection of the fusion tag expression in other streptococci including S. mutans, S. pneumoniae and S. sanguinis. Therefore, the expression cassettes we constructed will be a useful tool not only to investigate protein-protein interactions in Fap1 biogenesis in S. parasanguinis, but also to study protein functions in other gram-positive bacteria in which pVT1666 replicates.

Regulation of Fimbrial Expression

Fimbria-mediated interaction with the host elicits both innate and adaptive immune responses, and thus their expression may not always be beneficial in vivo. Furthermore, the metabolic drain of producing fimbriae is significant. It is not surprising, therefore, to find that fimbrial production in Escherichia coli and Salmonella enterica is under extensive environmental regulation. In many instances, fimbrial expression is regulated by phase variation, in which individual cells are capable of switching between fimbriate and afimbriate states to produce a mixed population. Mechanisms of phase variation vary considerably between different fimbriae and involve both genetic and epigenetic processes. Notwithstanding this, fimbrial expression is also sometimes controlled at the posttranscriptional level. In this chapter, we review key features of the regulation of fimbrial gene expression in E. coli and Salmonella. The occurrence and distribution of fimbrial operons vary significantly among E. coli pathovars and even among the many Salmonella serovars. Therefore, general principles are presented on the basis of detailed discussion of paradigms that have been extensively studied, including Pap, type 1 fimbriae, and curli. The roles of operon specific regulators like FimB or CsgD and of global regulatory proteins like Lrp, CpxR, and the histone-like proteins H-NS and IHF are reviewed as are the roles of sRNAs and of signalling nucleotide cyclic-di-GMP. Individual examples are discussed in detail to illustrate how the regulatory factors cooperate to allow tight control of expression of single operons. Molecular networks that allow coordinated expression between multiple fimbrial operons and with flagella in a single isolate are also presented. This chapter illustrates how adhesin expression is controlled, and the model systems also illustrate general regulatory principles germane to our overall understanding of bacterial gene regulation.

Messenger RNA Decay

This chapter discusses several topics relating to the mechanisms of mRNA decay. These topics include the following: important physical properties of mRNA molecules that can alter their stability; methods for determining mRNA half-lives; the genetics and biochemistry of proteins and enzymes involved in mRNA decay; posttranscriptional modification of mRNAs; the cellular location of the mRNA decay apparatus; regulation of mRNA decay; the relationships among mRNA decay, tRNA maturation, and ribosomal RNA processing; and biochemical models for mRNA decay. Escherichia coli has multiple pathways for ensuring the effective decay of mRNAs and mRNA decay is closely linked to the cell's overall RNA metabolism. Finally, the chapter highlights important unanswered questions regarding both the mechanism and importance of mRNA decay.

Adhesins of Diffusely Adherent and Enteroaggregative Escherichia coli

Epidemiological studies have implicated enteroaggregative Escherichia coli (EAEC) strains in acute and persistent diarrhea in children, in food-borne diarrhea outbreaks, and in traveler's diarrhea, and this group is recognized as an emerging pathotype of enteric disease. Diffusely adherent E. coli (DAEC) have been implicated as a cause of diarrhea, especially in children more than 2 years old, in both developing and developed countries. Although EAEC and DAEC strains appear to have different molecular equipment for attachment to host cell surfaces, identification and characterization of the gene clusters encoding adherence evidenced close relatedness between those determinants most frequently detected in isolates belonging to these two pathotypes of diarrheagenic E. coli. DAEC strains are a heterogeneous group of E. coli isolates, many of which express the related so-called Dr adhesins. The single designation is based on the identification of one similar cellular receptor for all these proteins. Although structurally different, they all recognize the Dr human blood group antigen on the decay-accelerating factor (DAF or CD55). These adhesins are encoded by a family of closely related operons, the first characterized and sequenced being the afa operon. Consequently, it has been suggested that this group of DAEC strains producing such adhesins be named the Afa/Dr DAEC family. Three distinct but closely related gene clusters coding for phenotypically and morphologically distinct aggregative adherence fimbriae (AAF) have been characterized. In each case, electron microscopy revealed that bacterial surfaces were surrounded by long, relatively flexible fimbrial structures.

Pathogenesis of Afa/Dr diffusely adhering Escherichia coli

Over the last few years, dramatic increases in our knowledge about diffusely adhering Escherichia coli (DAEC) pathogenesis have taken place. The typical class of DAEC includes E. coli strains harboring AfaE-I, AfaE-II, AfaE-III, AfaE-V, Dr, Dr-II, F1845, and NFA-I adhesins (Afa/Dr DAEC); these strains (i) have an identical genetic organization and (ii) allow binding to human decay-accelerating factor (DAF) (Afa/Dr(DAF) subclass) or carcinoembryonic antigen (CEA) (Afa/Dr(CEA) subclass). The atypical class of DAEC includes two subclasses of strains; the atypical subclass 1 includes E. coli strains that express AfaE-VII, AfaE-VIII, AAF-I, AAF-II, and AAF-III adhesins, which (i) have an identical genetic organization and (ii) do not bind to human DAF, and the atypical subclass 2 includes E. coli strains that harbor Afa/Dr adhesins or others adhesins promoting diffuse adhesion, together with pathogenicity islands such as the LEE pathogenicity island (DA-EPEC). In this review, the focus is on Afa/Dr DAEC strains that have been found to be associated with urinary tract infections and with enteric infection. The review aims to provide a broad overview and update of the virulence aspects of these intriguing pathogens. Epidemiological studies, diagnostic techniques, characteristic molecular features of Afa/Dr operons, and the respective role of Afa/Dr adhesins and invasins in pathogenesis are described. Following the recognition of membrane-bound receptors, including type IV collagen, DAF, CEACAM1, CEA, and CEACAM6, by Afa/Dr adhesins, activation of signal transduction pathways leads to structural and functional injuries at brush border and junctional domains and to proinflammatory responses in polarized intestinal cells. In addition, uropathogenic Afa/Dr DAEC strains, following recognition of beta(1) integrin as a receptor, enter epithelial cells by a zipper-like, raft- and microtubule-dependent mechanism. Finally, the presence of other, unknown virulence factors and the way that an Afa/Dr DAEC strain emerges from the human intestinal microbiota as a "silent pathogen" are discussed.

HrpA, a DEAH-box RNA helicase, is involved in mRNA processing of a fimbrial operon in Escherichia coli

Endonucleolytic cleavage of mRNA in the daa operon of Escherichia coli is responsible for co-ordinate regulation of genes involved in F1845 fimbrial biogenesis. Cleavage occurs by an unidentified endoribonuclease, is translation dependent and involves a unique recognition mechanism. Here, we present the results of a genetic strategy used to identify factors involved in daa mRNA processing. We used a reporter construct consisting of the daa mRNA processing region fused to the gene encoding green fluorescent protein (GFP). A mutant defective in daa mRNA processing and expressing high levels of GFP was isolated by flow cytometry. To determine the location of mutations, two different genetic approaches, Hfr crosses and P1 transductions, were used. The mutation responsible for the processing defect was subsequently mapped to the 32 min region of the E. coli chromosome. A putative DEAH-box RNA helicase-encoding gene at this position, hrpA, was able to restore the ability of the mutant to cleave daa mRNA. Site-directed mutagenesis of the hrpA regions predicted to encode nucleotide triphosphate binding and hydrolysis functions abolished the ability of the gene to restore the processing defect in the mutant. We propose that HrpA is a novel enzyme involved in mRNA processing in E. coli.

RNA processing and degradation in Bacillus subtilis

This review focuses on the enzymes and pathways of RNA processing and degradation in Bacillus subtilis, and compares them to those of its gram-negative counterpart, Escherichia coli. A comparison of the genomes from the two organisms reveals that B. subtilis has a very different selection of RNases available for RNA maturation. Of 17 characterized ribonuclease activities thus far identified in E. coli and B. subtilis, only 6 are shared, 3 exoribonucleases and 3 endoribonucleases. Some enzymes essential for cell viability in E. coli, such as RNase E and oligoribonuclease, do not have homologs in B. subtilis, and of those enzymes in common, some combinations are essential in one organism but not in the other. The degradation pathways and transcript half-lives have been examined to various degrees for a dozen or so B. subtilis mRNAs. The determinants of mRNA stability have been characterized for a number of these and point to a fundamentally different process in the initiation of mRNA decay. While RNase E binds to the 5' end and catalyzes the rate-limiting cleavage of the majority of E. coli RNAs by looping to internal sites, the equivalent nuclease in B. subtilis, although not yet identified, is predicted to scan or track from the 5' end. RNase E can also access cleavage sites directly, albeit less efficiently, while the enzyme responsible for initiating the decay of B. subtilis mRNAs appears incapable of direct entry. Thus, unlike E. coli, RNAs possessing stable secondary structures or sites for protein or ribosome binding near the 5' end can have very long half-lives even if the RNA is not protected by translation.

Transcriptional analysis of the sfa determinant revealing mmRNA processing events in the biogenesis of S fimbriae in pathogenic Escherichia coli

Among the virulence factors present in pathogenic extraintestinal Escherichia coli strains, expression of fimbrial adhesins is necessary for attachment to the host tissues and subsequent colonization. Occurrence of the sfa determinant coding for the S fimbriae is widespread among the uropathogens and meningitis isolates. The sfa operon consists of nine genes. In the biogenesis of S fimbriae, the proteins encoded by the sfa genes are presumably required in a specific stoichiometry. In the present work we studied how differential expression of the sfa operon genes occurs. Our findings indicate that a number of endoribonucleolytic cleavages occur in the mRNA from the sfa operon, and we detected the presence of different distinct transcriptional products, including sfaBA, sfaA, sfaADE, and sfaGSH. The sfaGSH transcript represents the three distal genes of the sfa operon, which code for the minor subunits of the S fimbriae. Analysis of the proteins in S fimbriae suggested that expression of the sfaGSH transcript provides equimolar amounts of the minor subunits. Furthermore, we showed that in the generation of the major sfaA transcript, the processing included RNase E endoribonuceolytic cleavage of the precursor sfaBA transcript. We suggest that posttranscriptional mRNA processing events result in differential gene expression important to achieve the stoichiometry necessary for fimbrial adhesin biogenesis.

The phylogenetic distribution of bacterial ribonucleases

Ribonucleases play key, often essential, roles in cellular metabolism. Nineteen ribonuclease activities, from 22 different proteins, have so far been described in bacteria, the majority of them from either Escherichia coli or Bacillus subtilis. Here we examine the phylogenetic distribution of all of these ribonucleases in 50 eubacterial and archaeal species whose genomes have been completely sequenced, with particular emphasis on the endoribonucleases. Although some enzymes are very highly conserved throughout evolution, there appears to be no truly universal ribonuclease. While some organisms, like E.coli, have a large selection of ribonucleases, many with overlapping functions, others seem to have relatively few or have many that remain to be discovered.

Characterization of the specific cleavage of ceiE7-mRNA of the bactericidal ColE7 operon

Posttranscriptional control of the bactericidal ColE7 operon has been implicated by a feedback endonucleolytic cleavage of its own mRNA. The cleavage site has been located at the coding region of ceiE7, the second cistron of the ColE7 cea-cei-cel polycistronic transcript. Interestingly, Im7 protein, the translation product of ceiE7, is required for the specific cleavage. It was found that both sequence (GAUCUGAUU) flanking the cleavage site and the putative T1 stem-loop structure distal to the coding region of ceiE7 gene play a critical role for the specific cleavage of ceiE7-mRNA. Furthermore, we have verified that a di-nucleotide GG sequence located at the topmost position of the loop region of the putative stem-loop structure is essential for the specific cleavage of ceiE7-mRNA. Thus, our data reveal the existence of a novel mRNA degradative machinery for the regulation of the expression of ColE7 operon.

Endonuclease cleavage of messenger RNA in Bacillus subtilis

A deletion derivative of the ermC gene was constructed that expresses a 254-nucleotide mRNA. The small size of this mRNA facilitated the detection of processing products that did not differ greatly in size from the full-length transcript. In the presence of erythromycin, which induces ribosome stalling near the 5' end of ermC mRNA, the 254-nucleotide mRNA was cleaved endonucleolytically at the site of ribosome stalling. Only the downstream product of this cleavage was detectable; the upstream product was apparently too unstable to be detected. The downstream cleavage product accumulated at times after rifampicin addition, suggesting that the stalled ribosome at the 5' end conferred stability to this RNA fragment. Neither Bs-RNase III nor RNase M5, the two known narrow-specificity endoribonucleases of Bacillus subtilis, was responsible for this cleavage. These results indicate the presence in B. subtilis of another specific endoribonuclease, which may be ribosome associated.

Characterization of AfaE adhesins produced by extraintestinal and intestinal human Escherichia coli isolates: PCR assays for detection of Afa adhesins that do or do not recognize Dr blood group antigens

Operons of the afa family are expressed by pathogenic Escherichia coli strains associated with intestinal and extraintestinal infections in humans and animals. The recently demonstrated heterogeneity of these operons (L. Lalioui, M. Jouve, P. Gounon, and C. Le Bouguénec, Infect. Immun. 67:5048-5059, 1999) was used to develop a new PCR assay for detecting all the operons of the afa family with a single genetic tool. This PCR approach was validated by investigating three collections of human E. coli isolates originating from the stools of infants with diarrhea (88 strains), the urine of patients with pyelonephritis (97 strains), and the blood of cancer patients (115 strains). The results obtained with this single test and those previously obtained with several PCR assays were closely correlated. The AfaE adhesins encoded by the afa operons are variable, particularly with respect to the primary sequence encoded by the afaE gene. The receptor binding specificities have not been determined for all of these adhesins; some recognize the Dr blood group antigen (Afa/Dr(+) adhesins) on the human decay-accelerating factor (DAF) as a receptor, and others (Afa/Dr(-) adhesins) do not. Thus, the afa operons detected in this study were characterized by subtyping the afaE gene using specific PCRs. In addition, the DAF-binding capacities of as-yet-uncharacterized AfaE adhesins were tested by various cellular approaches. The afaE8 subtype (Afa/Dr(-) adhesin) was found to predominate in afa-positive isolates from sepsis patients (75%); it was frequent in afa-positive pyelonephritis E. coli (55.5%) and absent from diarrhea-associated strains. In contrast, Afa/Dr(+) strains (regardless of the afaE subtype) were associated with both diarrhea (100%) and extraintestinal infections (44 and 25% in afa-positive pyelonephritis and sepsis strains, respectively). These data suggest that there is an association between the subtype of AfaE adhesin and the physiological site of the infection caused by afa-positive strains.

A tripeptide sequence within the nascent DaaP protein is required for mRNA processing of a fimbrial operon in Escherichia coli

The biogenesis of F1845 fimbriae, a member of the Dr family of Escherichia coli adhesins, is regulated by endonucleolytic cleavage of the daaABCDPE primary transcript and differential stability of the resulting cleavage products. Processing of daa mRNA is dependent upon translation of a small open reading frame, designated daaP, which flanks the daa processing site. Here, we demonstrate that daa mRNA processing is directly coupled to daaP translation. Cleavage of the daaA-E mRNA was shown to require the tripeptide Gly-Pro-Pro (GPP), encoded by daaP codons 49-51 downstream of the processing site. Processing also required active translation through RNA located upstream of the processing site; however, processing did not depend on the amino acid sequence encoded by the region of daaP upstream of the processing site. Finally, determination of the processing site was shown to involve its location relative to the codons encoding the GPP tripeptide. These data show that translation of daaP is required in cis to promote RNA processing. These data suggest a model involving interaction of the nascent GPP tripeptide portion of the DaaP polypeptide with the ribosome, triggering cleavage of the associated mRNA at a fixed distance upstream. A model of active involvement of the ribosome in this process is proposed.

Structural and functional lesions in brush border of human polarized intestinal Caco-2/TC7 cells infected by members of the Afa/Dr diffusely adhering family of Escherichia coli

Diffusely adhering Escherichia coli (DAEC) strains expressing F1845 fimbrial adhesin or Dr hemagglutinin belonging to the Afa/Dr family of adhesins infect cultured polarized human intestinal cells through recognition of the brush border-associated decay-accelerating factor (DAF; CD55) as a receptor. The wild-type Afa/Dr DAEC strain C1845 has been shown to induce brush border lesions by an adhesin-dependent mechanism triggering apical F-actin rearrangements. In the present study, we undertook to further characterize cell injuries following the interaction of wild-type Afa/Dr DAEC strains C1845 and IH11128 expressing fimbrial F1845 adhesin and Dr hemagglutinin, respectively, with polarized, fully differentiated Caco-2/TC7 cells. In both cases, bacterium-cell interaction was followed by rearrangement of the major brush border-associated cytoskeletal proteins F-actin, villin, and fimbrin, proteins which play a pivotal role in brush border assembly. In contrast, distribution of G-actin, actin-depolymerizing factor, and tubulin was not modified. Using draE mutants, we found that a mutant in which cysteine replaces aspartic acid at position 54 conserved binding capacity but failed to induce F-actin disassembly. Accompanying the cytoskeleton injuries, we found that the distribution of brush border-associated functional proteins sucrase-isomaltase (SI), dipeptidylpeptidase IV (DPPIV), glucose transporter SGLT1, and fructose transporter GLUT5 was dramatically altered. In parallel, SI and DPPIV enzyme activity decreased.

Afa/Dr diffusely adhering Escherichia coli C1845 infection promotes selective injuries in the junctional domain of polarized human intestinal Caco-2/TC7 cells

The Afa/Dr diffusely adhering Escherichia coli (DAEC) C1845 strain harboring the F1845 fimbrial adhesin interacts with the brush border-associated CD55 molecule and promotes elongation of brush border microvilli resulting from rearrangement of the F-actin network. This phenomenon involves the activation of a cascade of signaling coupled to the glycosylphosphatidylinositol-anchored receptor of the F1845 adhesin. We provide evidence that infection of the polarized human intestinal cell line Caco-2/TC7 by strain C1845 is followed by an increase in the paracellular permeability for [(3)H]mannitol without a decrease of the transepithelial resistance of the monolayers. Alterations in the distribution of tight-junction (TJ)-associated occludin and ZO-1 protein are observed, whereas the distribution of the zonula adherens-associated E-cadherin is not affected. Using the recombinant E. coli strains HB101(pSSS1) and -(pSSS1C) expressing the F1845 fimbrial adhesin, we demonstrate that the adhesin-CD55 interaction is not sufficient for the induction of structural and functional TJ lesions. Moreover, using the actin filament-stabilizing agent Jasplakinolide, we demonstrate that the C1845-induced functional alterations in TJs are independent of the C1845-induced apical cytoskeleton rearrangements. The results indicated that pathogenic factor(s) other than F1845 adhesin may be operant in Afa/Dr DAEC C1845.

Recruitment of CD55 and CD66e brush border-associated glycosylphosphatidylinositol-anchored proteins by members of the Afa/Dr diffusely adhering family of Escherichia coli that infect the human polarized intestinal Caco-2/TC7 cells

The Afa/Dr family of diffusely adhering Escherichia coli (Afa/Dr DAEC) includes bacteria expressing afimbrial adhesins (AFA), Dr hemagglutinin, and fimbrial F1845 adhesin. We show that infection of human intestinal Caco-2/TC7 cells by the Afa/Dr DAEC strains C1845 and IH11128 is followed by clustering of CD55 around adhering bacteria. Mapping of CD55 epitopes involved in CD55 clustering by Afa/Dr DAEC was conducted using CD55 deletion mutants expressed by stable transfection in CHO cells. Deletion in the short consensus repeat 1 (SCR1) domain abolished Afa/Dr DAEC-induced CD55 clustering. In contrast, deletion in the SCR4 domain does not modify Afa/Dr DAEC-induced CD55 clustering. We show that the brush border-associated glycosylphosphatidylinositol (GPI)-anchored protein CD66e (carcinoembryonic antigen) is recruited by the Afa/Dr DAEC strains C1845 and IH11128. This conclusion is based on the observations that (i) infection of Caco-2/TC7 cells by Afa/Dr DAEC strains is followed by clustering of CD66e around adhering bacteria and (ii) Afa/Dr DAEC strains bound efficiently to stably transfected HeLa cells expressing CD66e, accompanied by CD66e clustering around adhering bacteria. Inhibition assay using monoclonal antibodies directed against CD55 SCR domains, and polyclonal anti-CD55 and anti-CD66e antibodies demonstrate that CD55 and CD66e function as a receptors for the C1845 and IH11128 bacteria. Moreover, using structural draE gene mutants, we found that a mutant in which cysteine replaced aspartic acid at position 54 displayed conserved binding capacity but failed to induce CD55 and CD66e clustering. Taken together, these data give new insights into the mechanisms by which Afa/Dr DAEC induces adhesin-dependent cross talk in the human polarized intestinal epithelial cells by mobilizing brush border-associated GPI-anchored proteins known to function as transducing molecules.

Molecular cloning and characterization of the afa-7 and afa-8 gene clusters encoding afimbrial adhesins in Escherichia coli strains associated with diarrhea or septicemia in calves

The afa gene clusters, which encode proteins involved in adhesion to epithelial cells, from Escherichia coli strains associated with urinary and intestinal infections in humans have been characterized. Pathogenic isolates of bovine and porcine origin that possess afa-related sequences have recently been described. We report in this work the cloning and characterization of the afa-7 and afa-8 gene clusters from bovine isolates. Hybridization and sequencing experiments revealed that despite similarity in genetic organization, the afa-7 and afa-8 genes, and the well-characterized afa-3 operon expressed by human-pathogenic isolates, correspond to three different members of the afa family of gene clusters. However, like the afa-3 gene cluster, both the afa-7 and afa-8 gene clusters were found to encode an afimbrial adhesin (AfaE) and an invasin (AfaD). The AfaD peptides encoded by the three gene clusters were only 45% identical, but functional complementation experiments indicated that they belong to the same family of invasins. Hemagglutination and adhesion assays demonstrated that the AfaE-VII and AfaE-VIII adhesins bind to different receptors and that these receptors are not the human decay-accelerating factor recognized to be the receptor of all previously described AfaE adhesins. The AfaE-VIII adhesin is very similar to the M agglutinin of human-uropathogenic strains. We used PCR assays to screen 25 bovine strains for afaD and afaE genes of either the afa-7 or afa-8 gene cluster. The afa-8 gene cluster was highly prevalent in bovine isolates previously reported to carry afa-related sequences (23 of 24 strains), particularly in strains producing cytotoxic necrotizing factors (16 of 16 strains). The location of the afa-8 gene cluster on the plasmids or chromosome of these isolates suggests that it could be carried by a mobile element, facilitating its dissemination among bovine-pathogenic E. coli strains.