Dihydrothiazolo ring-fused 2-pyridone antimicrobial compounds treat Streptococcus pyogenes skin and soft tissue infection

We have developed GmPcides from a peptidomimetic dihydrothiazolo ring-fused 2-pyridone scaffold that have antimicrobial activities against a broad-spectrum of Gram-positive pathogens. Here we examine the treatment efficacy of GmPcides using skin and soft tissue infection (SSTI) and biofilm formation models by Streptococcus pyogenes. Screening our compound library for minimal inhibitory (MIC) and minimal bactericidal (MBC) concentrations identified GmPcide PS757 as highly active against S. pyogenes. Treatment of S. pyogenes biofilm with PS757 revealed robust efficacy against all phases of biofilm formation by preventing initial biofilm development, ceasing biofilm maturation and eradicating mature biofilm. In a murine model of S. pyogenes SSTI, subcutaneous delivery of PS757 resulted in reduced levels of tissue damage, decreased bacterial burdens and accelerated rates of wound-healing, which were associated with down-regulation of key virulence factors, including M protein and the SpeB cysteine protease. These data demonstrate that GmPcides show considerable promise for treating S. pyogenes infections.

Microbial co-occurrences on catheters from long-term catheterized patients

Catheter-associated urinary tract infections (CAUTIs), a common cause of healthcare-associated infections, are caused by a diverse array of pathogens that are increasingly becoming antibiotic resistant. We analyze the microbial occurrences in catheter and urine samples from 55 human long-term catheterized patients collected over one year. Although most of these patients were prescribed antibiotics over several collection periods, their catheter samples remain colonized by one or more bacterial species. Examination of a total of 366 catheter and urine samples identify 13 positive and 13 negative genus co-occurrences over 12 collection periods, representing associations that occur more or less frequently than expected by chance. We find that for many patients, the microbial species composition between collection periods is similar. In a subset of patients, we find that the most frequently sampled bacteria, Escherichia coli and Enterococcus faecalis, co-localize on catheter samples. Further, co-culture of paired isolates recovered from the same patients reveals that E. coli significantly augments E. faecalis growth in an artificial urine medium, where E. faecalis monoculture grows poorly. These findings suggest novel strategies to collapse polymicrobial CAUTI in long-term catheterized patients by targeting mechanisms that promote positive co-associations.

Vaccination with Acinetobacter baumannii adhesin Abp2D provides protection against catheter-associated urinary tract infection

Catheter-associated urinary tract infections (CAUTIs) contribute greatly to the burden of healthcare associated infections. Acinetobacter baumannii is a Gram-negative bacterium with high levels of antibiotic resistance that is of increasing concern as a CAUTI pathogen. A. baumannii expresses fibrinogen-binding adhesins (Abp1D and Abp2D) that mediate colonization and biofilm formation on catheters, which become coated with fibrinogen upon insertion. We developed a protein subunit vaccine against Abp1DRBD and Abp2DRBD and showed that vaccination significantly reduced bladder bacterial titers in a mouse model of CAUTI. We then determined that immunity to Abp2DRBD alone was sufficient for protection. Mechanistically, we defined the B cell response to Abp2DRBD vaccination and demonstrated that immunity was transferrable to naïve mice through passive immunization with Abp2DRBD-immune sera. This work represents a novel strategy in the prevention of A. baumannii CAUTI and has an important role to play in the global fight against antimicrobial resistance.

Structure-function correlates of fibrinogen binding by Acinetobacter adhesins critical in catheter-associated urinary tract infections

Multidrug-resistant Acinetobacter baumannii infections are an urgent clinical problem and can cause difficult-to-treat nosocomial infections. During such infections, like catheter-associated urinary tract infections (CAUTI), A. baumannii rely on adhesive, extracellular fibers, called chaperone-usher pathway (CUP) pili for critical binding interactions. The A. baumannii uropathogenic strain, UPAB1, and the pan-European subclone II isolate, ACICU, use the CUP pili Abp1 and Abp2 (previously termed Cup and Prp, respectively) in tandem to establish CAUTIs, specifically to facilitate bacterial adherence and biofilm formation on the implanted catheter. Abp1 and Abp2 pili are tipped with two domain tip adhesins, Abp1D and Abp2D, respectively. We discovered that both adhesins bind fibrinogen, a critical host wound response protein that is released into the bladder upon catheterization and is subsequently deposited on the catheter. The crystal structures of the Abp1D and Abp2D receptor-binding domains were determined and revealed that they both contain a large, distally oriented pocket, which mediates binding to fibrinogen and other glycoproteins. Genetic, biochemical, and biophysical studies revealed that interactions with host proteins are governed by several critical residues in and along the edge of the binding pocket, one of which regulates the structural stability of an anterior loop motif. K34, located outside of the pocket but interacting with the anterior loop, also regulates the binding affinity of the protein. This study illuminates the mechanistic basis of the critical fibrinogen-coated catheter colonization step in A. baumannii CAUTI pathogenesis.

Ring-fused 2-pyridones effective against multidrug-resistant Gram-positive pathogens and synergistic with standard-of-care antibiotics

The alarming rise of multidrug-resistant Gram-positive bacteria has precipitated a healthcare crisis, necessitating the development of new antimicrobial therapies. Here we describe a new class of antibiotics based on a ring-fused 2-pyridone backbone, which are active against vancomycin-resistant enterococci (VRE), a serious threat as classified by the Centers for Disease Control and Prevention, and other multidrug-resistant Gram-positive bacteria. Ring-fused 2-pyridone antibiotics have bacteriostatic activity against actively dividing exponential phase enterococcal cells and bactericidal activity against nondividing stationary phase enterococcal cells. The molecular mechanism of drug-induced killing of stationary phase cells mimics aspects of fratricide observed in enterococcal biofilms, where both are mediated by the Atn autolysin and the GelE protease. In addition, combinations of sublethal concentrations of ring-fused 2-pyridones and standard-of-care antibiotics, such as vancomycin, were found to synergize to kill clinical strains of VRE. Furthermore, a broad range of antibiotic resistant Gram-positive pathogens, including those responsible for the increasing incidence of antibiotic resistant healthcare-associated infections, are susceptible to this new class of 2-pyridone antibiotics. Given the broad antibacterial activities of ring-fused 2-pyridone compounds against Gram-positive (GmP) bacteria we term these compounds GmPcides, which hold promise in combating the rising tide of antibiotic resistant Gram-positive pathogens.

Limited effects of long-term daily cranberry consumption on the gut microbiome in a placebo-controlled study of women with recurrent urinary tract infections

BACKGROUND

Urinary tract infections (UTIs) affect 15 million women each year in the United States, with > 20% experiencing frequent recurrent UTIs. A recent placebo-controlled clinical trial found a 39% reduction in UTI symptoms among recurrent UTI sufferers who consumed a daily cranberry beverage for 24 weeks. Using metagenomic sequencing of stool from a subset of these trial participants, we assessed the impact of cranberry consumption on the gut microbiota, a reservoir for UTI-causing pathogens such as Escherichia coli, which causes > 80% of UTIs.

RESULTS

The overall taxonomic composition, community diversity, carriage of functional pathways and gene families, and relative abundances of the vast majority of observed bacterial taxa, including E. coli, were not changed significantly by cranberry consumption. However, one unnamed Flavonifractor species (OTU41), which represented ≤1% of the overall metagenome, was significantly less abundant in cranberry consumers compared to placebo at trial completion. Given Flavonifractor's association with negative human health effects, we sought to determine OTU41 characteristic genes that may explain its differential abundance and/or relationship to key host functions. Using comparative genomic and metagenomic techniques, we identified genes in OTU41 related to transport and metabolism of various compounds, including tryptophan and cobalamin, which have been shown to play roles in host-microbe interactions.

CONCLUSION

While our results indicated that cranberry juice consumption had little impact on global measures of the microbiome, we found one unnamed Flavonifractor species differed significantly between study arms. This suggests further studies are needed to assess the role of cranberry consumption and Flavonifractor in health and wellbeing in the context of recurrent UTI.

TRIAL REGISTRATION

Clinical trial registration number: ClinicalTrials.gov NCT01776021 .

Structural basis for usher activation and intramolecular subunit transfer in P pilus biogenesis in Escherichia coli

Chaperone-usher pathway (CUP) pili are extracellular proteinaceous fibers ubiquitously found on Gram-negative bacteria, and mediate host-pathogen interactions and biofilm formation critical in pathogenesis in numerous human diseases¹. During pilus assembly an outer membrane (OM) macromolecular machine called the usher catalyzes pilus biogenesis from the individual subunits that are delivered as chaperone-subunit complexes in the periplasm. The usher orchestrates pilus assembly using all five functional domains: a 24-stranded transmembrane β-barrel translocation domain (TD), a β-sandwich plug domain (PD), an amino-terminal periplasmic domain (NTD) and two carboxy-terminal periplasmic domains (CTD1 and CTD2)^2–6. Despite extensive structural and functional characterization, the mechanism by which the usher is activated to initiate pilus biogenesis is unknown. Here we present the crystal structure of the full-length PapC usher from Escherichia coli in complex with its cognate PapDG chaperone-subunit complex in a pre-activation state, elucidating molecular details of how the usher is specifically engaged by allosteric interactions with its substrate preceding activation and how the usher facilitates the transfer of subunits from the NTD to the CTDs during pilus assembly. This work elucidates the intricate workings of a molecular machine that catalyzes CUP pilus assembly and opens the door for the development of potent inhibitors to block pilus biogenesis.

Functional role of the type 1 pilus rod structure in mediating host-pathogen interactions

Uropathogenic E. coli (UPEC), which cause urinary tract infections (UTI), utilize type 1 pili, a chaperone usher pathway (CUP) pilus, to cause UTI and colonize the gut. The pilus rod, comprised of repeating FimA subunits, provides a structural scaffold for displaying the tip adhesin, FimH. We solved the 4.2 Å resolution structure of the type 1 pilus rod using cryo-electron microscopy. Residues forming the interactive surfaces that determine the mechanical properties of the rod were maintained by selection based on a global alignment of fimA sequences. We identified mutations that did not alter pilus production in vitro but reduced the force required to unwind the rod. UPEC expressing these mutant pili were significantly attenuated in bladder infection and intestinal colonization in mice. This study elucidates an unappreciated functional role for the molecular spring-like property of type 1 pilus rods in host-pathogen interactions and carries important implications for other pilus-mediated diseases.

Escherichia coli, or E. coli for short, is a type of bacteria commonly found in the guts of people and animals. Certain types of E. coli can cause urinary tract infections (UTIs): they travel from the digestive tract up to the bladder (and sometimes to the kidneys) where they provoke painful symptoms. To cause the infection, the bacteria must become solidly attached to the lining of the bladder; otherwise they will get flushed out whenever urine is expelled.

Pili are hair-like structures that cover a bacterium and allow it to attach to surfaces. E. coli has many different types of pili, but one seems particularly important in UTIs: type 1 pili. These pili are formed of subunits that assemble into a long coil-shaped rod, which is tipped by adhesive molecules that can stick to body surfaces. The current hypothesis is that the pili act as shock absorbers: when the bladder empties, the pili’s coil-like structure can unwind into a flexible straight fiber. This would take some of the forces off the adhesive molecules that are attached to the bladder, and help the bacteria to remain in place when urine flows out. However, the exact structure of type 1 pili is still unclear, and the essential role of their coil-like shape unconfirmed.

Here, Spaulding, Schreiber, Zheng et al. use a microscopy method called cryo-EM to reveal the structure of the type 1 pili at near atomic-level, and identify the key units necessary for their coiling properties. The experiments show that pili with certain mutations in these units unwind much more easily when the bacteria carrying them are ‘tugged on’ with molecular tweezers. The bacteria with mutant pili are also less able to cause UTIs in mice. The coiling ability of the type 1 pili is therefore essential for E. coli to invade and colonize the bladder.

Every year, over 150 million people worldwide experience a UTI; for 25% of women, the infection regularly returns. Antibiotics usually treat the problem but bacteria are becoming resistant to these drugs. New treatments could be designed if scientists understand what roles pili play in the infection mechanisms.

Bacterial virulence phenotypes of Escherichia coli and host susceptibility determine risk for urinary tract infections

Urinary tract infections (UTIs) are caused by uropathogenic Escherichia coli (UPEC) strains. In contrast to many enteric E. coli pathogroups, no genetic signature has been identified for UPEC strains. We conducted a high-resolution comparative genomic study using E. coli isolates collected from the urine of women suffering from frequent recurrent UTIs. These isolates were genetically diverse and varied in their urovirulence, that is, their ability to infect the bladder in a mouse model of cystitis. We found no set of genes, including previously defined putative urovirulence factors (PUFs), that were predictive of urovirulence. In addition, in some patients, the E. coli strain causing a recurrent UTI had fewer PUFs than the supplanted strain. In competitive experimental infections in mice, the supplanting strain was more efficient at colonizing the mouse bladder than the supplanted strain. Despite the lack of a clear genomic signature for urovirulence, comparative transcriptomic and phenotypic analyses revealed that the expression of key conserved functions during culture, such as motility and metabolism, could be used to predict subsequent colonization of the mouse bladder. Together, our findings suggest that UTI risk and outcome may be determined by complex interactions between host susceptibility and the urovirulence potential of diverse bacterial strains.

Narrowing the spectrum: the new frontier of precision antimicrobials

Antibiotics have become the standard of care for bacterial infections. However, rising rates of antibiotic-resistant infections are outpacing the development of new antimicrobials. Broad-spectrum antibiotics also harm beneficial microbial communities inhabiting humans. To combat antibiotic resistance and protect these communities, new precision antimicrobials must be engineered to target specific pathogens.

Selective depletion of uropathogenic E. coli from the gut by a FimH antagonist

Urinary tract infections (UTI) caused by uropathogenic E. coli (UPEC) affect 150 million people annually^1,2. Despite effective antibiotic therapy, 30–50% of patients experience recurrent UTI (rUTI)¹. Additionally, the growing prevelance of UPEC resistant to last-line antibiotic treatments, and more recently carbapenems and colistin, make UTIs a prime example of the antibiotic-resistance crisis and emphasize the need for new approaches to treat and prevent bacterial infections^3–5. UPEC strains establish reservoirs in the gut from which they are shed in the feces, can colonize the peri-urethral area or vagina and subsequently ascend through the urethra to the urinary tract, where they cause UTI⁶. UPEC isolates encode up to 16 distinct chaperone-usher pathway (CUP) pili and each pilus type likely enables colonization of a habitat in the host or environment⁷. For example, the type 1 pilus adhesin, FimH, binds mannose on the bladder surface, mediating bladder colonization. However, little is known regarding the mechanisms underlying UPEC persistence in the gut⁵. Using a mouse model, we found that F17-like and type 1 pili promote intestinal colonization and show distinct binding to epithelial cells distributed along colonic crypts. Phylogenomic and structural analyses reveal that F17-like pili are closely related to pilus types carried by intestinal pathogens, but are restricted to extra-intestinal pathogenic E. coli. Moreover, we show that targeting FimH with a high-affinity inhibitor, mannoside M4284, reduces intestinal colonization of genetically diverse UPEC isolates, while simultaneously treating UTI, without significantly disrupting the the structural configuration of the gut microbiota. By selectively depleting the intestinal UPEC reservoir, mannosides could significantly reduce the rate of UTI and rUTI.

Evolutionary fine-tuning of conformational ensembles in FimH during host-pathogen interactions

Positive selection in the two-domain type 1 pilus adhesin FimH enhances Escherichia coli fitness in urinary tract infection (UTI). We report a comprehensive atomic-level view of FimH in two-state conformational ensembles in solution, composed of one low-affinity tense (T) and multiple high-affinity relaxed (R) conformations. Positively selected residues allosterically modulate the equilibrium between these two conformational states, each of which engages mannose through distinct binding orientations. A FimH variant that only adopts the R state is severely attenuated early in a mouse model of uncomplicated UTI but is proficient at colonizing catheterized bladders in vivo or bladder transitional-like epithelial cells in vitro. Thus, the bladder habitat has barrier(s) to R state-mediated colonization possibly conferred by the terminally differentiated bladder epithelium and/or decoy receptors in urine. Together, our studies reveal the conformational landscape in solution, binding mechanisms, and adhesive strength of an allosteric two-domain adhesin that evolved "moderate" affinity to optimize persistence in the bladder during UTI.

Inflammation-Induced Adhesin-Receptor Interaction Provides a Fitness Advantage to Uropathogenic E. coli during Chronic Infection

Uropathogenic E. coli (UPEC) is the dominant cause of urinary tract infections, clinically described as cystitis. UPEC express CUP pili, which are extracellular fibers tipped with adhesins that bind mucosal surfaces of the urinary tract. Here we identify the role of the F9/Yde/Fml pilus for UPEC persistence in the inflamed urothelium. The Fml adhesin FmlH binds galactose β1-3 N-acetylgalactosamine found in core-1 and -2 O-glycans. Deletion of fmlH had no effect on UPEC virulence in an acute mouse model of cystitis. However, FmlH provided a fitness advantage during chronic cystitis, which is manifested as persistent bacteriuria, high bladder bacterial burdens, and chronic inflammation. In situ binding confirmed that FmlH bound avidly to the inflamed, but not the naive bladder. In accordance with its pathogenic profile, vaccination with FmlH significantly protected mice from chronic cystitis. Thus, UPEC employ separate CUP pili to adapt to the rapidly changing niche during bladder infection.

A tale of two pili: assembly and function of pili in bacteria

Bacterial pili have long been recognized as mediators of initial host-pathogen interactions important for the progression of Gram-negative bacterial diseases. An appreciation of the role of pili on virulence in Gram-positive bacteria and the unique properties of their biogenesis is a rapidly emerging area of research. In this review, we focus on recent advances in one of the longest-studied Gram-negative pilus systems, the chaperone/usher assembled pili, along with the newcomer to the field, the sortase-assembled pili of Gram-positive bacteria. In both systems, a wealth of new structural and molecular details has emerged recently. In light of this, we explore similarities between chaperone/usher and sortase-assembled pilus biogenesis and highlight paradigms unique to each, with the goal of using knowledge of each system to raise new questions and inform future studies of the other.

The differential affinity of the usher for chaperone-subunit complexes is required for assembly of complete pili

Attachment to host cells via adhesive surface structures is a prerequisite for the pathogenesis of many bacteria. Uropathogenic Escherichia coli assemble P and type 1 pili for attachment to the host urothelium. Assembly of these pili requires the conserved chaperone/usher pathway, in which a periplasmic chaperone controls the folding of pilus subunits and an outer membrane usher provides a platform for pilus assembly and secretion. The usher has differential affinity for pilus subunits, with highest affinity for the tip-localized adhesin. Here, we identify residues F21 and R652 of the P pilus usher PapC as functioning in the differential affinity of the usher. R652 is important for high-affinity binding to the adhesin whereas F21 is important for limiting affinity for the PapA major rod subunit. PapC mutants in these residues are specifically defective for pilus assembly in the presence of PapA, demonstrating that differential affinity of the usher is required for assembly of complete pili. Analysis of PapG deletion mutants demonstrated that the adhesin is not required to initiate P pilus biogenesis. Thus, the differential affinity of the usher may be critical to ensure assembly of functional pilus fibres.

Adaptor function of PapF depends on donor strand exchange in P-pilus biogenesis of Escherichia coli

P-pilus biogenesis occurs via the highly conserved chaperone-usher pathway and involves the strict coordination of multiple subunit proteins. All nonadhesin structural P-pilus subunits possess the same topology, consisting of two domains: an incomplete immunoglobulin-like fold (pilin body) and an N-terminal extension. Pilus subunits form interactions with one another through donor strand exchange, occurring at the usher, in which the N-terminal extension of an incoming subunit completes the pilin body of the preceding subunit, allowing the incorporation of the subunit into the pilus fiber. In this study, pilus subunits in which the N-terminal extension was either deleted or swapped with that of another subunit were used to examine the role of each domain of PapF in functions involving donor strand exchange and hierarchical assembly. We found that the N-terminal extension of PapF is required to adapt the PapG adhesin to the tip of the fiber. The pilin body of PapF is required to efficiently initiate assembly of the remainder of the pilus, with the assistance of the N-terminal extension. Thus, distinct functions were assigned to each region of the PapF subunit. In conclusion, all pilin subunits possess the same overall architectural topology; however, each N-terminal extension and pilin body has specific functions in pilus biogenesis.

Intracellular bacterial communities of uropathogenic Escherichia coli in urinary tract pathogenesis

Urinary tract infections in young, healthy women frequently recur, despite their traditional classification as acute infections. Conventional wisdom dictates that uropathogens causing recurrent infections in such individuals come from the fecal or vaginal flora, in the same manner as the initial infection. However, recent studies of uropathogenic Escherichia coli have found that it can carry out a complex developmental program within the superficial epithelial cells of the mouse bladder, forming intracellular bacterial communities with many biofilm-like properties. These intracellular biofilms allow the bacteria to outlast a strong host immune response to establish a dormant reservoir of pathogens inside the bladder cells. Re-emergence of bacteria from this reservoir might be the source of recurrent infection.

Quantitative studies of the binding of the class II PapG adhesin from uropathogenic Escherichia coli to oligosaccharides

Binding of the class II PapG adhesin, found at the tip of filamentous pili on Escherichia coli, to the carbohydrate moiety of globoseries glycolipids in the human kidney is a key step in development of pyelonephritis, a severe form of urinary tract infection. An assay based on surface plasmon resonance for quantification of the binding of the class II PapG adhesin to oligosaccharides has been developed. Using this assay dissociation constants ranging from 80 to 540 microM were determined for binding of the PapG adhesin to di-pentasaccharide fragments from the globoseries of glycolipids. A series of galabiose derivatives, modified at the anomeric position, O-2' or O-3', was also investigated. The anomeric position appeared to be the most promising for development of improved inhibitors of PapG-mediated adhesion of E. coli. p-Methoxyphenyl galabioside was found to be most potent (K(d)=140 microM), and binds to PapG almost as well as the Forssman pentasaccharide.

Structural basis of the interaction of the pyelonephritic E. coli adhesin to its human kidney receptor

PapG is the adhesin at the tip of the P pilus that mediates attachment of uropathogenic Escherichia coli to the uroepithelium of the human kidney. The human specific allele of PapG binds to globoside (GbO4), which consists of the tetrasaccharide GalNAc beta 1-3Gal alpha 1-4Gal beta 1-4Glc linked to ceramide. Here, we present the crystal structures of a binary complex of the PapG receptor binding domain bound to GbO4 as well as the unbound form of the adhesin. The biological importance of each of the residues involved in binding was investigated by site-directed mutagenesis. These studies provide a molecular snapshot of a host-pathogen interaction that determines the tropism of uropathogenic E. coli for the human kidney and is critical to the pathogenesis of pyelonephritis.

Structural basis of chaperone function and pilus biogenesis

Many Gram-negative pathogens assemble architecturally and functionally diverse adhesive pili on their surfaces by the chaperone-usher pathway. Immunoglobulin-like periplasmic chaperones escort pilus subunits to the usher, a large protein complex that facilitates the translocation and assembly of subunits across the outer membrane. The crystal structure of the PapD-PapK chaperone-subunit complex, determined at 2.4 angstrom resolution, reveals that the chaperone functions by donating its G(1) beta strand to complete the immunoglobulin-like fold of the subunit via a mechanism termed donor strand complementation. The structure of the PapD-PapK complex also suggests that during pilus biogenesis, every subunit completes the immunoglobulin-like fold of its neighboring subunit via a mechanism termed donor strand exchange.

Periplasmic chaperone recognition motif of subunits mediates quaternary interactions in the pilus

The class of proteins collectively known as periplasmic immunoglobulin-like chaperones play an essential role in the assembly of a diverse set of adhesive organelles used by pathogenic strains of Gram-negative bacteria. Herein, we present a combination of genetic and structural data that sheds new light on chaperone-subunit and subunit-subunit interactions in the prototypical P pilus system, and provides new insights into how PapD controls pilus biogenesis. New crystallographic data of PapD with the C-terminal fragment of a subunit suggest a mechanism for how periplasmic chaperones mediate the extraction of pilus subunits from the inner membrane, a prerequisite step for subunit folding. In addition, the conserved N- and C-terminal regions of pilus subunits are shown to participate in the quaternary interactions of the mature pilus following their uncapping by the chaperone. By coupling the folding of subunit proteins to the capping of their nascent assembly surfaces, periplasmic chaperones are thereby able to protect pilus subunits from premature oligomerization until their delivery to the outer membrane assembly site.

Outer-membrane PapC molecular usher discriminately recognizes periplasmic chaperone-pilus subunit complexes

P pili are highly ordered composite structures consisting of thin fibrillar tips joined end-to-end to rigid helical rods. The production of these virulence-associated structures requires a periplasmic chaperone (PapD) and an outer membrane protein (PapC) that is the prototype member of a newly recognized class of proteins that we have named "molecular ushers." Two in vitro assays showed that the preassembly complexes that PapD forms with the three most distal tip fibrillar proteins (PapG, PapF, and PapE) bound to PapC. The relative affinity of each complex for PapC was found to correlate with the final position of the subunit type in the tip fibrillum. In contrast, the complexes PapD forms with the major component of the pilus rod, PapA, or the pilus rod initiating protein, PapK, did not recognize PapC. The in vitro data argue that differential targeting of chaperone-subunit complexes to PapC may be part of a mechanism to ensure the correctly ordered assembly of adhesive composite pili.

A sequence at the inside end of IS50 down regulates transposition

Deletion analysis has shown that the segment at the IS50 inside (I) end that is needed for efficient transposition is approximately 19 bp long. Dam methylation at two 5' GATC sequences within this segment decreases I-end transposition activity. A third 5' GATC sequence is present at bp 21-24 of the I end. The comparisons presented here show that extension of the I end from 19 to 24 bp decreases its transposition activity in dam cells 5- to 50-fold, depending on the overall transposon structure.

Saturation mutagenesis of the inside end of insertion sequence IS50

A 19-bp segment at the inside (I) end of IS50 (Tn5) is needed for efficient transposition. The importance of each position was assayed by making at least one base substitution at each position by either chemical-or oligodeoxyribonucleotide-directed mutagenesis. Mutant I ends were paired with a wild-type (wt) segment from the outside (O) end of IS50 and the transposase (tnp) gene was placed either between the ends or 1200 bp from the O end. The frequency of transposition of the resultant elements to bacteriophage lambda was measured. At least one substitution at each of the 19 I-end positions decreased transposition activity to less than 25% of wt, and most substitutions (25 of 28) decreased it to less than 5% of wt from one or both donor plasmids. These results show that each position in the I end is important during transposition.

Factors affecting transposition activity of IS50 and Tn5 ends

The partially matched I and O ends of IS50 (the insertion sequence of the transposon Tn5) are needed for transposition, probably as the sites upon which the cis-acting transposase and host proteins act. To better understand how transposition is regulated we made a series of IS50-related elements in which the positions of the ends and of the transposase gene were varied systematically. Assays of these elements showed that the I and O ends differ inherently in transposition activity. Other workers showed that methylation, at DNA N6-adenine methyltransferase (Dam) recognition sites within the I end and the transposase tnp gene promoter, inhibits transposase synthesis and also I end activity. We show that the effect of Dammediated methylation on an I end depends on the end's orientation relative to the tnp gene. Further, in dam+ cells oriented like -tnp----in relation to the first and second ends) are (O, I) greater than (O, O) greater than or equal to (I, O) greater than (I, I). In dam- cells the relative activities are (O, I) = (I, O) = (I, I) greater than (O, O). Our results are consistent with a model orginally developed for IS10, in which hemi-methylation resulting from passage of a replication fork regulates transposition.