New Perspectives on Escherichia coli Signal Peptidase I Substrate Specificity: Investigating Why the TasA Cleavage Site Is Incompatible with LepB Cleavage.

Escherichia coli signal peptidase I (LepB) has been shown to inefficiently cleave secreted proteins with aromatic amino acids at the second position after the signal peptidase cleavage site (P2'). The Bacillus subtilis exported protein TasA contains a phenylalanine at P2', which in B. subtilis is cleaved by a dedicated archaeal-organism-like signal peptidase, SipW. We have previously shown that when the TasA signal peptide is fused to maltose binding protein (MBP) up to the P2' position, the TasA-MBP fusion protein is cleaved very inefficiently by LepB. However, the precise reason why the TasA signal peptide hinders cleavage by LepB is not known. In this study, a set of 11 peptides were designed to mimic the inefficiently cleaved secreted proteins, wild-type TasA and TasA-MBP fusions, to determine whether the peptides interact with and inhibit the function of LepB. The binding affinity and inhibitory potential of the peptides against LepB were assessed by surface plasmon resonance (SPR) and a LepB enzyme activity assay. Molecular modeling of the interaction between TasA signal peptide and LepB indicated that the tryptophan residue at P2 (two amino acids before the cleavage site) inhibited the active site serine-90 residue on LepB from accessing the cleavage site. Replacing the P2 tryptophan with alanine (W26A) allowed for more efficient processing of the signal peptide when the TasA-MBP fusion was expressed in E. coli. The importance of this residue to inhibit signal peptide cleavage and the potential to design LepB inhibitors based on the TasA signal peptide are discussed. IMPORTANCE Signal peptidase I is an important drug target, and understanding its substrate is critically important to develop new bacterium-specific drugs. To that end, we have a unique signal peptide that we have shown is refractory to processing by LepB, the essential signal peptidase I in E. coli, but previously has been shown to be processed by a more human-like signal peptidase found in some bacteria. In this study, we demonstrate how the signal peptide can bind but is unable to be processed by LepB, using a variety of methods. This can inform the field on how to better design drugs that can target LepB and understand the differences between bacterial and human-like signal peptidases.

The role of signal sequence proximal residues in the mature region of bacterial secreted proteins in E. coli.

Secreted proteins contain an N-terminal signal peptide to guide them through the secretion pathway. Once the protein is translocated, the signal peptide is removed by a signal peptidase, such as signal peptidase I. The signal peptide has been extensively studied and reviewed; however, the mature region has not been the focus of review. Here we cover the experimental evidence that highlights the important role of the mature region amino acid residues in both the efficiency and the ability of secreted proteins to be successfully exported via secretion pathways and cleaved by signal peptidase I.

Expression of the Bacillus subtilis TasA signal peptide leads to cell death in Escherichia coli due to inefficient cleavage by LepB.

Bacillus subtilis has five type I signal peptidases, one of these, SipW, is an archaeal-like peptidase. SipW is expressed in an operon (tapA-sipW-tasA) and is responsible for removing the signal peptide from two proteins: TapA and TasA. It is unclear from the signal peptide sequence of TasA and TapA, why an archaeal-like signal peptidase is required for their processing. Bioinformatic analysis of TasA and TapA indicates that both contain highly similar signal peptide cleavage sites, both predicted to be cleaved by Escherichia coli signal peptidase I, LepB. We show that expressing full length TasA in E. coli is toxic and leads to cell death. To determine if this phenotype is due to the inability of the E. coli LepB to process the TasA signal peptide, we fused the TasA signal peptide and two amino acids of mature TasA (up to P2') to both maltose binding protein (MBP) and ß-lactamase (Bla). We observed a defect in secretion, indicated by an abundance of unprocessed protein with both TasA-MBP and TasA-Bla fusions. A series of mutations in both TasA-MBP and TasA-Bla were made around the junction of the TasA signal peptide and the fusion protein. Both of these studies indicate that residues around the predicted TasA signal sequence cleavage site, particularly the sequence from P3 to P2', inhibit processing by LepB. The cell death observed when TasA and TasA signal sequence fusion proteins are expressed is likely due to the TasA signal peptide blocking LepB and thereby the general secretion pathway.

A drug candidate for Alzheimer's and Huntington's disease, PBT2, can be repurposed to render Neisseria gonorrhoeae susceptible to natural cationic antimicrobial peptides.

BACKGROUND: Neisseria gonorrhoeae is a Gram-negative bacterial pathogen that causes gonorrhoea. No vaccine is available to prevent gonorrhoea and the emergence of MDR N. gonorrhoeae strains represents an immediate public health threat. OBJECTIVES: To evaluate whether PBT2/zinc may sensitize MDR N. gonorrhoeae to natural cationic antimicrobial peptides. METHODS: MDR strains that contain differing resistance mechanisms against numerous antibiotics were tested in MIC assays. MIC assays were performed using the broth microdilution method according to CLSI guidelines in a microtitre plate. Serially diluted LL-37 or PG-1 was tested in combination with a sub-inhibitory concentration of PBT2/zinc. Serially diluted tetracycline was also tested with sub-inhibitory concentrations of PBT2/zinc and LL-37. SWATH-MS proteomic analysis of N. gonorrhoeae treated with PBT2/zinc, LL-37 and/or tetracycline was performed to determine the mechanism(s) of N. gonorrhoeae susceptibility to antibiotics and peptides. RESULTS: Sub-inhibitory concentrations of LL-37 and PBT2/zinc synergized to render strain WHO-Z susceptible to tetracycline, whereas the killing effect of PG-1 and PBT2/zinc was additive. SWATH-MS proteomic analysis suggested that PBT2/zinc most likely leads to a loss of membrane integrity and increased protein misfolding and, in turn, results in bacterial death. CONCLUSIONS: Here we show that PBT2, a candidate Alzheimer's and Huntington's disease drug, can be repurposed to render MDR N. gonorrhoeae more susceptible to the endogenous antimicrobial peptides LL-37 and PG-1. In the presence of LL-37, PBT2/zinc can synergize with tetracycline to restore tetracycline susceptibility to gonococci resistant to this antibiotic.

Evolution for improved secretion and fitness may be the selective pressures leading to the emergence of two NDM alleles.

The New Delhi metallo-ß-lactamase (NDM-1) mediates resistance to ß-lactam antibiotics. NDM-1 was likely formed as the result of a gene fusion between sequences encoding the first six amino acids of cytoplasm-localised aminoglycosidase, AphA6, and a periplasmic metallo-ß -lactamase. We show that NDM-1 has an atypical signal peptide and is inefficiently secreted. Two new blaNDM-1 alleles that have polymorphisms in the signal peptide; NDM-1(P9R), a proline to arginine substitution, and NDM-2, a proline to alanine substitution (P28A) were studied. Here, we show that both the P9R and P28A substitutions improve secretion compared to NDM-1 and display higher resistance to some ß-lactam antibiotics. Mass spectrometry analysis of these purified NDM proteins showed that the P28A mutation in NDM-2 creates new signal peptide cleavage sites at positions 27 and 28. For NDM-1, we detected a signal peptide cleavage site between L21/M22 of the precursor protein. We find no evidence that NDM-1 is a lipoprotein, as has been reported elsewhere. In addition, expression of NDM-2 improves the fitness of E. coli, compared to NDM-1, in the absence of antibiotic selection. This study shows how optimization of the secretion efficiency of NDM-1 leads to increased resistance and increased fitness.

Structural, Biochemical, and In Vivo Characterization of MtrR-Mediated Resistance to Innate Antimicrobials by the Human Pathogen Neisseria gonorrhoeae.

Neisseria gonorrhoeae responds to host-derived antimicrobials by inducing the expression of the mtrCDE-encoded multidrug efflux pump, which expels microbicides, such as bile salts, fatty acids, and multiple extrinsically administered drugs, from the cell. In the absence of these cytotoxins, the TetR family member MtrR represses the mtrCDE genes. Although antimicrobial-dependent derepression of mtrCDE is clear, the physiological inducers of MtrR are unknown. Here, we report the crystal structure of an induced form of MtrR. In the binding pocket of MtrR, we observed electron density that we hypothesized was N-cyclohexyl-3-aminopropanesulfonic acid (CAPS), a component of the crystallization reagent. Using the MtrR-CAPS structure as an inducer-bound template, we hypothesized that bile salts, which bear significant chemical resemblance to CAPS, are physiologically relevant inducers. Indeed, characterization of MtrR-chenodeoxycholate and MtrR-taurodeoxycholate interactions, both in vitro and in vivo, revealed that these bile salts, but not glyocholate or taurocholate, bind MtrR tightly and can act as bona fide inducers. Furthermore, two residues, W136 and R176, were shown to be important in binding chenodeoxycholate but not taurodeoxycholate, suggesting different binding modes of the bile salts. These data provide insight into a crucial mechanism utilized by the pathogen to overcome innate human defenses.IMPORTANCENeisseria gonorrhoeae causes a significant disease burden worldwide, and a meteoric rise in its multidrug resistance has reduced the efficacy of antibiotics previously or currently approved for therapy of gonorrheal infections. The multidrug efflux pump MtrCDE transports multiple drugs and host-derived antimicrobials from the bacterial cell and confers survival advantage on the pathogen within the host. Transcription of the pump is repressed by MtrR but relieved by the cytosolic influx of antimicrobials. Here, we describe the structure of induced MtrR and use this structure to identify bile salts as physiological inducers of MtrR. These findings provide a mechanistic basis for antimicrobial sensing and gonococcal protection by MtrR through the derepression of mtrCDE expression after exposure to intrinsic and clinically applied antimicrobials.

Efficient function of signal peptidase 1 of Escherichia coli is partly determined by residues in the mature N-terminus of exported proteins.

Exported proteins require an N-terminal signal peptide to direct them from the cytoplasm to the periplasm. Once the protein has been translocated across the cytoplasmic membrane, the signal peptide is cleaved by a signal peptidase, allowing the remainder of the protein to fold into its mature state in the periplasm. Signal peptidase I (LepB) cleaves non-lipoproteins and recognises the sequence Ala-X-Ala. Amino acids present at the N-terminus of mature, exported proteins have been shown to affect the efficiency at which the protein is exported. Here we investigated a bias against aromatic amino acids at the second position in the mature protein (P2'). Maltose binding protein (MBP) was mutated to introduce aromatic amino acids (tryptophan, tyrosine and phenylalanine) at P2'. All mutants with aromatic amino acids at P2' were exported less efficiently as indicated by a slight increase in precursor protein in vivo. Binding of LepB to peptides that encompass the MBP cleavage site were analysed using surface plasmon resonance. These studies showed peptides with an aromatic amino acid at P2' had a slower off rate, due to a significantly higher binding affinity for LepB. These data are consistent with the accumulation of small amounts of preMBP in purified protein samples. Hence, the reason for the lack of aromatic amino acids at P2' in E. coli is likely due to interference with efficient LepB activity. These data and previous bioinformatics strongly suggest that aromatic amino acids are not preferred at P2' and this should be incorporated into signal peptide prediction algorithms.

Control of gdhR Expression in Neisseria gonorrhoeae via Autoregulation and a Master Repressor (MtrR) of a Drug Efflux Pump Operon.

The MtrCDE efflux pump of Neisseria gonorrhoeae contributes to gonococcal resistance to a number of antibiotics used previously or currently in treatment of gonorrhea, as well as to host-derived antimicrobials that participate in innate defense. Overexpression of the MtrCDE efflux pump increases gonococcal survival and fitness during experimental lower genital tract infection of female mice. Transcription of mtrCDE can be repressed by the DNA-binding protein MtrR, which also acts as a global regulator of genes involved in important metabolic, physiologic, or regulatory processes. Here, we investigated whether a gene downstream of mtrCDE, previously annotated gdhR in Neisseria meningitidis, is a target for regulation by MtrR. In meningococci, GdhR serves as a regulator of genes involved in glucose catabolism, amino acid transport, and biosynthesis, including gdhA, which encodes an l-glutamate dehydrogenase and is located next to gdhR but is transcriptionally divergent. We report here that in N. gonorrhoeae, expression of gdhR is subject to autoregulation by GdhR and direct repression by MtrR. Importantly, loss of GdhR significantly increased gonococcal fitness compared to a complemented mutant strain during experimental murine infection. Interestingly, loss of GdhR did not influence expression of gdhA, as reported for meningococci. This variance is most likely due to differences in promoter localization and utilization between gonococci and meningococci. We propose that transcriptional control of gonococcal genes through the action of MtrR and GdhR contributes to fitness of N. gonorrhoeae during infection.IMPORTANCE The pathogenic Neisseria species are strict human pathogens that can cause a sexually transmitted infection (N. gonorrhoeae) or meningitis or fulminant septicemia (N. meningitidis). Although they share considerable genetic information, little attention has been directed to comparing transcriptional regulatory systems that modulate expression of their conserved genes. We hypothesized that transcriptional regulatory differences exist between these two pathogens, and we used the gdh locus as a model to test this idea. For this purpose, we studied two conserved genes (gdhR and gdhA) within the locus. Despite general conservation of the gdh locus in gonococci and meningococci, differences exist in noncoding sequences that correspond to promoter elements or potential sites for interacting with DNA-binding proteins, such as GdhR and MtrR. Our results indicate that implications drawn from studying regulation of conserved genes in one pathogen are not necessarily translatable to a genetically related pathogen.

Signal peptidase I processed secretory signal sequences: Selection for and against specific amino acids at the second position of mature protein.

Signal peptides direct proteins from the cytoplasm to the periplasm. These N-terminal peptides are cleaved upon entry to the periplasm by either signal peptidase I, or signal peptidase II for lipoproteins. Signal peptidase I is a serine protease that has either a serine-lysine or serine-histidine catalytic dyad present in the active site. The recognition site for signal peptide cleavage by signal peptidase I has been defined primarily by an Ala-X-Ala motif at the C-terminal end of the signal peptide, one amino acid away from the cleavage site. We used a verified set of signal peptidase I cleaved proteins from E. coli to look for novel conserved features, focusing on the N-terminus of the mature protein. We observed a striking bias for the presence of acidic residues at second position of the mature protein (P2'), and a complete absence of aromatic amino acids at the same position. Whole genome analysis of the predicted set of all E. coli and B. subtilis secreted proteins confirmed the same strong bias for acidic residues at P2' of the mature protein, and against aromatic amino acids at the same position. When these studies were extended to archaeal genomes (M. voltae and S. tokodaii) and the yeast genome from S. cerevisiae, this bias was not observed. E. coli and B. subtilis primarily express a signal peptidase I contains a serine-lysine catalytic dyad, whilst those of archaeal and eukaryotic origin generally have a serine-histidine catalytic dyad. This difference may explain the differential bias for acidic residues and against aromatic residues at P2'. These observations suggest additional key residues that may favor or prevent signal sequence recognition or cleavage by signal peptidase I, and thereby facilitate more accurate in silico prediction of signal peptidase I cleavage sites.

Dueling regulatory properties of a transcriptional activator (MtrA) and repressor (MtrR) that control efflux pump gene expression in Neisseria gonorrhoeae.

UNLABELLED: MtrA is a member of the AraC family of transcriptional regulators and has been shown to play an important role in enhancing transcription of the mtrCDE operon, which encodes a tripartite multidrug efflux pump, when gonococci are exposed to a sublethal level of antimicrobials. Heretofore, the DNA-binding properties of MtrA were unknown. In order to understand how MtrA activates mtrCDE expression, we successfully purified MtrA and found that it could bind specifically to the mtrCDE promoter region. The affinity of MtrA for the mtrCDE promoter increased 2-fold in the presence of a known effector and substrate of the MtrCDE pump, the nonionic detergent Triton X-100 (TX-100). When placed in competition with MtrR, the transcriptional repressor of mtrCDE, MtrA was found to bind with apparent lower affinity than MtrR to the same region. However, preincubation of MtrA with TX-100 prior to addition of the promoter-containing DNA probe increased MtrA binding and greatly reduced its dissociation from the promoter upon addition of MtrR. Two independent approaches (DNase I footprinting and a screen for bases important in MtrA binding) defined the MtrA-binding site 20-30 bp upstream of the known MtrR-binding site. Collectively, these results suggest that the MtrA and MtrR-binding sites are sterically close and that addition of an effector increases the affinity of MtrA for the mtrCDE promoter such that MtrR binding is negatively impacted. Our results provide a mechanism for transcriptional activation of mtrCDE by MtrA and highlight the complexity of transcriptional control of drug efflux systems possessed by gonococci. IMPORTANCE: Antibiotic resistance in Neisseria gonorrhoeae has been increasing in recent years, such that in 2007 the Centers for Disease Control and Prevention listed N. gonorrhoeae as a "superbug." One of the major contributors to antibiotic resistance in N. gonorrhoeae is the MtrCDE efflux pump. Until now, most work on the regulation of the genes encoding this efflux pump has been done on the transcriptional repressor, MtrR. This study is the first one to purify and define the DNA-binding ability of the transcriptional activator, MtrA. Understanding how levels of the MtrCDE efflux pump are regulated increases our knowledge of gonococcal biology and how the gonococcus can respond to various stresses, including antimicrobials.

A novel mechanism of high-level, broad-spectrum antibiotic resistance caused by a single base pair change in Neisseria gonorrhoeae.

UNLABELLED: The MtrC-MtrD-MtrE multidrug efflux pump of Neisseria gonorrhoeae confers resistance to a diverse array of antimicrobial agents by transporting these toxic compounds out of the gonococcus. Frequently in gonococcal strains, the expression of the mtrCDE operon is differentially regulated by both a repressor, MtrR, and an activator, MtrA. The mtrR gene lies 250 bp upstream of and is transcribed divergently from the mtrCDE operon. Previous research has shown that mutations in the mtrR coding region and in the mtrR-mtrCDE intergenic region increase levels of gonococcal antibiotic resistance and in vivo fitness. Recently, a C-to-T transition mutation 120 bp upstream of the mtrC start codon, termed mtr120, was identified in strain MS11 and shown to be sufficient to confer high levels of antimicrobial resistance when introduced into strain FA19. Here we report that this mutation results in a consensus -10 element and that its presence generates a novel promoter for mtrCDE transcription. This newly generated promoter was found to be stronger than the wild-type promoter and does not appear to be subject to MtrR repression or MtrA activation. Although rare, the mtr120 mutation was identified in an additional clinical isolate during sequence analysis of antibiotic-resistant strains cultured from patients with gonococcal infections. We propose that cis-acting mutations can develop in gonococci that significantly alter the regulation of the mtrCDE operon and result in increased resistance to antimicrobials. IMPORTANCE: Gonorrhea is the second most prevalent sexually transmitted bacterial infection and a worldwide public health concern. As there is currently no vaccine against Neisseria gonorrhoeae, appropriate diagnostics and subsequent antibiotic therapy remain the primary means of infection control. However, the effectiveness of antibiotic treatment is constantly challenged by the emergence of resistant strains, mandating a thorough understanding of resistance mechanisms to aid in the development of new antimicrobial therapies and genetic methods for antimicrobial resistance testing. This study was undertaken to characterize a novel mechanism of antibiotic resistance regulation in N. gonorrhoeae. Here we show that a single base pair mutation generates a second, stronger promoter for mtrCDE transcription that acts independently of the known efflux system regulators and results in high-level antimicrobial resistance.

Directed evolution of efficient secretion in the SRP-dependent export of TolB.

Signal sequence non-optimal codons have been shown to be important for the folding and efficient export of maltose binding protein (MBP), a SecB dependent protein. In this study, we analysed the importance of signal sequence non-optimal codons of TolB, a signal recognition particle (SRP) dependent exported protein. The protein production levels of wild type TolB (TolB-wt) and a mutant allele of TolB in which all signal sequence non-optimal codons were changed to a synonymous optimal codon (TolB-opt), revealed that TolB-opt production was 12-fold lower than TolB-wt. This difference could not be explained by changes in mRNA levels, or plasmid copy number, which was the same in both strains. A directed evolution genetic screen was used to select for mutants in the TolB-opt signal sequence that resulted in higher levels of TolB production. Analysis of the 46 independent TolB mutants that reverted to wild type levels of expression revealed that at least four signal sequence non-optimal codons were required. These results suggest that non-optimal codons may be required for the folding and efficient export of all proteins exported via the Sec system, regardless of whether they are dependent on SecB or SRP for delivery to the inner membrane.

Coupling between codon usage, translation and protein export in Escherichia coli.

Proteins destined for export via the Sec-dependent pathway are synthesized with a short N-terminal signal peptide. A requirement for export is that the proteins are in a translocationally competent state. This is a loosely folded state that allows the protein to pass through the SecYEG apparatus and pass into the periplasm. In order to maintain pre-secretory proteins in an export-competent state, there are many factors that slow the folding of the pre-secretory protein in the cytoplasm. These include cytoplasmic chaperones, such as SecB, and the signal recognition particle, which bind the pre-secretory protein and direct it to the cytoplasmic membrane for export. Recently, evidence has been published that non-optimal codons in the signal sequence are important for a time-critical early event to allow the correct folding of pre-secretory proteins. This review details the recent developments in folding of the signal peptide and the pre-secretory protein.

Signal sequence non-optimal codons are required for the correct folding of mature maltose binding protein.

Non-optimal codons are generally characterised by a low concentration of isoaccepting tRNA and a slower translation rate compared to optimal codons. In a previous study, we reported a 20-fold reduction in maltose binding protein (MBP) level when the non-optimal codons in the signal sequence were optimised. In this study, we report that the 20-fold reduction is rescued when MBP is expressed at 28 degrees C instead of 37 degrees C, suggesting that the signal sequence optimised MBP protein (MBP-opt) may be misfolded, and is being degraded at 37 degrees C. Consistent with this idea, transient induction of the heat shock proteases prior to MBP expression at 28 degrees C restores the 20-fold difference, demonstrating that the difference in production levels is due to post-translational degradation of MBP-opt by the heat-shock proteases. Analysis of the structure of purified MBP-wt and MBP-opt grown at 28 degrees C showed that although they have similar secondary structure content, MBP-opt is more resistant to thermal unfolding than is MBP-wt. The two proteins also exhibit different tryptic fragment profiles, further confirming that they are folded into conformationally different states. This is the first study to demonstrate that signal sequence non-optimal codons can influence the folding of the mature exported protein.

Biased codon usage in signal peptides: a role in protein export.

The signal peptide of proteins exported via the general secretory pathway encodes structural features that enable the targeting and export of the protein to the periplasm. Recent studies have shown biased codon usage at the second amino acid position and a high usage of non-optimal codons within the signal peptide. Altering these biases in codon usage can have deleterious effects on protein folding and export. We propose that these codon-usage biases act in concert to optimize the export process through modulating ribosome spacing on the transcript. This highlights a new aspect of protein export and implies that codon usage in the signal peptide encodes signals that are important for protein targeting and export to the periplasm.

Secretory signal sequence non-optimal codons are required for expression and export of beta-lactamase.

In this study we altered the codon usage in the signal sequence of the bla gene, encoding beta-lactamase in Escherichia coli. Changing all of the thirteen non-optimal codons to optimal lowered expression 4-fold as measured by minimum inhibitory concentration (MIC) to the beta-lactam antibiotic ampicillin. The difference in ampicillin resistance was reduced at 28 degrees C compared to expression at 37 degrees C, suggesting that the optimised bla allele is misfolded and degraded by heat-shock regulated proteases. A screen was carried out, designed specifically to identify revertants with changes in codon usage resulting in higher MIC to ampicillin. The nine revertants revealed by this method all had optimal to non-optimal codon changes in the signal sequence. These results, and those of our previous study with maltose binding protein model system, confirm that non-optimal codons are important for expression and export of secretory proteins via both the SecB-dependent and -independent pathways.

Selection for efficient translation initiation biases codon usage at second amino acid position in secretory proteins.

The definition of a typical sec-dependent bacterial signal peptide contains a positive charge at the N-terminus, thought to be required for membrane association. In this study the amino acid distribution of all Escherichia coli secretory proteins were analysed. This revealed that there was a statistically significant bias for lysine at the second codon position (P2), consistent with a role for the positive charge in secretion. Removal of the positively charged residue P2 in two different model systems revealed that a positive charge is not required for protein export. A well-characterized feature of large amino acids like lysine at P2 is inhibition of N-terminal methionine removal by methionyl amino-peptidase (MAP). Substitution of lysine at P2 for other large or small amino acids did not affect protein export. Analysis of codon usage revealed that there was a bias for the AAA lysine codon at P2, suggesting that a non-coding function for the AAA codon may be responsible for the strong bias for lysine at P2 of secretory signal sequences. We conclude that the selection for high translation initiation efficiency maybe the selective pressure that has led to codon and consequent amino acid usage at P2 of secretory proteins.

Experimental confirmation of a key role for non-optimal codons in protein export.

Non-optimal codons are defined by low usage and low abundance of corresponding tRNA, and have an established role in translational pausing to allow the correct folding of proteins. Our previous work reported a striking abundance of non-optimal codons in the signal sequences of secretory proteins exported via the sec-dependent pathway in Escherichia coli. In the current study the signal sequence of maltose-binding protein (MBP) was altered so that non-optimal codons were substituted with the most optimal codon from their synonymous codon family. The expression of MBP from the optimized allele (malE-opt) was significantly less than wild-type malE. Expression of MBP from malE-opt was partially restored in a range of cytoplasmic and periplasmic protease deficient strains, confirming that reduced expression of MBP in malE-opt was due to its preferential degradation by cytoplasmic and periplasmic proteases. These data confirm a novel role for non-optimal codon usage in secretion by slowing the rate of translation across the N-terminal signal sequence to facilitate proper folding of the secreted protein.

Genetic analysis of a plasmid encoding haemocin production in Haemophilus paragallinarum.

The full sequence of plasmid p250, isolated from Haemophilus paragallinarum strain HP250, has been obtained. The plasmid contains seven ORFs: a putative integrase, a putative replication protein (repB) and five ORFs similar to those from the haemocin (bacteriocin) hmcDCBAI operon from Haemophilus influenzae. Of 19 other non-plasmid-containing H. paragallinarum strains screened (11 serovar reference strains and 8 field isolates), 17 strains produced haemocin and were resistant to killing by strain HP250. These strains, unlike strain HP250, have a chromosomally encoded haemocin operon. A number of other members of the family Pasteurellaceae were tested for haemocin sensitivity. Pasteurella avium, Pasteurella volantium and Pasteurella species A, all non-pathogenic bacteria found in the respiratory tract of chickens suffering from respiratory diseases, were sensitive to H. paragallinarum haemocin. However, amongst the pathogenic Pasteurellaceae, 50 % of P. multocida isolates and all five isolates of Pasteurella haemolytica tested were sensitive to the haemocin. Given the prevalence of haemocin production in H. paragallinarum strains, it may play a role in aiding colonization by inhibiting other Gram-negative bacteria that are associated with the respiratory tract in chickens. The origin of replication from plasmid p250 has been used to generate an Escherichia coli-H. paragallinarum shuttle vector which may be useful in genetically manipulating H. paragallinarum.