Mapping of export signals of Pseudomonas aeruginosa pilin with alkaline phosphatase fusions

Systematic functional analysis of the Com pilus in Streptococcus sanguinis: a minimalistic type 4 filament dedicated to DNA uptake in monoderm bacteria

IMPORTANCE

Type 4 filaments (T4F) are nanomachines ubiquitous in prokaryotes, centered on filamentous polymers of type 4 pilins. T4F are exceptionally versatile and widespread virulence factors in bacterial pathogens. The mechanisms of filament assembly and the many functions they facilitate remain poorly understood because of the complexity of T4F machineries. This hinders the development of anti-T4F drugs. The significance of our research lies in characterizing the simplest known T4F-the Com pilus that mediates DNA uptake in competent monoderm bacteria-and showing that four protein components universally conserved in T4F are sufficient for filament assembly. The Com pilus becomes a model for elucidating the mechanisms of T4F assembly.

Mechanism of assembly of type 4 filaments: everything you always wanted to know (but were afraid to ask)

Type 4 filaments (T4F) are a superfamily of filamentous nanomachines - virtually ubiquitous in prokaryotes and functionally versatile - of which type 4 pili (T4P) are the defining member. T4F are polymers of type 4 pilins, assembled by conserved multi-protein machineries. They have long been an important topic for research because they are key virulence factors in numerous bacterial pathogens. Our poor understanding of the molecular mechanisms of T4F assembly is a serious hindrance to the design of anti-T4F therapeutics. This review attempts to shed light on the fundamental mechanistic principles at play in T4F assembly by focusing on similarities rather than differences between several (mostly bacterial) T4F. This holistic approach, complemented by the revolutionary ability of artificial intelligence to predict protein structures, led to an intriguing mechanistic model of T4F assembly.

Exceptionally widespread nanomachines composed of type IV pilins: the prokaryotic Swiss Army knives

Prokaryotes have engineered sophisticated surface nanomachines that have allowed them to colonize Earth and thrive even in extreme environments. Filamentous machineries composed of type IV pilins, which are associated with an amazing array of properties ranging from motility to electric conductance, are arguably the most widespread since distinctive proteins dedicated to their biogenesis are found in most known species of prokaryotes. Several decades of investigations, starting with type IV pili and then a variety of related systems both in bacteria and archaea, have outlined common molecular and structural bases for these nanomachines. Using type IV pili as a paradigm, we will highlight in this review common aspects and key biological differences of this group of filamentous structures.

Using type IV pili as a paradigm, we review common genetic, structural and mechanistic features (many) as well as differences (few) of the exceptionally widespread and functionally versatile prokaryotic nano-machines composed of type IV pilins.

Cell-free production of integral membrane aspartic acid proteases reveals zinc-dependent methyltransferase activity of the Pseudomonas aeruginosa prepilin peptidase PilD

Integral membrane aspartic acid proteases are receiving growing recognition for their fundamental roles in cellular physiology of eukaryotes and prokaryotes, and may be medically important pharmaceutical targets. The Gram-negative Pseudomonas aeruginosa PilD and the archaeal Methanococcus voltae FlaK were synthesized in the presence of unilamellar liposomes in a cell-free translation system. Cosynthesis of PilD with its full-length substrate, PilA, or of FlaK with its full-length substrate, FlaB2, led to complete cleavage of the substrate signal peptides. Scaled-up synthesis of PilD, followed by solubilization in dodecyl-β-d-maltoside and chromatography, led to a pure enzyme that retained both of its known biochemical activities: cleavage of the PilA signal peptide and S-adenosyl methionine-dependent methylation of the mature pilin. X-ray fluorescence scans show for the first time that PilD is a zinc-binding protein. Zinc is required for the N-terminal methylation of the mature pilin, but not for signal peptide cleavage. Taken together, our work identifies the P. aeruginosa prepilin peptidase PilD as a zinc-dependent N-methyltransferase and provides a new platform for large-scale synthesis of PilD and other integral membrane proteases important for basic microbial physiology and virulence.

Minor pilins of the type IV pilus system participate in the negative regulation of swarming motility

Pseudomonas aeruginosa exhibits distinct surface-associated behaviors, including biofilm formation, flagellum-mediated swarming motility, and type IV pilus-driven twitching. Here, we report a role for the minor pilins, PilW and PilX, components of the type IV pilus assembly machinery, in the repression of swarming motility. Mutating either the pilW or pilX gene alleviates the inhibition of swarming motility observed for strains with elevated levels of the intracellular signaling molecule cyclic di-GMP (c-di-GMP) due to loss of BifA, a c-di-GMP-degrading phosphodiesterase. Blocking PilD peptidase-mediated processing of PilW and PilX renders the unprocessed proteins defective for pilus assembly but still functional in c-di-GMP-mediated swarming repression, indicating our ability to separate these functions. Strains with mutations in pilW or pilX also fail to exhibit the increase in c-di-GMP levels observed when wild-type (WT) or bifA mutant cells are grown on a surface. We also provide data showing that c-di-GMP levels are increased upon PilY1 overexpression in surface-grown cells and that this c-di-GMP increase does not occur in the absence of the SadC diguanylate cyclase. Increased levels of endogenous PilY1, PilX, and PilA are observed when cells are grown on a surface compared to liquid growth, linking surface growth and enhanced signaling via SadC. Our data support a model wherein PilW, PilX, and PilY1, in addition to their role(s) in type IV pilus biogenesis, function to repress swarming via modulation of intracellular c-di-GMP levels. By doing so, these pilus assembly proteins contribute to P. aeruginosa's ability to coordinately regulate biofilm formation with its two surface motility systems.

Transcriptional activation of the tad type IVb pilus operon by PypB in Yersinia enterocolitica

Type IV pili are virulence factors in various bacteria and mediate, among other functions, the colonization of diverse surfaces. Various subclasses of type IV pili have been identified, but information on pilus expression, biogenesis, and the associated phenotypes is sparse for the genus Yersinia. We recently described the identification of PypB as a transcriptional regulator in Yersinia enterocolitica. Here we show that the pypB gene is associated with the tad locus, a genomic island that is widespread among bacterial and archaeal species. The genetic linkage of pypB with the tad locus is conserved throughout the yersiniae but is not found among other bacteria carrying the tad locus. We show that the genes of the tad locus form an operon in Y. enterocolitica that is controlled by PypB and that pypB is part of this operon. The tad genes encode functions necessary for the biogenesis of the Flp subfamily of type IVb pili initially described for Aggregatibacter actinomycetemcomitans to mediate a tight-adherence phenotype. In Y. enterocolitica, the Flp pilin protein shows some peculiarities in its amino acid sequence that imply similarities as well as differences compared to typical motifs found in the Flp subtype of type IVb pili. Flp is expressed and processed after PypB overproduction, resulting in microcolony formation but not in increased adherence to biotic or abiotic surfaces. Our data describe the transcriptional regulation of the tad type IVb pilus operon by PypB in Y. enterocolitica but fail to show most previously described phenotypes associated with this type of pilus in other bacteria.

PilA localization affects extracellular polysaccharide production and fruiting body formation in Myxococcus xanthus

Myxococcus xanthus is a Gram-negative bacterium capable of complex developmental processes involving vegetative swarming and fruiting body formation. Social (S-) gliding motility, one of the two motility systems used by M. xanthus, requires at least two cell surface structures: type IV pili (TFP) and extracellular polysaccharides (EPS). Extended TFP that are composed of thousands of copies of PilA retract upon binding to EPS and thereby pull the cell forward. TFP also act as external sensor to regulate EPS production. In this study, we generated a random PilA mutant library and identified one derivative, SW1066, which completely failed to undergo developmental processes. Detailed characterization revealed that SW1066 produced very little EPS but wild-type amounts of PilA. These mutated PilA subunits, however, are unable to assemble into functional TFP despite their ability to localize to the membrane. By preventing the mutated PilA of SW1066 to translocate from the cytoplasm to the membrane, fruiting body formation and EPS production were restored to the levels observed in mutant strains lacking PilA. This apparent connection between PilA membrane accumulation and reduction in surface EPS implies that specific cellular PilA localization are required to maintain the EPS level necessary to sustain normal S-motility in M. xanthus.

Architectures and biogenesis of non-flagellar protein appendages in Gram-negative bacteria

Bacteria commonly expose non-flagellar proteinaceous appendages on their outer surfaces. These extracellular structures, called pili or fimbriae, are employed in attachment and invasion, biofilm formation, cell motility or protein and DNA transport across membranes. Over the past 15 years, the power of molecular and structural techniques has revolutionalized our understanding of the biogenesis, structure, function and mode of action of these bacterial organelles. Here, we review the five known classes of Gram-negative non-flagellar appendages from a biosynthetic and structural point of view.

Type IV pili: e pluribus unum?

The widespread role of pili as colonization factors in pathogens has long been recognized in Gram-negative bacteria and more recently in Gram-positive bacteria, making the study of these hair-like filaments a perennial hot topic for research. No other pili are found in as many or as diverse bacteria as type IV pili. This is likely a consequence of their ancient origin and unique ability to promote multiple and strikingly different phenotypes such as attachment to surfaces, aggregation, uptake of DNA during transformation, motility, etc. Two decades of investigations in several model species have shed some light on the structure of these filaments and the molecular basis of some of the properties they confer. Moreover, recent discoveries have led to a better knowledge of the genetic basis and molecular mechanisms of type IV pili biogenesis. This brings us a few steps closer to understanding how these filaments are produced, but leaves us wondering whether (as in the famous motto that inspired the title) out of the many models studied will emerge one unifying mechanism.

Post-transcriptional cross-talk between pro- and anti-colonization pili biosynthesis systems in Vibrio cholerae

The pathogen Vibrio cholerae modulates the expression of many genes in order to transition from its environmental reservoir to its niche in the human host. Among these are genes encoding two related Type IV pili, the mannose-sensitive haemagglutinin (MSHA) pilus, which aids V. cholerae persistence in aquatic environments but causes clearance of bacteria by host immune defences, and the toxin co-regulated pilus (TCP) required for colonization. These antagonistic effects are resolved transcriptionally by the regulator ToxT, which represses msh genes while activating tcp genes during infection. We show that these two pili systems are also intertwined post-transcriptionally through the ToxT-regulated pre-pilin peptidase TcpJ. We found that the major MSHA pilin, MshA, was degraded in V. cholerae in a TcpJ-dependent fashion. In a heterologous Escherichia coli system, TcpJ can recognize both MshA and its cognate substrate, the TCP subunit TcpA, but that processing by TcpJ causes the degradation of MshA. Through site-directed mutagenesis and chimeric pilin analysis, we show that this process targets a combination of MshA N-terminal motifs and depends on the proteolytic activity of TcpJ. Moreover, overexpression of tcpJ partially restored the ability of bacteria unable to transcriptionally downregulate msh genes to colonize infant mice. These findings describe co-ordinated proteolysis as a regulatory mechanism in V. cholerae and illustrate this organism's adaptability in the face of dramatic environmental changes.

Export of the pseudopilin XcpT of the Pseudomonas aeruginosa type II secretion system via the signal recognition particle-Sec pathway

Type IV pilins and pseudopilins are found in various prokaryotic envelope protein complexes, including type IV pili and type II secretion machineries of gram-negative bacteria, competence systems of gram-positive bacteria, and flagella and sugar-binding structures in members of the archaeal kingdom. The precursors of these proteins have highly conserved N termini, consisting of a short, positively charged leader peptide, which is cleaved off by a dedicated peptidase during maturation, and a hydrophobic stretch of approximately 20 amino acid residues. Which pathway is involved in the inner membrane translocation of these proteins is unknown. We used XcpT, the major pseudopilin from the type II secretion machinery of Pseudomonas aeruginosa, as a model to study this process. Transport of an XcpT-PhoA hybrid was shown to occur in the absence of other Xcp components in P. aeruginosa and in Escherichia coli. Experiments with conditional sec mutants and reporter-protein fusions showed that this transport process involves the cotranslational signal recognition particle targeting route and is dependent on a functional Sec translocon.

A systematic genetic analysis in Neisseria meningitidis defines the Pil proteins required for assembly, functionality, stabilization and export of type IV pili

Although type IV pili (Tfp) are among the commonest virulence factors in bacteria, their biogenesis by complex machineries of 12-15 proteins, and thereby their function remains poorly understood. Interestingly, some of these proteins were reported to merely antagonize the retraction of the fibres powered by PilT, because piliation could be restored in their absence by a mutation in the pilT gene. The recent identification of the 15 Pil proteins dedicated to Tfp biogenesis in Neisseria meningitidis offered us the unprecedented possibility to define their exact contribution in this process. We therefore systematically introduced a pilT mutation into the corresponding non-piliated mutants and characterized them for the rescue of Tfp and Tfp-mediated virulence phenotypes. We found that in addition to the pilin, the main constituent of Tfp, only six Pil proteins were required for the actual assembly of the fibres, because apparently normal fibres were restored in the remaining mutants. Restored fibres were surface-exposed, except in the pilQ/T mutant in which they were trapped in the periplasm, suggesting that the PilQ secretin was the sole Pil component necessary for their emergence on the surface. Importantly, although in most mutants the restored Tfp were not functional, the pilG/T and pilH/T mutants could form bacterial aggregates and adhere to human cells efficiently, suggesting that Tfp stabilization and functional maturation are two discrete steps. These findings have numerous implications for understanding Tfp biogenesis/function and provide a useful groundwork for the characterization of the precise function of each Pil protein in this process.

The RpoT regulon of Pseudomonas putida DOT-T1E and its role in stress endurance against solvents

Pseudomonas putida encodes 20 extracytoplasmic sigma factors (ECFs). In this study, we show that one of these ECFs, known as ECF-Pp12 (PP3006), plays a role in tolerance of toluene and other organic solvents. Based on this finding, we have called the gene that encodes this new ECF rpoT. The rpoT gene forms an operon with the preceding gene and with the gene located downstream. The translated gene product of the open reading frame PP3005 is an inner membrane protein, whereas the PP3007 protein is periplasmic. A nonpolar DeltarpoT mutant was generated by homologous recombination, and survival of the mutant was tested under various stress conditions. The mutant strain was hypersensitive to toluene and other solvents but just as tolerant as the wild type of stress imposed by heat, antibiotics, NaCl, paraquat, sodium dodecyl sulfate, H(2)O(2), and benzoate. In the DeltarpoT mutant background, expression of around 50 transcriptional units was affected: 31 cistrons were upregulated, and 23 cistrons were downregulated. This indicates that about 1% of all P. putida genes are under the direct or indirect influence of RpoT. The rpoT gene controls the expression of a number of membrane proteins, including components of the respiratory chains, porins, transporters, and multidrug efflux pumps. Hypersensitivity of the P. putida RpoT-deficient mutant to organic solvents can be attributed to the fact that in the DeltarpoT strain, expression of the toluene efflux pump ttgGHI genes is severalfold lower than in the parental strain.

Active-site residues in the type IV prepilin peptidase homologue PibD from the archaeon Sulfolobus solfataricus

Archaeal preflagellin peptidases and bacterial type IV prepilin peptidases belong to a family of aspartic acid proteases that cleave the leader peptides of precursor proteins with type IV prepilin signal sequences. The substrate repertoire of PibD from the crenarchaeon Sulfolobus solfataricus is unusually diverse. In addition to flagellin, PibD cleaves three sugar-binding proteins unique to this species and a number of proteins with unknown function. Here we demonstrate that PibD contains two aspartic acid residues that are essential for cleavage activity. An additional pair of aspartic acids in a large cytoplasmic loop is also important for function and is possibly involved in leader peptide recognition. Combining the results of transmembrane segment predictions and cysteine-labeling experiments, we suggest a membrane topology model for PibD with the active-site aspartic acid residues exposed to the cytosol.

Genome-wide identification of Pseudomonas aeruginosa exported proteins using a consensus computational strategy combined with a laboratory-based PhoA fusion screen

The Gram-negative pathogen Pseudomonas aeruginosa encodes multiple protein export systems, the substrates of which contain export signals such as N-terminal signal peptides. Here we report the first genome-wide computational and laboratory screen for N-terminal signal peptides in this important opportunistic pathogen. The computational identification of signal peptides was based on a consensus between multiple predictive tools and showed that 38% of the P. aeruginosa PAO1 proteome was predicted to encode exported proteins, most of which utilize cleavable type I signal peptides or uncleavable transmembrane helices. In addition, known and novel lipoproteins (type II), twin arginine transporter (TAT), and prepilin peptidase substrates (type IV) were also identified. A laboratory-based screen using the alkaline phosphatase (PhoA) fusion method was then used to test our predictions. In total, 310 nonredundant PhoA fusions were successfully identified, 296 of which possess a predicted export signal. Analysis of the PhoA fusion proteins lacking an export signal revealed that three proteins have alternate translation start sites that encode signal peptides, two proteins may use an unknown export signal, and the remaining nine proteins are likely cytoplasmic proteins and represent false positives associated with the PhoA screen. Our approach to identify exported proteins illustrates how computational and laboratory-based methods are complementary, where computational analyses provide a large number of accurate predictions while laboratory methods both confirm predictions and reveal unique cases meriting further analysis.

Cell envelope components contributing to biofilm growth and survival of Pseudomonas putida in low-water-content habitats

Bacteria in terrestrial habitats frequently reside as biofilm communities on surfaces that are unsaturated, i.e. biofilms are covered in water films varying in thickness depending on the environmental conditions. Water availability in these habitats is influenced by the osmolarity of the water (solute stress) and by cellular dehydration imposed by matric stress, which increases as water content decreases. Unfortunately, we understand relatively little about the molecular mechanisms required for bacterial growth in low-water-content habitats. Here, we describe the use of mini-Tn5-'phoA to identify genes in Pseudomonas putida that are matric water stress controlled and to generate mutants defective in desiccation tolerance. We identified 20 genes that were induced by a matric stress but not by a thermodynamically equivalent solute stress, 11 genes were induced by both a matric and a solute stress, three genes were induced by a solute stress and three genes were repressed by a matric stress. Their patterns of expression were analysed in laboratory media, and their contribution to desiccation tolerance was evaluated. Twenty-six genes were homologous to sequences present in the completed P. putida KT2440 genome sequence or plasmid pWWO sequence that are involved in protein fate, nutrient or solute acquisition, energy generation, motility, alginate biosynthesis or cell envelope structure, and the function of five could not be predicted from the sequence. Together, these genes and their importance to desiccation tolerance provide a view of the environment perceived by bacteria in low-water-content habitats, and suggest that the mechanisms for adaptation for growth in low-water-content habitats are different from those for growth in high-osmolarity habitats.

The aminoglycoside 6'-N-acetyltransferase type Ib encoded by Tn1331 is evenly distributed within the cell's cytoplasm

The multiresistance transposon Tn1331, which mediates resistance to several aminoglycosides and beta-lactams, includes the aac(6')-Ib, aadA1, bla(OXA-9), and bla(TEM-1) genes. The nucleotide sequence of aac(6')-Ib includes a region identical to that of the bla(TEM-1) gene. This region encompasses the promoter and the initiation codon followed by 15 nucleotides. Since there were three possible translation initiation sites, the amino acid sequence at the N terminus of the aminoglycoside 6'-N-acetyltransferase type Ib [AAC(6')-Ib] was determined and was found to be SIQHF. This result indicated that aac(6')-Ib includes a translational fusion: the first five amino acids of the leader peptide of the TEM beta-lactamase are fused to the rest of the AAC(6')-Ib protein. This gene fusion could have formed during the genesis of Tn1331 as a consequence of the generation of a 520-nucleotide duplication (M. E. Tolmasky, Plasmid 24:218-226, 1990). An identical gene isolated from a Serratia marcescens strain has been previously described (G. Tran van Nhieu and E. Collatz, J. Bacteriol. 169:5708-5714, 1987). Extraction of the periplasmic proteins of E. coli harboring aac(6')-Ib by spheroplast formation showed that most of the AAC(6')-Ib protein is present in the cytoplasm. A genetic fusion to phoA confirmed these results. AAC(6')-Ib was shown to be evenly distributed inside the cell's cytoplasm by fluorescent microscopy with an AAC(6')-Ib-cyan fluorescent protein fusion.

The fla gene cluster is involved in the biogenesis of flagella in Halobacterium salinarum

In this study, a flagella-related protein gene cluster is described for Halobacterium salinarum. The fla gene cluster is located upstream of the flagellin genes flgB1-3 and oriented in the opposite direction. It consists of nine open reading frames (ORFs): htpIX, a member of the halobacterial transducer protein gene family, and the genes flaD-K. The genes flaD, E, G, H, I and J share high homologies with genes from other Archaea. Interestingly, flaK shows similarities to bacterial genes involved in the regulation of flagellar synthesis. The ORFs of flaH, flaI and flaK contain sequences coding for nucleotide binding sites. Furthermore, flaI contains a motif called the bacterial type II secretion protein E signature, indicating a functional relation to members of the bacterial pili type IV-type II secretion protein superfamily. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that the genes flaE to flaK are transcribed into one polycistronic message. In frame deletion mutants of flaI were generated by gene replacement. The deletion strain lacks motility and belongs to the fla(-) mutant class, indicating that it is deficient in flagellar biogenesis. The overall amount of flagellin protein in Delta flaI cells is reduced, although transcription of the flagellin genes is unaffected. Therefore, the flaI gene product is involved in the biosynthesis, transport or assembly of flagella in H. salinarum.

The archaeal flagellum: a different kind of prokaryotic motility structure

The archaeal flagellum is a unique motility apparatus distinct in composition and likely in assembly from the bacterial flagellum. Gene families comprised of multiple flagellin genes co-transcribed with a number of conserved, archaeal-specific accessory genes have been identified in several archaea. However, no homologues of any bacterial genes involved in flagella structure have yet been identified in any archaeon, including those archaea in which the complete genome sequence has been published. Archaeal flagellins possess a highly conserved hydrophobic N-terminal sequence that is similar to that of type IV pilins and clearly unlike that of bacterial flagellins. Also unlike bacterial flagellins but similar to type IV pilins, archaeal flagellins are initially synthesized with a short leader peptide that is cleaved by a membrane-located peptidase. With recent advances in genetic transfer systems in archaea, knockouts have been reported in several genes involved in flagellation in different archaea. In addition, techniques to isolate flagella with attached hook and anchoring structures have been developed. Analysis of these preparations is under way to identify minor structural components of archaeal flagella. This and the continued isolation and characterization of flagella mutants should lead to significant advances in our knowledge of the composition and assembly of archaeal flagella.

Components and dynamics of fiber formation define a ubiquitous biogenesis pathway for bacterial pili

Type IV pili (Tfp) are a unique class of multifunctional surface organelles in Gram-negative bacteria, which play important roles in prokaryotic cell biology. Although components of the Tfp biogenesis machinery have been characterized, it is not clear how they function or interact. Using Neisseria gonorrhoeae as a model system, we report here that organelle biogenesis can be resolved into two discrete steps: fiber formation and translocation of the fiber to the cell surface. This conclusion is based on the capturing of an intermediate state in which the organelle is retained within the cell owing to the simultaneous absence of the secretin family member and biogenesis component PilQ and the twitching motility/pilus retraction protein PilT. This finding is the first demonstration of a specific translocation defect associated with loss of secretin function, and additionally confirms the role of PilT as a conditional antagonist of stable pilus fiber formation. These findings have important implications for Tfp structure and function and are pertinent to other membrane translocation systems that utilize a highly related set of components.

The type 4 prepilin peptidases comprise a novel family of aspartic acid proteases

Type 4 prepilins or prepilin-like-proteins are secreted by a wide range of bacterial species and are required for a variety of functions including type 4 pilus formation, toxin and other enzyme secretion, gene transfer, and biofilm formation. A distinctive feature of these proteins is the presence of a specialized leader peptide that is cleaved off by a cognate membrane-bound type 4 prepilin peptidase (TFPP) during the process of secretion. In this report we show that the TFPPs represent a novel family of bilobed aspartate proteases that is unlike any other protease. The active site pairs of aspartic acids of the two TFPPs in Vibrio cholerae are found at positions 125 and 189 of TcpJ and 147 and 212 of VcpD. Corresponding aspartate residues are completely conserved throughout this extensive peptidase family.

Bacterial type II protein export and pilus biogenesis: more than just homologies?

Protein export by Gram-negative bacteria requires devoted machineries to allow for the passage of hydrolytic enzymes and toxins through the cell envelope. The Type II export machinery has a number of distinct characteristics, which include its role as an extension of Sec-dependent secretion, its ability to recognize and export fully folded substrates efficiently and, perhaps most significantly, the relationship between a subset of its gene products with the Type IV pilus-biogenesis apparatus. An important question is whether we can extrapolate our knowledge, albeit limited, of Type IV pilus biogenesis to understand the structure and function of the Type II export apparatus. This and other questions relating to the energetics of assembly and specificity of the apparatus are addressed in this article.

Involvement of the cis/trans isomerase Cti in solvent resistance of Pseudomonas putida DOT-T1E

Pseudomonas putida DOT-T1E is a solvent-resistant strain that is able to grow in the presence of high concentrations of toluene. We have cloned and sequenced the cti gene of this strain, which encodes the cis/trans isomerase, termed Cti, that catalyzes the cis-trans isomerization of esterified fatty acids in phospholipids, mainly cis-oleic acid (C(16:1,9)) and cis-vaccenic acid (C(18:1,11)), in response to solvents. To determine the importance of this cis/trans isomerase for solvent resistance a Cti-null mutant was generated and characterized. This mutant showed a longer lag phase when grown with toluene in the vapor phase; however, after the lag phase the growth rate of the mutant strain was similar to that of the wild type. The mutant also showed a significantly lower survival rate when shocked with 0.08% (vol/vol) toluene. In contrast to the wild-type strain, which grew in liquid culture medium at temperatures up to 38.5 degrees C, the Cti-null mutant strain grew significantly slower at temperatures above 37 degrees C. An in-frame fusion of the Cti protein with the periplasmic alkaline phosphatase suggests that this constitutively expressed enzyme is located in the periplasm. Primer extension studies confirmed the constitutive expression of Cti. Southern blot analysis of total DNA from various pseudomonads showed that the cti gene is present in all the tested P. putida strains, including non-solvent-resistant ones, and in some other Pseudomonas species.

Transcriptional regulation of type 4 pilin genes and the site-specific recombinase gene, piv, in Moraxella lacunata and Moraxella bovis

Moraxella lacunata and Moraxella bovis use type 4 pili to adhere to epithelial tissues of the cornea and conjunctiva. Primer extension analyses were used to map the transcriptional start sites for the genes encoding the major pilin subunits (tfpQ/I) and the DNA invertase (piv), which determines pilin type expression. tfpQ/I transcription starts at a sigma54-dependent promoter (tfpQ/Ip2) and, under certain growth conditions, this transcription is accompanied by weaker upstream transcription that starts at a potential sigma70-dependent promoter (tfpQ/Ip1). piv is expressed in both M. lacunata and M. bovis from a putative sigma70-dependent promoter (pivp) under all conditions assayed. Sigma54-dependent promoters require activators in order to initiate transcription; therefore, it is likely that tfpQ/Ip2 is also regulated by an activator in Moraxella. Primer extension assays with RNA isolated from Escherichia coli containing the subcloned pilin inversion region from M. lacunata showed that pivp is used for the expression of piv; however, tfpQ/Ip2 is not used for the transcription of tfpQ/I. Transcription from tfpQ/Ip2 was activated in E. coli when the sensor (PilS) and response regulator (PilR) proteins of type 4 pilin transcription in Pseudomonas aeruginosa were expressed from a plasmid. These results suggest that the expression of the type 4 pilin in M. lacunata and M. bovis is regulated not only by a site-specific DNA inversion system but also by a regulatory system which is functionally analogous to the PilS-PilR two-component system of P. aeruginosa.

Structure-function relationship of type-IV prepilin peptidase of Pseudomonas aeruginosa--a review

The bifunctional enzyme prepilin peptidase (PilD) from Pseudomonas aeruginosa is a key determinant in both type-IV pilus biogenesis and extracellular protein secretion, in its roles as a leader peptidase and MTase. It is responsible for endopeptidic cleavage of the unique leader peptides that characterize type-IV pilin precursors, as well as proteins with homologous leader sequences that are essential components of the general secretion pathway found in a variety of Gram-negative pathogens. Following removal of the leader peptides, the same enzyme is responsible for the second posttranslational modification that characterizes the type-IV pilins and their homologues, namely N-methylation of the newly exposed N-terminal amino acid residue. This review discusses some of the work begun in order to answer questions regarding the structure-function relationships of the active sites of this unique enzyme.

Outer membrane translocation arrest of the TcpA pilin subunit in rfb mutants of Vibrio cholerae O1 strain 569B

The toxin-coregulated pilus (TCP) of Vibrio cholerae is a type 4-related fimbrial adhesin and a useful model for the study of type 4 pilus biogenesis and related bacterial macromolecular transport pathways. Transposon mutagenesis of the putative perosamine biosynthesis genes in the rfb operon of V. cholerae 569B eliminates lipopolysaccharide (LPS) O-antigen biosynthesis but also leads to a specific defect in TCP export. Localization of TcpA is made difficult by the hydrophobic nature of this bundle-forming pilin, which floats anomalously in sucrose density gradients, but the processed form of TcpA can be found in membrane and periplasmic fractions prepared from these strains. While TcpA cannot be detected by surface immunogold labelling in transmission electron microscope preparations, EDTA pretreatment facilitates immunofluorescent antibody labelling of whole cells, and ultrathin cryosectioning techniques confirm membrane and periplasmic accumulation of TcpA. Salt and detergent extraction, protease accessibility, and chemical cross-linking experiments suggest that although TcpA has not been assembled on the cell surface, subunit interactions are otherwise identical to those within TCP. In addition, TcpA-mediated fucose-resistant hemagglutination of murine erythrocytes is preserved in whole-cell lysates, suggesting that TcpA has obtained its mature conformation. These data localize a stage of type 4 pilin translocation to the outer membrane, at which stage export failure leads to the accumulation of pilin subunits in a configuration similar to that within the mature fiber. Possible candidates for the outer membrane defect are discussed.

The toxin-co-regulated pilus of Vibrio cholerae O1: a model for type 4 pilus biogenesis?

The toxin-co-regulated pilus (TCP), an important colonization factor of Vibrio cholerae, is similar to the type 4 pilus produced by a variety of pathogenic Gram-negative bacteria. The putative translocation and assembly machinery of TCP has broad similarities with known pilin and nonpilin export mechanisms.

Sorting of protein A to the staphylococcal cell wall

The cell wall of gram-positive bacteria can be thought of as representing a unique cell compartment, which contains anchored surface proteins that require specific sorting signals. Some biologically important products are anchored in this way, including protein A and fibronectin binding protein of Staphylococcus aureus and streptococcal M protein. Studies of staphylococcal protein A and Escherichia coli alkaline phosphatase show that the signal both necessary and sufficient for cell wall anchoring consists of an LPXTGX motif, a C-terminal hydrophobic domain, and a charged tail. These sequence elements are conserved in many surface proteins from different gram-positive bacteria. We propose the existence of a hitherto undescribed sorting mechanism that positions proteins on the surface of gram-positive bacteria.

Components of the protein-excretion apparatus of Pseudomonas aeruginosa are processed by the type IV prepilin peptidase

In the Gram-negative pathogen Pseudomonas aeruginosa, mutants in the gene for the prepilin peptidase (pilD) are pleiotropic, as they not only fail to process pilin but also accumulate in the periplasm, in their mature form, several toxins and hydrolytic enzymes that are normally exported to the external medium (excreted). We have suggested that this excretion defect is due to the lack of PilD-dependent processing of proteins that share sequences in common with the prepilin subunit and that are components of a protein-excretion machinery. In this paper we report the isolation and characterization of transposon-induced excretion mutants with phenotypes similar to that of a pilD gene mutant. Using oligonucleotide probes designed to recognize sequences encoding the cleavage site of the type IV prepilins, we have isolated four linked genes with the predicted putative PilD-dependent cleavage site. Site-specific mutations within these genes have shown that they are required for protein excretion, and PilD-dependent processing of at least one of the four encoded proteins was demonstrated. Evidence suggests that similar components play a role in protein excretion in a wide variety of Gram-negative bacteria.

Neisseria gonorrhoeae prepilin export studied in Escherichia coli

The pilE gene of Neisseria gonorrhoeae MS11 and a series of pilE-phoA gene fusions were expressed in Escherichia coli. The PhoA hybrid proteins were shown to be located in the membrane fraction of the cells, and the prepilin product of the pilE gene was shown to be located exclusively in the cytoplasmic membrane. Analysis of the prepilin-PhoA hybrids showed that the first 20 residues of prepilin can function as an efficient export (signal) sequence. This segment of prepilin includes an unbroken sequence of 8 hydrophobic or neutral residues that form the N-terminal half of a 16-residue hydrophobic region of prepilin. Neither prepilin nor the prepilin-PhoA hybrids were processed by E. coli leader peptidase despite the presence of two consensus cleavage sites for this enzyme just after this hydrophobic region. Comparisons of the specific molecular activities of the four prepilin-PhoA hybrids and analysis of their susceptibility to proteolysis by trypsin and proteinase K in spheroplasts allow us to propose two models for the topology of prepilin in the E. coli cytoplasmic membrane. The bulk of the evidence supports the simplest of the two models, in which prepilin is anchored in the membrane solely by the N-terminal hydrophobic domain, with the extreme N terminus facing the cytoplasm and the longer C terminus facing the periplasm.

Processing of TCP pilin by TcpJ typifies a common step intrinsic to a newly recognized pathway of extracellular protein secretion by gram-negative bacteria

Biogenesis of the Vibrio cholerae toxin-coregulated pilus (TCP) requires the activities of at least seven accessory proteins. We demonstrate that a portion of this pathway involves a novel processing step in which a hydrophilic leader peptide is proteolytically removed from TcpA by the gene product characterized in this report, TcpJ, to yield the mature, export-competent form of the pilin. Cleavage of the pilin leader peptide is independent of known signal peptidases as demonstrated by pilin-processing profiles in Escherichia coli strains conditionally defective for production of leader peptidase or grown in the presence of the antibiotic globomycin. Additionally, pilin cleavage did not rely on the SecA protein, as evidenced by TcpA processing in azide-treated cells. These results suggest that TcpJ is representative of a new class of proteins involved in SecA-independent proteolytic cleavage of a set of atypical leader peptides during extracellular export.

Characterisation of a Pseudomonas aeruginosa twitching motility gene and evidence for a specialised protein export system widespread in eubacteria

Type-4 fimbriae (pili) are associated with a phenomenon known as twitching motility, which appears to be involved with bacterial translocation across solid surfaces. Pseudomonas aeruginosa mutants which produce fimbriae, but which have lost the twitching motility function, display altered colony morphology and resistance to fimbrial-specific bacteriophage. We have used phenotypic complementation of such mutants to isolate a region of DNA involved in twitching motility. This region was physically mapped to a SpeI fragment around 20 min on the P. aeruginosa PAO chromosome, remote from the major fimbrial locus (around 75 min) where the structural subunit-encoding gene (fimA/pilA) and ancillary genes required for fimbrial assembly (pilB, C and D) are found. A gene, pilT, within the twitching motility region is predicted to encode a 344-amino acid protein which has strong homology to a variety of other bacterial proteins. These include the P. aeruginosa PilB protein, the ComG ORF-1 protein from the Bacillus subtilis comG operon (necessary for competence), the PulE protein from the Klebsiella oxytoca (formerly K. pneumoniae) pulC-O operon (involved in pullulanase export), and the VirB-11 protein from the virB operon (involved in virulence) which is located on the Agrobacterium tumefaciens Ti plasmid. We have also identified other sets of homologies between P. aeruginosa fimbrial assembly (Pil) proteins and B. subtilis Com and K. oxytoca Pul proteins, which suggest that these are all related members of a specialised protein export pathway which is widespread in the eubacteria.

Organization of the fimbrial gene region of Bacteroides nodosus: class I and class II strains

The fimbrial subunit genes of Bacteroides nodosus may be divided into two distinct classes, based on the sequence of the major subunit gene fimA (accompanying paper--Mattick et al., 1991). The genetic organization of the fibrial gene region in these two classes is also distinct. Upstream of fimA in both classes in opposite transcriptional orientation is the gene aroA which encodes amino acid biosynthetic enzyme 5-enolpyruvylshikimate-3-phosphate synthase. However, downstream of fimA the two classes are quite different until homology is restored at a bidirectional transcription termination signal separating the fimbrial operon from a gene clpB, which appears to encode the regulatory subunit of an ATP-dependent protease. Between aroA and clpB class I strains contain, apart from fimA, only one other gene (fimB). Sequence and polymerase chain reaction analyses indicate that fimB does not have a separate promoter but rather is co-transcribed with fimA at a level attenuated by the strength of the transcription termination signal in the intergenic region. In class II strains fimA is followed by a more extended region containing three genes, which appear to have the same transcriptional arrangement as fimB. The second of these genes (fimD) may represent a functional analogue of fimB although there is no close sequence homology. The first gene (fimC) has no obvious similarity to either fimB or fimD. Beyond fimD, at the 3' end of the class II-specific region, is a variant fimbrial subunit gene (fimZ) which is virtually identical in serogroups D and H and which appears to represent a duplicate, possibly redundant, gene closely related to the progenitor of the more divergent structural subunit fimA gene found in these strains. Comparisons of the predicted fimZ product with those of fimA in class I and class II strains, as well as of the boundaries of the class-specific regions, suggest that the class II sequences evolved in another type 4 fimbriate species and were subsequently substituted in the B. nodosus genome by lateral transfer. Analysis of the sequences flanking fimA in different strains indicates that recombinational exchange of both fimA and the entire operon has also occurred between strains, and is possibly a mechanism for disseminating structural diversity in the population.

Pilin expression and processing in pilus mutants of Neisseria gonorrhoeae: critical role of Gly-1 in assembly

Spontaneous mutants of Neisseria gonorrheae failing to express pili or having diminished levels of piliation were studied with regard to pilin expression. All mutants displayed altered pilin processing detectable as the release of soluble, truncated pilin molecules (S-pilin). Of particular interest was the finding, in one mutant, that substitution of serine for glycine at position -1 of propilin, a highly conserved residue among N-metPhe and related pilins, abolished pilus expression but not S-pilin release. The degree of S-pilin processing and the levels of membrane-associated pilin varied among the different classes of mutants, suggesting that each was blocked at a distinct step of pilus biogenesis. The data support a model in which increased S-pilin processing is a result of a decreased rate of pilus polymerization.

Multiple roles of the pilus biogenesis protein pilD: involvement of pilD in excretion of enzymes from Pseudomonas aeruginosa

In Pseudomonas aeruginosa, the genes pilB, pilC, and pilD encode proteins necessary for posttranslational modification and assembly of pilin monomers into pilus organelles (D. Nunn, S. Bergman, and S. Lory, J. Bacteriol. 172:2911-2919, 1990). We show that PilD, encoding a putative pilin-specific leader peptidase, also controls export of alkaline phosphatase, phospholipase C, elastase, and exotoxin A. pilD mutants accumulate these proteins in the periplasmic space, while secretion of periplasmic and outer membrane proteins appears to be normal. The periplasmic form of exotoxin A was fully mature in size, contained all cysteines in disulfide bonds, and was toxic in a tissue culture cytotoxicity assay, suggesting that in pilD mutants, exotoxin A was folded into its native conformation. The function of the other two accessory proteins, PilB and PilC, appears to be restricted to pilus biogenesis, and strains carrying mutations in their respective genes do not show an export defect. These studies show that in addition to cleaving the leader sequence from prepilin, PilD has an additional role in secretion of proteins that are released from P. aeruginosa into the surrounding media. PilD most likely functions as a protease that is involved in processing and assembly of one or more components of the membrane machinery necessary for the later stages of protein extracellular localization.

Vibrio cholerae O395 tcpA pilin gene sequence and comparison of predicted protein structural features to those of type 4 pilins

Vibrio cholerae O1 expresses a pilus that is coordinately regulated with cholera toxin production and hence termed TCP, for toxin-coregulated pilus. Insertion of Tn5 IS50L::phoA (TnphoA) into the major pilin subunit gene, tcpA, has previously been shown to render the strain avirulent as a result of its inability to colonize. One such insertion was isolated and used as a probe to screen for clones containing the intact tcpA gene. The DNA sequence of tcpA was determined by using the intact gene and several tcpA-phoA gene fusions. The deduced protein sequence agreed completely with that previously determined for the TcpA N terminus and with the size of the mature pilin protein. The reported homology with N-methylphenylalanine (type 4) pilins near the N terminus was extended and shown to include components of the atypical leader peptide as well as overall predicted structural similarities in other regions of the pilins. In contrast to the modified N-terminal phenylalanine residue found in all characterized type 4 pilins, the corresponding position in tcpA contains a Met codon, thus implying that the previously uncharacterized amino acid corresponding to the N-terminal position of the mature TcpA pilin is a modified form of methionine. Except for this difference, mature TcpA has the overall predicted structural motifs shared among type 4 pilins.

Purification and characterization of Bacillus thuringiensis var. tenebrionis insecticidal proteins produced in E. coli

Native and single amino acid variants of the Bacillus thuringiensis var. tenebrionis insecticidal proteins were expressed in Escherichia coli, purified and examined for biological and biochemical properties. A novel, pH dependent, preferential precipitation method was implemented to purify Escherichia coli produced Bacillus thuringiensis var. tenebrionis proteins, which are active against Colorado potato beetle (Leptinotarsa decemlineata) larvae. Cysteine residues of the native Bacillus thuringiensis var. tenebrionis protein were replaced by serine residues by site-directed mutagenesis to investigate the biological and structural importance of the individual cysteine residues. Sulfhydryl determination of the native and amino acid variant Bacillus thuringiensis var. tenebrionis proteins revealed that the native protein contains no disulfide bonds. Modification of the carboxyl terminal cysteine residue (amino acid 540) caused complete inactivation of the protein. Native, truncated and single amino acid variants (other than at amino acid 540) exhibited insecticidal activities comparable to each other and to solubilized crystals from the original strain.

A phoA structural gene mutation that conditionally affects formation of the enzyme bacterial alkaline phosphatase

The phoA503 mutant was identified as a mutant that shows a novel phoA regulatory phenotype. The phoA503 allele dramatically reduces the synthesis of bacterial alkaline phosphatase activity during Pi starvation in an otherwise wild-type host and during the logarithmic growth phase in a phoR or phoU background. Near-normal amounts of enzyme activity are found in phoR phoA503 or phoU phoA503 mutants when starved for carbon, nitrogen, or sulfur or during the stationary phase, however. Marker rescue and DNA sequence analysis located the phoA503 mutation to the phoA coding region. It is a C-to-T transition that would cause a substitution of Val for Ala-22 in the mature protein. Transcriptional and translational lacZ fusions to both wild-type and mutant alleles demonstrated that phoA gene expression is unaltered. Also, the mutant protein was secreted and processed as efficiently as the wild type. Furthermore, the subunits appeared to dimerize and to be stable in the periplasm. But, greater than 98% of the dimers were inactive and found exclusively as isozyme 1. An activation of preformed phoA503 dimers occurred during the stationary phase with the concomitant conversion into isozymes 2 and 3. We propose that the phoA503 mutation affects a late stage in the formation of active enzyme. An unknown change when Pi is present during stationary-phase growth leads to formation of active dimers, which is responsible for this new conditional phenotype.

Products of three accessory genes, pilB, pilC, and pilD, are required for biogenesis of Pseudomonas aeruginosa pili

The polar pili of Pseudomonas aeruginosa are composed of monomers of the pilin structural subunits. The biogenesis of pili involves the synthesis of pilin precursor, cleavage of a six-amino-acid leader peptide, membrane translocation, and assembly of monomers into a filamentous structure extending from the bacterial surface. This report describes three novel genes necessary for the formation of pili. DNA sequences adjacent to pilA, the pilin structural gene, were cloned and mutagenized with transposon Tn5. Each of the insertions were introduced into the chromosome of P. aeruginosa PAK by gene replacement. The effect of the Tn5 insertions in the bacterial chromosome on pilus assembly was assessed by electron microscopy and sensitivity of mutants to a pilus-specific bacteriophage. The resultant mutants were also tested for synthesis and membrane localization of the pilin antigen in order to define the genes required for maturation, export, and assembly of pilin. A 4.0-kilobase-pair region of DNA adjacent to the pilin structural gene was found to be essential for formation of pili. This region was sequenced and found to contain three open reading frames coding for 62-, 38- to 45-, and 28- to 32-kilodalton proteins (pilB, pilC, and pilD, respectively). Three proteins of similar molecular weight were expressed in Escherichia coli from the 4.0-kilobase-pair fragment flanking pilA with use of a T7 promoter-polymerase expression system. The results of the analyses of the three genes and the implications for pilin assembly and maturation are discussed.

Expression of the Pseudomonas aeruginosa toxA positive regulatory gene (regA) in Escherichia coli

The regA gene is a positive regulatory gene that regulates toxin A production in Pseudomonas aeruginosa at the transcriptional level. The product of the regA gene was examined in Escherichia coli with the expression vector pT7-7. A 1.3-kilobase AvaI-HindIII fragment containing the regA gene was cloned into the pT7-7 vector. A recombinant plasmid (pAH1) encoded a 29-kilodalton protein. The molecular weight of this protein correlated closely with the predicted molecular weight of the RegA protein. Production of the RegA protein in E. coli required both an E. coli promoter and an E. coli ribosome-binding site. Two in-frame deletion derivatives in which certain regions of the regA gene were expressed from the T7 promoter encoded 26- and 18-kilodalton fusion proteins, respectively. The RegA protein and the two fusion proteins were localized to the inner membrane of E. coli. Neither RegA protein nor the two fusion proteins showed DNA-binding activity to the 410-base-pair fragment containing the upstream region of toxA when synthesized in E. coli.

Alkaline phosphatase fusions: sensors of subcellular location

Alkaline phosphatase fusions allow genes to be identified solely on the basis of their protein products being exported from the cytoplasm. Thus, the use of such fusions helps render biological processes which involve cell envelope and secreted proteins accessible to a sophisticated genetic analysis. Furthermore, alkaline phosphatase fusions can be used to locate export signals. Specifying such signals is an important component of studies on the structure of individual cell envelope proteins. The basis of the alkaline phosphatase fusion approach is the finding that the activity of the enzyme responds differently to different environments. Thus, the activity of the fusion protein gives evidence as to its location. This general approach of using sensor proteins which vary in their function, depending on their environment, could be extended to the study of other sorts of problems. It may be that certain enzymes will provide an assay for localization to a particular subcellular compartment, if the environment of the compartment differs from that of others. For instance, the lysosome is more acidic than other intracellular organelles. A gene fusion system employing a reporter enzyme that could show activity only at the pH of the lysosome could allow the detection of signals determining lysosomal localization. Analogous types of enzymes may be used as probes for other subcellular compartments.

Secretion of extracellular proteins by Pseudomonas aeruginosa

Pseudomonas aeruginosa is a bacterial species of commercial value secreting numerous extracellular proteins, involved in pathogenesis. Most strains produce at least a lipase, a phospholipase, an alkaline phosphatase, an exotoxin and 2 proteases (elastase and alkaline protease). Various mechanisms for secretion of exoproteins appear to exist in P aeruginosa. Genetic analysis has led to the identification of 2 secretion pathways: i) a "general" secretion pathway, defined by the xcp mutations, which mediates secretion of most extracellular proteins, and; ii) an independent secretion pathway specific for alkaline protease. Our present knowledge on the pathways and components of the secretion machinery in P aeruginosa is reviewed in this article.

A gene required for transfer of T-DNA to plants encodes an ATPase with autophosphorylating activity

The virB operon of the Agrobacterium tume-faciens pTiA6NC plasmid likely plays a role in directing T-DNA transfer events at the bacterial membrane, as determined previously by mutagenesis and cellular fractionation studies and by DNA sequence analysis of the approximately 12-kilobase-pair operon. The DNA sequence analysis also revealed consensus mononucleotide binding domains in the deduced virB5 and virB11 gene products, suggesting that one or both of these proteins couple energy, by means of nucleotide triphosphate (NTP) hydrolysis, to T-DNA transport. In this report, the product of virB11, an essential virulence gene, was overproduced in Escherichia coli and purified by using immunoaffinity chromatography. The immunoaffinity purified protein, as well as NaDodSO4/polyacrylamide gel-eluted protein, bound and hydrolyzed ATP in the absence of DNA effectors. VirB11 protein also demonstrated in vitro autophosphorylation activity. VirB11 protein was localized primarily to the cytoplasmic membrane by immunoblot analysis of membrane fractions. The deduced VirB11 protein exhibits sequence similarity to comG ORF1, a protein required for uptake of DNA by competent Bacillus subtilis cells. These findings suggest that phosphorylation may serve to activate a component(s) of the A. tumefaciens T-DNA transport apparatus and may also represent a general activation mechanism of other bacterial DNA transport systems.

Regions of toxin A involved in toxin A excretion in Pseudomonas aeruginosa

Toxin A is excreted by Pseudomonas aeruginosa as a mature 66,583-dalton protein. In this study, we used molecular cloning and deletion analysis to define specific regions of the toxin molecule involved in its excretion. Subclones that express either the amino terminus, the carboxy terminus, or toxin A molecules with internal deletions were constructed. The hypotoxigenic mutant PAO-T1 was used as a host for the expression of the toxin constructs. When overexpressed (by the presence of extra copies of the toxin A-positive regulatory gene, regA, in trans), toxin A-cross-reactive materials produced by most of these constructs were detected in the supernatant of PAO-T1. The supernatant of P. aeruginosa PAO-T1 contained proteolytic activity that degraded toxin A-derived products but not the intact toxin molecule. A single SalI intragenic deletion (coding for the leader peptide, the first 30 amino acids, and the last 305 amino acids of the toxin) resulted in a relatively stable product in the supernatant of PAO-T1. The product of the carboxy terminus construct (which codes for the last 305 amino acids of the toxin) was detected in the lysate of PAO-T1 only. The data suggest that the amino terminus region of toxin A (the leader peptide plus the first 30 amino acid of the mature protein) is sufficient for its excretion, and that a second region, amino acids 309 through 413, protects an internally truncated toxin A molecule from the proteolytic activity in the supernatant of P. aeruginosa PAO-T1.

Isolation, molecular characterization and expression of the ushB gene of Salmonella typhimurium which encodes a membrane-bound UDP-sugar hydrolase

The UDP-sugar hydrolase of Salmonella typhimurium has previously been reported to be located in both the inner and the outer membrane. We have cloned the gene, designated ushB, encoding this enzyme and determined its nucleotide sequence. No significant sequence homology with the periplasmic UDP-sugar hydrolase of Escherichia coli was found at either the DNA or protein level. However, a sequence is detectable, in the E. coli genome, which weakly hybridizes with a specific ushB probe. Polypeptide analysis has allowed the identification of the Salmonella hydrolase which has an Mr of 28,349 as compared to an Mr of 60,767 for the E. coli hydrolase. Most of the protein (approximately 90%) is located in the inner membrane. Two independent membrane fractionation procedures indicate that the remainder may be associated with the outer membrane. The deduced primary structure indicates the presence of an N-terminal signal peptide, although certain features of the region surrounding the putative processing site indicate that processing may be inefficient, or may not occur. Experiments with several inhibitors of signal peptidase function fail to demonstrate the appearance of a precursor form.