National audit of non-melanoma skin cancer excisions performed by plastic surgery in the UK

A national, multi-centre audit of non-melanoma skin cancer excisions by plastic surgery.

Compensating for over-production inhibition of the Hsmar1 transposon in Escherichia coli using a series of constitutive promoters

Background

Transposable elements (TEs) are a diverse group of self-mobilizing DNA elements. Transposition has been exploited as a powerful tool for molecular biology and genomics. However, transposition is sometimes limited because of auto-regulatory mechanisms that presumably allow them to cohabit within their hosts without causing excessive genomic damage. The papillation assay provides a powerful visual screen for hyperactive transposases. Transposition is revealed by the activation of a promoter-less lacZ gene when the transposon integrates into a non-essential gene on the host chromosome. Transposition events are detected as small blue speckles, or papillae, on the white background of the main Escherichia coli colony.

Results

We analysed the parameters of the papillation assay including the strength of the transposase transcriptional and translational signals. To overcome certain limitations of inducible promoters, we constructed a set of vectors based on constitutive promoters of different strengths to widen the range of transposase expression. We characterized and validated our expression vectors with Hsmar1, a member of the mariner transposon family. The highest rate of transposition was observed with the weakest promoters. We then took advantage of our approach to investigate how the level of transposition responds to selected point mutations and the effect of joining the transposase monomers into a single-chain dimer.

Conclusions

We generated a set of vectors to provide a wide range of transposase expression which will be useful for screening libraries of transposase mutants. The use of weak promoters should allow screening for truly hyperactive transposases rather than those that are simply resistant to auto-regulatory mechanisms, such as overproduction inhibition (OPI). We also found that mutations in the Hsmar1 dimer interface provide resistance to OPI in bacteria, which could be valuable for improving bacterial transposon mutagenesis techniques.

Targeted DNA transposition in vitro using a dCas9-transposase fusion protein

Homology-directed genome engineering is limited by transgene size. Although DNA transposons are more efficient with large transgenes, random integrations are potentially mutagenic. Here we present an in vitro mechanistic study that demonstrates efficient Cas9 targeting of the mariner transposon Hsmar1. Integrations were unidirectional and tightly constrained to one side of the sgRNA binding site. Further analysis of the nucleoprotein intermediates demonstrated that the transposase and Cas9 moieties can bind their respective substrates independently or in concert. Kinetic analysis of the reaction in the presence of the Cas9 target-DNA revealed a delay between first and second strand cleavage at the transposon end. This step involves a significant conformational change that may be hindered by the properties of the interdomainal linker. Otherwise, the transposase moiety behaved normally and was proficient for integration in vitro and in Escherichia coli. Specific integration into the lacZ gene in E. coli was obscured by a high background of random integrations. Nevertheless, Cas9 is an attractive candidate for transposon-targeting because it has a high affinity and long dwell-time at its target site. This will facilitate a future optogenetic strategy for the temporal control of integration, which will increase the ratio of targeted to untargeted events.

Preoperative oral antibiotic bowel preparation in elective resectional colorectal surgery reduces rates of surgical site infections: a single-centre experience with a cost-effectiveness analysis

INTRODUCTION

Surgical site infections cause considerable postoperative morbidity and mortality. The aim of this study was to determine the effect on surgical site infection rates following introduction of a departmental oral antibiotic bowel preparation protocol.

METHODS

A prospective single-centre study was performed for elective colorectal resections between May 2016-April 2018; with a control group with mechanical bowel preparation and treatment group with oral antibiotic bowel preparation (neomycin and metronidazole) and mechanical bowel preparation. The primary outcome of surgical site infection and secondary outcomes of anastomotic leak, length of stay and mortality rate were analysed using Fisher's exact test and independent samples t-tests. A cost-effectiveness analysis was also performed.

RESULTS

A total of 311 patients were included; 156 in the mechanical bowel preparation group and 155 in the mechanical bowel preparation plus oral antibiotic bowel preparation group. The study included 180 (57.9%) men and 131 (42.1%) women with a mean age of 68 years. There was a significant reduction in surgical site infection rates (mechanical bowel preparation 16.0% vs mechanical bowel preparation plus oral antibiotic bowel preparation 4.5%; P = 0.001) and mean length of stay (mechanical bowel preparation 10.2 days vs mechanical bowel preparation plus oral antibiotic bowel preparation 8.2 days; P = 0.012). There was also a reduction in anastomotic leak and mortality rates. Subgroup analyses demonstrated significantly reduced surgical site infection rates in laparoscopic resections (P = 0.008). There was an estimated cost saving of £239.13 per patient and £37,065 for our institution over a one-year period.

CONCLUSION

Oral antibiotic bowel preparation is a feasible and cost-effective intervention shown to significantly reduce the rates of surgical site infection and length of stay in elective colorectal surgery.

Human SETMAR is a DNA sequence-specific histone-methylase with a broad effect on the transcriptome

Transposons impart dynamism to the genomes they inhabit and their movements frequently rewire the control of nearby genes. Occasionally, their proteins are domesticated when they evolve a new function. SETMAR is a protein methylase with a sequence-specific DNA binding domain. It began to evolve about 50 million years ago when an Hsmar1 transposon integrated downstream of a SET-domain methylase gene. Here we show that the DNA-binding domain of the transposase targets the enzyme to transposon-end remnants and that this is capable of regulating gene expression, dependent on the methylase activity. When SETMAR was modestly overexpressed in human cells, almost 1500 genes changed expression by more than 2-fold (65% up- and 35% down-regulated). These genes were enriched for the KEGG Pathways in Cancer and include several transcription factors important for development and differentiation. Expression of a similar level of a methylase-deficient SETMAR changed the expression of many fewer genes, 77% of which were down-regulated with no significant enrichment of KEGG Pathways. Our data is consistent with a model in which SETMAR is part of an anthropoid primate-specific regulatory network centered on the subset of genes containing a transposon end.

Transposase subunit architecture and its relationship to genome size and the rate of transposition in prokaryotes and eukaryotes

Cut-and-paste transposons are important tools for mutagenesis, gene-delivery and DNA sequencing applications. At the molecular level, the most thoroughly understood are Tn5 and Tn10 in bacteria, and mariner and hAT elements in eukaryotes. All bacterial cut-and-paste transposases characterized to date are monomeric prior to interacting with the transposon end, while all eukaryotic transposases are multimers. Although there is a limited sample size, we proposed that this defines two pathways for transpososome assembly which distinguishes the mechanism of the bacterial and eukaryotic transposons. We predicted that the respective pathways would dictate how the rate of transposition is related to transposase concentration and genome size. Here, we have tested these predictions by creating a single-chain dimer version of the bacterial Tn5 transposase. We show that artificial dimerization switches the transpososome assembly pathway from the bacterial-style to the eukaryotic-style. Although this had no effect in vitro, where the transposase does not have to search far to locate the transposon ends, it increased the rate of transposition in bacterial and HeLa cell assays. However, in contrast to the mariner elements, the Tn5 single-chain dimer remained unaffected by over-production inhibition, which is an emergent property of the transposase subunit structure in the mariner elements.

The roles of the human SETMAR (Metnase) protein in illegitimate DNA recombination and non-homologous end joining repair

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Full length SETMAR expression has no effect on DNA repair and integration in vivo.

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SETMAR putative nuclease activity is not required in vivo.

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Separate expression of the SET and MAR domains affects DNA repair and integration.

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SETMAR isoform with a truncated SET-domain is specific to species containing the MAR domain.

SETMAR is a fusion between a SET-domain methyltransferase gene and a mariner-family transposase gene, which is specific to anthropoid primates. However, the ancestral SET gene is present in all other mammals and birds. SETMAR is reported to be involved in transcriptional regulation and a diverse set of reactions related to DNA repair. Since the transcriptional effects of SETMAR depend on site-specific DNA binding, and are perturbed by inactivating the methyltransferase, we wondered whether we could differentiate the effects of the SET and MAR domains in DNA repair assays. We therefore generated several stable U2OS cell lines expressing either wild type SETMAR or truncation or point mutant variants. We tested these cell lines with in vivo plasmid-based assays to determine the relevance of the different domains and activities of SETMAR in DNA repair. Contrary to previous reports, we found that wild type SETMAR had little to no effect on the rate of cell division, DNA integration into the genome or non-homologous end joining. Also contrary to previous reports, we failed to detect any effect of a strong active-site mutation that should have knocked out the putative nuclease activity of SETMAR.

A case of hepato-biliary infection secondary to cryptosporidium in a patient on tacrolimus

Introduction.

Cryptosporidium infection is known to cause hepato-biliary involvement, mainly in association with T-cell immune deficiency. Hepato-biliary involvement in association with milder immunosuppression is less well described. We describe the first case, to our knowledge, of Cryptosporidium hominis hepato-biliary infection associated with tacrolimus in a patient with nephrotic syndrome.

Case presentation.

A 14 year old girl who had been on tacrolimus for nephrotic syndrome presented with diarrhea due to C. hominis. Nineteen days after her initial presentation she attended hospital with abdominal pain and deranged liver function tests. An ultrasound scan showed a thickened gall bladder. Her symptoms settled and her liver function tests returned to normal after treatment with nitazoxanide.

Conclusion.

Cryptosporidium should be considered in the differential diagnosis of both diarrhea and hepato-biliary symptoms and abnormal liver function tests, even in the presence of relatively mild immunosuppression. Nitazoxanide was an effective treatment in this case.

A single active site in the mariner transposase cleaves DNA strands of opposite polarity

The RNase H structural fold defines a large family of nucleic acid metabolizing enzymes that catalyze phosphoryl transfer reactions using two divalent metal ions in the active site. Almost all of these reactions involve only one strand of the nucleic acid substrates. In contrast, cut-and-paste transposases cleave two DNA strands of opposite polarity, which is usually achieved via an elegant hairpin mechanism. In the mariner transposons, the hairpin intermediate is absent and key aspects of the mechanism by which the transposon ends are cleaved remained unknown. Here, we characterize complexes involved prior to catalysis, which define an asymmetric pathway for transpososome assembly. Using mixtures of wild-type and catalytically inactive transposases, we show that all the catalytic steps of transposition occur within the context of a dimeric transpososome. Crucially, we find that each active site of a transposase dimer is responsible for two hydrolysis and one transesterification reaction at the same transposon end. These results provide the first strong evidence that a DDE/D active site can hydrolyze DNA strands of opposite polarity, a mechanism that has rarely been observed with any type of nuclease.

The Invertebrate Lysozyme Effector ILYS-3 Is Systemically Activated in Response to Danger Signals and Confers Antimicrobial Protection in C. elegans

Little is known about the relative contributions and importance of antibacterial effectors in the nematode C. elegans, despite extensive work on the innate immune responses in this organism. We report an investigation of the expression, function and regulation of the six ilys (invertebrate-type lysozyme) genes of C. elegans. These genes exhibited a surprising variety of tissue-specific expression patterns and responses to starvation or bacterial infection. The most strongly expressed, ilys-3, was investigated in detail. ILYS-3 protein was expressed constitutively in the pharynx and coelomocytes, and dynamically in the intestine. Analysis of mutants showed that ILYS-3 was required for pharyngeal grinding (disruption of bacterial cells) during normal growth and consequently it contributes to longevity, as well as being protective against bacterial pathogens. Both starvation and challenge with Gram-positive pathogens resulted in ERK-MAPK-dependent up-regulation of ilys-3 in the intestine. The intestinal induction by pathogens, but not starvation, was found to be dependent on MPK-1 activity in the pharynx rather than in the intestine, demonstrating unexpected communication between these two tissues. The coelomocyte expression appeared to contribute little to normal growth or immunity. Recombinant ILYS-3 protein was found to exhibit appropriate lytic activity against Gram-positive cell wall material.

Innate immune defenses against bacterial pathogenesis depend on the activation of antibacterial factors. We examined the expression and relative importance of a gene family encoding six invertebrate-type lysozymes in the much-studied nematode C. elegans. The ilys genes exhibit distinct patterns of tissue-specific expression and response to pathogenic challenge and/or starvation. The most abundantly expressed, ilys-3, exhibits constitutive pharyngeal expression, which we show is essential for efficient disruption of bacteria under non-pathogenic growth conditions, and consequently it contributes to normal longevity. ilys-3 is also strongly up-regulated in intestinal cells after starvation or exposure to Gram-positive pathogens such as Microbacterium nematophilum and acts as a ‘slow-effector’ in limiting pathogenic damage from intestinal infections. We show that this induction by pathogens depends on the action of an ERK-MAPK cascade, which acts in pharyngeal rather than intestinal cells; this implies communication between pharynx and intestine. Tagged ILYS-3 protein was detected mainly in recycling endosomes of intestinal cells and in the intestinal lumen after starvation. ILYS-3 was also expressed in coelomocytes (scavenger cells) but we found that these cells make little or no contribution to defense. We examined the enzymatic properties of recombinant ILYS-3 protein, finding that it has lytic activity against M. nematophilum cell-walls.

Crosstalk between transposase subunits during cleavage of the mariner transposon

Mariner transposition is a complex reaction that involves three recombination sites and six strand breaking and joining reactions. This requires precise spatial and temporal coordination between the different components to ensure a productive outcome and minimize genomic instability. We have investigated how the cleavage events are orchestrated within the mariner transpososome. We find that cleavage of the non-transferred strand is completed at both transposon ends before the transferred strand is cleaved at either end. By introducing transposon-end mutations that interfere with cleavage, but leave transpososome assembly unaffected, we demonstrate that a structural transition preceding transferred strand cleavage is coordinated between the two halves of the transpososome. Since mariner lacks the DNA hairpin intermediate, this transition probably reflects a reorganization of the transpososome to allow the access of different monomers onto the second pair of strands, or the relocation of the DNA within the same active site between two successive hydrolysis events. Communication between transposase subunits also provides a failsafe mechanism that restricts the generation of potentially deleterious double-strand breaks at isolated sites. Finally, we identify transposase mutants that reveal that the conserved WVPHEL motif provides a structural determinant of the coordination mechanism.

One to rule them all: A highly conserved motif in mariner transposase controls multiple steps of transposition

The development of transposon-based genome manipulation tools can benefit greatly from understanding transposons’ inherent regulatory mechanisms. The Tc1-mariner transposons, which are being widely used in biotechnological applications, are subject to a self-inhibitory mechanism whereby increasing transposase expression beyond a certain point decreases the rate of transposition. In a recent paper, Liu and Chalmers performed saturating mutagenesis on the highly conserved WVPHEL motif in the mariner-family transposase from the Hsmar1 element. Curiously, they found that the majority of all possible single mutations were hyperactive. Biochemical characterizations of the mutants revealed that the hyperactivity is due to a defect in communication between transposase subunits, which normally regulates transposition by reducing the rate of synapsis. This provides important clues for improving transposon-based tools. However, some WVPHEL mutants also showed features that would be undesirable for most biotechnological applications: they showed uncontrolled DNA cleavage activities and defects in the coordination of cleavage between the two transposon ends. The study illustrates how the knowledge of inhibitory mechanisms can help improve transposon tools but also highlights an important challenge, which is to specifically target a regulatory mechanism without affecting other important functions of the transposase.

Hyperactive mariner transposons are created by mutations that disrupt allosterism and increase the rate of transposon end synapsis

New applications for transposons in vertebrate genetics have spurred efforts to develop hyperactive variants. Typically, a genetic screen is used to identify several hyperactive point mutations, which are then incorporated in a single transposase gene. However, the mechanisms responsible for the increased activity are unknown. Here we show that several point mutations in the mariner transposase increase their activities by disrupting the allostery that normally serves to downregulate transposition by slowing synapsis of the transposon ends. We focused on the conserved WVPHEL amino acid motif, which forms part of the mariner transposase dimer interface. We generated almost all possible single substitutions of the W, V, E and L residues and found that the majority are hyperactive. Biochemical analysis of the mutations revealed that they disrupt signals that pass between opposite sides of the developing transpososome in response to transposon end binding. In addition to their role in allostery, the signals control the initiation of catalysis, thereby preventing non-productive double-strand breaks. Finally, we note that such breaks may explain the puzzling ‘self-inflicted wounds’ at the ends of the Mos1 transposon in Drosophila.

Accuracy and efficiency define Bxb1 integrase as the best of fifteen candidate serine recombinases for the integration of DNA into the human genome

Background

Phage-encoded serine integrases, such as φC31 integrase, are widely used for genome engineering. Fifteen such integrases have been described but their utility for genome engineering has not been compared in uniform assays.

Results

We have compared fifteen serine integrases for their utility for DNA manipulations in mammalian cells after first demonstrating that all were functional in E. coli. Chromosomal recombination reporters were used to show that seven integrases were active on chromosomally integrated DNA in human fibroblasts and mouse embryonic stem cells. Five of the remaining eight enzymes were active on extra-chromosomal substrates thereby demonstrating that the ability to mediate extra-chromosomal recombination is no guide to ability to mediate site-specific recombination on integrated DNA. All the integrases that were active on integrated DNA also promoted DNA integration reactions that were not mediated through conservative site-specific recombination or damaged the recombination sites but the extent of these aberrant reactions varied over at least an order of magnitude. Bxb1 integrase yielded approximately two-fold more recombinants and displayed about two fold less damage to the recombination sites than the next best recombinase; φC31 integrase.

Conclusions

We conclude that the Bxb1 and φC31 integrases are the reagents of choice for genome engineering in vertebrate cells and that DNA damage repair is a major limitation upon the utility of this class of site-specific recombinase.

Temperature triggers immune evasion by Neisseria meningitidis

Neisseria meningitidis has multiple strategies to evade complement-mediated killing, which contribute to its ability to cause septicaemic disease and meningitis. However, the meningococcus is primarily an obligate commensal of the human nasopharynx, and it is unclear why the bacterium has evolved exquisite mechanisms to avoid host immunity. Here we demonstrate that mechanisms of meningococcal immune evasion and resistance against complement increase in response to an elevation in ambient temperature. We have identified three independent RNA thermosensors located in the 5′-UTRs of genes necessary for capsule biosynthesis, the expression of factor H binding protein, and sialylation of lipopolysaccharide, which are essential for meningococcal resistance against immune killing^1,2. Therefore increased temperature (which occurs during inflammation) acts as a ‘danger signal’ for the meningococcus which enhances defence against human immune killing. Infection with viral pathogens, such as influenza, leads to inflammation in the nasopharynx with an elevated temperature and recruitment of immune effectors^3,4. Thermoregulation of immune defence could offer an adaptive advantage to the meningococcus during co-infection with other pathogens, and promote the emergence of virulence in an otherwise commensal bacterium.

The autoregulation of a eukaryotic DNA transposon

How do DNA transposons live in harmony with their hosts? Bacteria provide the only documented mechanisms for autoregulation, but these are incompatible with eukaryotic cell biology. Here we show that autoregulation of Hsmar1 operates during assembly of the transpososome and arises from the multimeric state of the transposase, mediated by a competition for binding sites. We explore the dynamics of a genomic invasion using a computer model, supported by in vitro and in vivo experiments, and show that amplification accelerates at first but then achieves a constant rate. The rate is proportional to the genome size and inversely proportional to transposase expression and its affinity for the transposon ends. Mariner transposons may therefore resist post-transcriptional silencing. Because regulation is an emergent property of the reaction it is resistant to selfish exploitation. The behavior of distantly related eukaryotic transposons is consistent with the same mechanism, which may therefore be widely applicable.

DOI:http://dx.doi.org/10.7554/eLife.00668.001

Transposons are regions of mobile DNA that can jump from one location in the genome to another. This represents a genetic burden to the host because there is always the risk that the transposon will inactivate a cellular gene. However, a greater problem is that transposition is accompanied by an increase in the number of copies of the transposon. Since each new copy will be a source of further new copies, amplification of transposons is necessarily exponential. The fact that eukaryotic cells are able to tolerate DNA transposons suggests the existence of regulatory mechanisms to defuse the inevitable genomic melt-down. Host-mediated epigenetic modifications and RNA interference will provide some level of protection. However, they are by no means completely effective and a well-adapted genomic parasite, such as a transposon, might be expected to have its own mechanism of regulation.

Now, Claeys Bouuaert, Lipkow and colleagues have used a computer model in combination with in vivo and in vitro experiments to search for this mechanism. Their experiments reveal how a DNA transposon is down-regulated by its own transposase. The transposase is the enzyme that catalyzes the ‘jump’ or transposition. It binds to specific sites at either end of the transposon and brings these together to make up a nucleoprotein complex called the transpososome. It is within this complex that the chemical steps of the reaction take place. When the number of transposons increases, so does the concentration of transposase. Claeys Bouuaert et al. show that the binding sites become saturated at a relatively low transposase concentration and that negative regulation arises from the resulting competition. Thus, the rate of transposition decreases as the number of transposons increases. They further use the computer model to explore how the amplification of the transposon is affected by transposon-specific and cellular-specific factors.

Claeys Bouuaert, Lipkow and colleagues based their study predominantly on a resurrected copy of the Hsmar1 transposon, which was active in the human genome 50 million years ago. However, they also tested two distantly related eukaryotic transposons and observed that their behavior was similar, which suggests that this could be a general mechanism that controls the activity of jumping genes. They also note that their competition mechanism is conceptually similar to the immunological ‘prozone effect’. This is a recurrent theme in protein chemistry and demonstrates once again that less is in fact sometimes more.

DOI:http://dx.doi.org/10.7554/eLife.00668.002

Hsmar1 transposition is sensitive to the topology of the transposon donor and the target

Hsmar1 is a member of the Tc1-mariner superfamily of DNA transposons. These elements mobilize within the genome of their host by a cut-and-paste mechanism. We have exploited the in vitro reaction provided by Hsmar1 to investigate the effect of DNA supercoiling on transposon integration. We found that the topology of both the transposon and the target affect integration. Relaxed transposons have an integration defect that can be partially restored in the presence of elevated levels of negatively supercoiled target DNA. Negatively supercoiled DNA is a better target than nicked or positively supercoiled DNA, suggesting that underwinding of the DNA helix promotes target interactions. Like other Tc1-mariner elements, Hsmar1 integrates into 5′-TA dinucleotides. The direct vicinity of the target TA provides little sequence specificity for target interactions. However, transposition within a plasmid substrate was not random and some TA dinucleotides were targeted preferentially. The distribution of intramolecular target sites was not affected by DNA topology.

Etanercept, infliximab and adalimumab for the treatment of psoriatic arthritis: a systematic review and economic evaluation

BACKGROUND

Etanercept, infliximab and adalimumab are licensed in the UK for the treatment of active and progressive psoriatic arthritis (PsA) in adults who have an inadequate response to standard treatment.

OBJECTIVE

To determine the clinical effectiveness, safety and cost-effectiveness of these biologic agents in the treatment of active and progressive PsA.

DATA SOURCES

Systematic reviews were performed, with data sought from 10 electronic databases (MEDLINE, EMBASE, Cochrane Central Register of Controlled Trials, Science Citation Index, Conference Proceedings Citation Index - Science, ClinicalTrials.gov, metaRegister of Current Controlled Trials, NHS Economic Evaluation Database, Health Economic Evaluations Database and EconLit) up to June 2009.

REVIEW METHODS

Full paper manuscripts of titles/abstracts considered relevant were obtained and assessed for inclusion by two reviewers according to criteria on study design, interventions, participants and outcomes. Data on study and participant characteristics, efficacy outcomes, adverse effects, costs to the health service and cost-effectiveness were extracted, along with baseline data where reported. The primary efficacy outcomes were measures of anti-inflammatory response, skin lesion response and functional status, and the safety outcome was the incidence of serious adverse events. The primary measure of cost-effectiveness was incremental cost per additional quality-adjusted life-year (QALY). Standard meta-analytic techniques were applied to efficacy data. Published cost-effectiveness studies and the economic analyses submitted to the National Institute for Health and Clinical Excellence (NICE) by the biologic manufacturers were reviewed. An economic model was developed by updating the model produced by the York Assessment Group for the previous NICE appraisal of biologics in PsA.

RESULTS

Pooled estimates of effect demonstrated a significant improvement in patients with PsA for all joint disease and functional status outcomes at 12-14 weeks' follow-up. The biologic treatment significantly reduced joint symptoms for etanercept [relative risk (RR) 2.60, 95% confidence interval (CI) 1.96 to 3.45], infliximab (RR 3.44, 95% CI 2.53 to 4.69) and adalimumab (RR 2.24, 95% CI 1.74 to 2.88), with 24-week data demonstrating maintained treatment effects. Trial data demonstrated a significant effect of all three biologics on skin disease at 12 or 24 weeks. Evidence synthesis found that infliximab appeared to be most effective across all outcomes of joint and skin disease. The response in joint disease was greater with etanercept than with adalimumab, whereas the response in skin disease was greater with adalimumab than with etanercept, although these differences are not statistically significant. Under base-case assumptions, etanercept was the most likely cost-effective strategy for patients with PsA and mild-to-moderate psoriasis if the threshold for cost-effectiveness was £20,000 or £30,000 per QALY. All biologics had a similar probability of being cost-effective for patients with PsA and moderate-to-severe psoriasis at a threshold of £20,000 per QALY.

LIMITATIONS

Limited available efficacy data and difficulty in assessing PsA activity and its response to biologic therapy.

CONCLUSIONS

The data indicated that etanercept, infliximab and adalimumab were efficacious in the treatment of PsA compared with placebo, with beneficial effects on joint symptoms, functional status and skin. Short-term data suggested that these biologic agents can delay joint disease progression and evidence to support their use in the treatment of PsA is convincing. Future research would benefit from long-term observational studies with large sample sizes of patients with PsA to demonstrate that beneficial effects are maintained, along with further monitoring of the safety profiles of the biologic agents.

FUNDING

The National Institute for Health Research Health Technology Assessment programme.

The transposon-like Correia elements encode numerous strong promoters and provide a potential new mechanism for phase variation in the meningococcus

Neisseria meningitidis is the primary causative agent of bacterial meningitis. The genome is rich in repetitive DNA and almost 2% is occupied by a diminutive transposon called the Correia element. Here we report a bioinformatic analysis defining eight subtypes of the element with four distinct types of ends. Transcriptional analysis, using PCR and a lacZ reporter system, revealed that two ends in particular encode strong promoters. The activity of the strongest promoter is dictated by a recurrent polymorphism (Y128) at the right end of the element. We highlight examples of elements that appear to drive transcription of adjacent genes and others that may express small non-coding RNAs. Pair-wise comparisons between three meningococcal genomes revealed that no more than two-thirds of Correia elements maintain their subtype at any particular locus. This is due to recombinational class switching between elements in a single strain. Upon switching subtype, a new allele is available to spread through the population by natural transformation. This process may represent a hitherto unrecognized mechanism for phase variation in the meningococcus. We conclude that the strain-to-strain variability of the Correia elements, and the large number of strong promoters encoded by them, allows for potentially widespread effects within the population as a whole. By defining the strength of the promoters encoded by the eight subtypes of Correia ends, we provide a resource that allows the transcriptional effects of a particular subtype at a given locus to be predicted.

Delivering the goods: viral and non-viral gene therapy systems and the inherent limits on cargo DNA and internal sequences

Viruses have long been considered to be the most promising tools for human gene therapy. However, the initial enthusiasm for the use of viruses has been tarnished in the light of potentially fatal side effects. Transposons have a long history of use with bacteria in the laboratory and are now routinely applied to eukaryotic model organisms. Transposons show promise for applications in human genetic modification and should prove a useful addition to the gene therapy tool kit. Here we review the use of viruses and the limitations of current approaches to gene therapy, followed by a more detailed analysis of transposon length and the physical properties of internal sequences, which both affect transposition efficiency. As transposon length increases, transposition decreases: this phenomenon is known as length-dependence, and has implications for vector cargo capacity. Disruption of internal sequences, either via deletion of native DNA or insertion of exogenous DNA, may reduce or enhance genetic mobility. These effects may be related to host factor binding, essential spacer requirements or other influences yet to be elucidated. Length-dependence is a complex phenomenon driven not simply by the distance between the transposon ends, but by host proteins, the transposase and the properties of the DNA sequences encoded within the transposon.

Transposition of the human Hsmar1 transposon: rate-limiting steps and the importance of the flanking TA dinucleotide in second strand cleavage

Hsmar1 is a member of the mariner family of DNA transposons. Although widespread in nature, their molecular mechanism remains obscure. Many other cut-and-paste elements use a hairpin intermediate to cleave the two strands of DNA at each transposon end. However, this intermediate is absent in mariner, suggesting that these elements use a fundamentally different mechanism for second-strand cleavage. We have taken advantage of the faithful and efficient in vitro reaction provided by Hsmar1 to characterize the products and intermediates of transposition. We report different factors that particularly affect the reaction, which are the reaction pH and the transposase concentration. Kinetic analysis revealed that first-strand nicking and integration are rapid. The rate of the reaction is limited in part by the divalent metal ion-dependent assembly of a complex between transposase and the transposon end(s) prior to the first catalytic step. Second-strand cleavage is the rate-limiting catalytic step of the reaction. We discuss our data in light of a model for the two metal ion catalytic mechanism and propose that mariner excision involves a significant conformational change between first- and second-strand cleavage at each transposon end. Furthermore, this conformational change requires specific contacts between transposase and the flanking TA dinucleotide.

Base flipping in tn10 transposition: an active flip and capture mechanism

The bacterial Tn5 and Tn10 transposases have a single active site that cuts both strands of DNA at their respective transposon ends. This is achieved using a hairpin intermediate that requires the DNA to change conformation during the reaction. In Tn5 these changes are controlled in part by a flipped nucleoside that is stacked on a tryptophan residue in a hydrophobic pocket of the transposase. Here we have investigated the base flipping mechanism in Tn10 transposition. As in Tn5 transposition, we find that base flipping takes place after the first nick and is required for efficient hairpin formation and resolution. Experiments with an abasic substrate show that the role of base flipping in hairpin formation is to remove the base from the DNA helix. Specific interactions between the flipped base and the stacking tryptophan residue are required for hairpin resolution later in the reaction. We show that base flipping in Tn10 transposition is not a passive reaction in which a spontaneously flipped base is captured and retained by the protein. Rather, it is driven in part by a methionine probe residue that helps to force the flipped base from the base stack. Overall, it appears that base flipping in Tn10 transposition is similar to that in Tn5 transposition.

IDEA (Intellectual Disability Exploring Answers): A population-based database for intellectual disability in Western Australia

Despite the demands it places on individuals, families and the community, intellectual disability (ID) is a neglected area of public health. Accurate estimates of prevalence are sparse and range from 0.5 to 3.0%. The cause of the condition is unknown in at least 50% of cases. This paper describes the Intellectual Disability Exploring Answers (IDEA) database set up in Western Australia to provide an infrastructure for research and to facilitate the planning of service provision for people with ID. Since 1953 a database for ID has been maintained in Western Australia, a state with a population of 1.95 million in an area of 2.52 million km2. The current IDEA database aims to obtain ongoing population-based ascertainment of ID from providers of clinical and educational services, with the potential for linkage to a network of other state databases. The average prevalence of ID for children born in Western Australia over the years 1983-1996 was 15.2 per 1000 live births, with 50% ascertained only through the education system. During this time period 60% of cases were male. Of children with an ID born in Western Australia in 1980-1999 and surviving to 1 year, 30.1% had a birth defect, and the prevalence ratio of birth defects in this group compared to the population with no birth defects was 6.5 (CI 6.3-6.8).

A swimming pool-associated outbreak of cryptosporidiosis in Staffordshire, England, October to December 2007

In October 2007 an increase in laboratory-confirmed cryptosporidiosis cases in Staffordshire, England prompted an outbreak investigation. Case ascertainment included interviewing suspected cases and contacts and obtaining faecal specimens from those with diarrhoea for laboratory identification. Over a three-month period we identified 57 cases of cryptosporidiosis (39 confirmed) distributed across 36 households. The majority of cases (69%) were younger than 20 years. The most plausible exposure was multiple swimming episodes (56% of cases) in 13 local public swimming pools. One large swimming pool was most frequently visited by swimmers and considered a significant contributor to transmission because of substandard filtration and maintenance systems. Control measures focused on inspecting and improving operating standards at swimming pools, hygiene information to swimmers, and early detection and exclusion of cases. The rapid case investigation described in this paper provided adequate information for the early detection and control of a typical seasonal swimming pool related cryptosporidiosis outbreak. Ensuring adequate filtration standards at public swimming pools particularly before the high use periods of late summer and autumn remains a priority.

A swimming pool-associated outbreak of cryptosporidiosis in Staffordshire, England, October to December 2007

In October 2007 an increase in laboratory-confirmed cryptosporidiosis cases in Staffordshire, England prompted an outbreak investigation. Case ascertainment included interviewing suspected cases and contacts and obtaining faecal specimens from those with diarrhoea for laboratory identification. Over a three-month period we identified 57 cases of cryptosporidiosis (39 confirmed) distributed across 36 households. The majority of cases (69%) were younger than 20 years. The most plausible exposure was multiple swimming episodes (56% of cases) in 13 local public swimming pools. One large swimming pool was most frequently visited by swimmers and considered a significant contributor to transmission because of substandard filtration and maintenance systems. Control measures focused on inspecting and improving operating standards at swimming pools, hygiene information to swimmers, and early detection and exclusion of cases. The rapid case investigation described in this paper provided adequate information for the early detection and control of a typical seasonal swimming pool related cryptosporidiosis outbreak. Ensuring adequate filtration standards at public swimming pools particularly before the high use periods of late summer and autumn remains a priority.

Outbreak of cryptosporidiosis among veterinary students

In January 2007, six veterinary students became infected with Cryptosporidium species, and records indicated that another student had been diagnosed in November 2006. It was established that the seven students had worked with cattle from the same farm. Microbiological tests indicated that they were infected with Cryptosporidium parvum. Subtyping by sequence analysis indicated a common source of infection for five of the students, but there was insufficient material to type the other two samples. Investigations indicated that the outbreak was caused by a lapse in hygiene, particularly handwashing, on a farm with enzootic C parvum in calves.

Fetal cardiac diagnosis and its influence on the pregnancy and newborn--a tertiary centre experience

OBJECTIVES

To evaluate the impact of an abnormal fetal cardiac scan on the management of the pregnancy and the outcome of the newborn.

METHODS

We reviewed all pregnancies that were referred to the Fetal Cardiac Unit for assessment to determine if the finding of a cardiac abnormality influenced the pregnancy and fetus, timing and mode of delivery, the treatment and outcome of the newborn. Diagnoses were confirmed by echocardiography following the baby's delivery.

RESULTS

Between January 2005 and July 2006, there were 251 detailed fetal cardiac scans carried out on at risk pregnancies or those with suspected abnormal scans in 127 fetuses. Seven of the 92 mothers with abnormal fetal cardiac scans opted for termination. Two were successfully treated during the pregnancy for hydrops fetalis arising from a tachyarrhythmia. One was induced early because of deterioration of fetal well-being and increasing cardiac size. Twenty-six infants required a prostaglandin infusion prior to surgery. Two required intensive care for associated malformations. There were 24 survivors following complex surgery, and 2 deaths. Two infants with severe tricuspid valve incompetence from a dysplastic valve died, one associated with a septicaemia and the other where surgery was delayed because of prematurity and low birth weight. There was no maternal morbidity or mortality.

CONCLUSIONS

Early detection of fetal cardiac malformation allows for careful counselling of the parents, ongoing antenatal review with a planned site and timing of delivery, and anticipatory postnatal care for optimum outcomes. The importance of careful screening is emphasized to allow for referral of mothers with potentially abnormal scans to an appropriate tertiary centre for confirmation and management.

Outbreak of Cryptosporidium parvum among children after a school excursion to an adventure farm, south west England

We present the results of an outbreak investigation on a Cryptosporidium parvum outbreak among 35 people, (27 students and 8 teachers), who participated in a school excursion to an outdoor adventure farm in South West England, from 22 to 26 May 2006. A cohort study was implemented to investigate possible sources of infection during the farm visit. The most likely transmission route was contact with faecally contaminated surface water following heavy rainfall, or consumption of water from the private well. Disinfection of the water reservoir was by chlorination, to which cryptosporidium is resistant. Supplemental disinfection or filtration methods of private water supplies on livestock farms may be needed. This study highlights the fact that epidemiological investigations of outbreaks as a result of environmental exposures are complex but important to inform the public and health professionals of the risks posed by private water supplies and outdoor activities. This is particularly so after heavy rainfall, as this may result in an increased effluent from faecally contaminated land, causing a wide variety of pathogens to wash into surface water and potentially, private wells. This poses risks for public health.

Transposition of Mboumar-9: identification of a new naturally active mariner-family transposon

Although mariner transposons are widespread in animal genomes, the vast majority harbor multiple inactivating mutations and only two naturally occurring elements are known to be active. Previously, we discovered a mariner-family transposon, Mboumar, in the satellite DNA of the ant Messor bouvieri. Several copies of the transposon contain a full-length open reading frame, including Mboumar-9, which has 64% nucleotide identity to Mos1 of Drosophila mauritiana. To determine whether Mboumar is currently active, we expressed and purified the Mboumar-9 transposase and demonstrate that it is able to catalyze the movement of a transposon from one plasmid to another in a genetic in vitro hop assay. The efficiency is comparable to that of the well-characterized mariner transposon Mos1. Transposon insertions were precise and were flanked by TA duplications, a hallmark of mariner transposition. Mboumar has been proposed to have a role in the evolution and maintenance of satellite DNA in M. bouvieri and its activity provides a means to examine the involvement of the transposon in the genome dynamics of this organism.

Functional coupling between the two active sites during Tn 10 transposition buffers the mutation of sequences critical for DNA hairpin processing

DNA processing reactions often involve multiple components acting in concert to achieve the desired outcome. However, it is usually difficult to know how the components communicate and cooperate to orchestrate an ordered series of events. We address this question in the context of the Tn 10 transposition reaction, in which the DNA cleavage and joining events occur within a higher-order complex containing a transposase dimer, two transposon ends and the DNA-bending host-factor IHF (Integration Host Factor). Previously it was shown that the complex is asymmetric. The a side consists of an IHF protomer initially immobilized by a DNA-loop, but subsequently used to promote conformational changes required for the cleavage steps. The beta side of the complex was considered to fulfil a more passive role. Here we show that the a side of the complex promotes coupled conformational changes at both transposon ends, while the a and beta sides communicate and cooperate to dominate different phases of the transposition reaction. Together, these effects provide for a robust response to critical changes in the transposon end. These findings also explain the intriguing genetic phenotypes of a series of previously reported Tn10 mutants and have consequences for the evolution of new elements.

Base-flipping dynamics in a DNA hairpin processing reaction

Many enzymes that repair or modify bases in double-stranded DNA gain access to their substrates by base flipping. Although crystal structures provide stunning snap shots, biochemical approaches addressing the dynamics have proven difficult, particularly in complicated multi-step reactions. Here, we use protein-DNA crosslinking and potassium permanganate reactivity to explore the base-flipping step in Tn5 transposition. We present a model to suggest that base flipping is driven by a combination of factors including DNA bending and the intrusion of a probe residue. The forces are postulated to act early in the reaction to create a state of tension, relieved by base flipping after cleavage of the first strand of DNA at the transposon end. Elimination of the probe residue retards the kinetics of nicking and reduces base flipping by 50%. Unexpectedly, the probe residue is even more important during the hairpin resolution step. Overall, base flipping is pivotal to the hairpin processing reaction because it performs two opposite but closely related functions. On one hand it disrupts the double helix, providing the necessary strand separation and steric freedom. While on the other, transposase appears to position the second DNA strand in the active site for cleavage using the flipped base as a handle.

The human SETMAR protein preserves most of the activities of the ancestral Hsmar1 transposase

Transposons have contributed protein coding sequences to a unexpectedly large number of human genes. Except for the V(D)J recombinase and telomerase, all remain of unknown function. Here we investigate the activity of the human SETMAR protein, a highly expressed fusion between a histone H3 methylase and a mariner family transposase. Although SETMAR has demonstrated methylase activity and a DNA repair phenotype, its mode of action and the role of the transposase domain remain obscure. As a starting point to address this problem, we have dissected the activity of the transposase domain in the context of the full-length protein and the isolated transposase domain. Complete transposition of an engineered Hsmar1 transposon by the transposase domain was detected, although the extent of the reaction was limited by a severe defect for cleavage at the 3' ends of the element. Despite this problem, SETMAR retains robust activity for the other stages of the Hsmar1 transposition reaction, namely, site-specific DNA binding to the transposon ends, assembly of a paired-ends complex, cleavage of the 5' end of the element in Mn(2+), and integration at a TA dinucleotide target site. SETMAR is unlikely to catalyze transposition in the human genome, although the nicking activity may have a role in the DNA repair phenotype. The key activity for the mariner domain is therefore the robust DNA-binding and looping activity which has a high potential for targeting the histone methylase domain to the many thousands of specific binding sites in the human genome provided by copies of the Hsmar1 transposon.

Etanercept and efalizumab for the treatment of psoriasis: a systematic review

OBJECTIVES

To evaluate the clinical effectiveness, safety, tolerability and cost-effectiveness of etanercept and efalizumab for the treatment of moderate to severe chronic plaque psoriasis.

DATA SOURCES

Major electronic databases and several Internet resources were searched up to April 2004.

REVIEW METHODS

Systematic reviews were undertaken of the efficacy, safety and economic reviews of etanercept and efalizumab. An existing systematic review of the efficacy and safety of other treatments was also updated. Economic models supplied by the manufacturers of etanercept and efalizumab were critiqued. An economic model was then developed of etanercept and efalizumab in the treatment of moderate to severe chronic plaque psoriasis.

RESULTS

The review of the clinical evidence identified a total of 39 published and three unpublished studies: eight randomised controlled trials (RCTs) of the efficacy of etanercept (three trials) and efalizumab (five); 10 studies of the adverse effects of the interventions; and 24 RCTs of the efficacy of the other treatments for moderate to severe psoriasis. The trials of the efficacy of the interventions were all double-blind and placebo-controlled trials and generally of good quality, but three of the five efalizumab trials were poorly reported. A total of 1347 patients were included in the etanercept trials and 2963 in the efalizumab trials. Data on the efficacy of etanercept 25 mg twice a week for 12 weeks were available from three RCTs. On average, active treatment resulted in 62% of patients achieving a Psoriasis Area and Severity Index (PASI) 50, 33% achieving a PASI 75, 11% achieving a PASI 90 and 40% were assessed as clear or almost clear. These figures are not adjusted for changes relative to placebo. Improvement in quality of life as assessed by mean percentage change in Dermatology Life Quality Index (DLQI) was around 59% with etanercept 25 mg twice a week compared with 9% with placebo, and all mean differences that could be calculated were statistically significantly in favour of etanercept. Data on the efficacy of etanercept 50 mg twice a week for 12 weeks were available from two RCTs. Across the two trials, the proportion of patients achieving PASI 50, 75 and 90 was 76, 49 and 21%, respectively; the pooled relative risks were all statistically significantly in favour of etanercept. The findings for mean PASI after treatment, mean percentage change in PASI from baseline and mean percentage change in DLQI also demonstrated the efficacy of etanercept treatment. Evidence from one RCT indicates that the response to etanercept is maintained post-treatment, at least in the medium term, and data from uncontrolled follow-up phases reflect and extend these findings. Efalizumab at a dose of 1 mg/kg once a week subcutaneously was studied in five RCTs. Across these trials, 12 weeks of active treatment resulted in an average of 55% of patients achieving PASI 50, 27% PASI 75, 4.3% PASI 90 and 27% clear or minimal psoriasis status. These figures are not adjusted for changes relative to placebo. There is no evidence from RCTs that the response to efalizumab 1 mg/kg once a week is maintained when treatment continues beyond 12 weeks, and long-term follow-up data relate to a range of doses and are poorly reported and so cannot be used to draw even tentative conclusions regarding the long-term efficacy of efalizumab. Uncontrolled data from trial follow-up suggest that time to relapse may be around 60 days. No data indicating the existence or absence of any rebound in psoriasis after discontinuation of efalizumab were identified. There is no evidence relating to the efficacy of efalizumab upon retreatment. A mixed treatment comparison analysis found a higher response rate in terms of PASI 50, 75 and 90 with etanercept than with efalizumab. Injection site reactions appear to be the most common adverse effects of etanercept. Overall, etanercept appears to be well tolerated in short- and long-term use, although many of the long-term data are not from patients with psoriasis. Headache, chills and, to a lesser extent, nausea, myalgia, pain and fever are the common adverse events associated with efalizumab. Overall, withdrawal rates due to adverse events are low. Longer term data for efalizumab are not readily available for evaluation, but the adverse events data up to 3 years appear to reflect those over 12 weeks and to remain stable. Unfortunately, few data for serious infections and serious adverse events with efalizumab are available. For the primary analysis comparing etanercept, efalizumab and supportive care, the results of the York Model suggest that the biological therapies would only be cost-effective for all patients with moderate to severe psoriasis if the NHS were willing to pay over pound 60,000 per QALY gained. In patients with poor baseline quality of life (fourth quartile DLQI), efalizumab, etanercept 25 mg (intermittent), etanercept 25 mg (continuous) and etanercept 50 mg (intermittent) would be cost-effective as part of a treatment sequence if the NHS were willing to pay pound 45,000, pound 35,000, pound 45,000 and pound 65,000 per QALY gained, respectively. In patients who are also at high risk of inpatient hospitalisation (21 days per annum), these therapies would be cost-effective as part of a sequence as long as the NHS were willingness to pay pound 25,000, pound 20,000, pound 25,000 and pound 45,000 per QALY gained, respectively. As part of a secondary analysis including a wider range of systemic therapies as comparators, the York Model found that it would only be cost-effective to use etanercept and efalizumab in a sequence after methotrexate, ciclosporin and Fumaderm.

CONCLUSIONS

Clinical trial data indicate that both etanercept and efalizumab are efficacious in patients who are eligible for systemic therapy, but the economic evaluation demonstrates that these biological therapies are likely to be cost-effective only in patients with poor baseline QoL and who are at risk of hospitalisation. Efficacy trials conducted in the specific population for which etanercept and efalizumab are licensed are required, as are long-term comparisons of etanercept and efalizumab with other treatments for moderate to severe psoriasis. Long-term efficacy trials and safety/tolerability data for patients treated with etanercept or efalizumab are required, as are trials on the response of specific subtypes of psoriasis to different drugs. Research on the rate of inpatient hospitalisation in patients with moderate to severe psoriasis is warranted, and the effect of treatment on this rate.

The changing epidemiology of cryptosporidiosis in North West England

Between 1996 and 2000, rates of cryptosporidiosis in North West England were significantly higher than overall in England and Wales, particularly during the first half of each year. In addition, during the second quarter of each year in this period, up to 40% of all cases recorded in England and Wales were from the North West Region. In 2001, cryptosporidiosis dramatically decreased throughout the United Kingdom and the springtime excess of cases formerly seen in the North West was no longer apparent. This changed epidemiology was due to a decline in cases of Cryptosporidium parvum (formerly genotype 2), associated with zoonotic transmission. Although the initial loss of a spring peak of infection corresponded with the outbreak of foot-and-mouth disease throughout the United Kingdom, its continued absence relates to major structural changes in the North West public water supply. This study highlights the far-reaching public health benefit of local working relationships in addressing re-occurring disease issues.

Cyclic changes in the affinity of protein-DNA interactions drive the progression and regulate the outcome of the Tn10 transposition reaction

The Tn10 transpososome is a DNA processing machine in which two transposon ends, a transposase dimer and the host protein integration host factor (IHF), are united in an asymmetrical complex. The transitions that occur during one transposition cycle are not limited to chemical cleavage events at the transposon ends, but also involve a reorganization of the protein and DNA components. Here, we demonstrate multiple pathways for Tn10 transposition. We show that one series of events is favored over all others and involves cyclic changes in the affinity of IHF for its binding site. During transpososome assembly, IHF is bound with high affinity. However, the affinity for IHF drops dramatically after cleavage of the first transposon end, leading to IHF ejection and unfolding of the complex. The ejection of IHF promotes cleavage of the second end, which is followed by restoration of the high affinity state which in turn regulates target interactions.

lpt6, a gene required for addition of phosphoethanolamine to inner-core lipopolysaccharide of Neisseria meningitidis and Haemophilus influenzae

We previously described a gene, lpt3, required for the addition of phosphoethanolamine (PEtn) at the 3 position on the beta-chain heptose (HepII) of the inner-core Neisseria meningitidis lipopolysaccharide (LPS), but it has long been recognized that the inner-core LPS of some strains possesses PEtn at the 6 position (PEtn-6) on HepII. We have now identified a gene, lpt6 (NMA0408), that is required for the addition of PEtn-6 on HepII. The lpt6 gene is located in a region previously identified as Lgt-3 and is associated with other LPS biosynthetic genes. We screened 113 strains, representing all serogroups and including disease and carriage strains, for the lpt3 and lpt6 genes and showed that 36% contained both genes, while 50% possessed lpt3 only and 12% possessed lpt6 only. The translated amino acid sequence of lpt6 has a homologue (72.5% similarity) in a product of the Haemophilus influenzae Rd genome sequence. Previous structural studies have shown that all H. influenzae strains investigated have PEtn-6 on HepII. Consistent with this, we found that, among 70 strains representing all capsular serotypes and nonencapsulated H. influenzae strains, the lpt6 homologue was invariably present. Structural analysis of LPS from H. influenzae and N. meningitidis strains where lpt6 had been insertionally inactivated revealed that PEtn-6 on HepII could not be detected. The translated amino acid sequences from the N. meningitidis and H. influenzae lpt6 genes have conserved residues across their lengths and are part of a family of proven or putative PEtn transferases present in a wide range of gram-negative bacteria.

Early intermediates of mariner transposition: catalysis without synapsis of the transposon ends suggests a novel architecture of the synaptic complex

The mariner family is probably the most widely distributed family of transposons in nature. Although these transposons are related to the well-studied bacterial insertion elements, there is evidence for major differences in their reaction mechanisms. We report the identification and characterization of complexes that contain the Himar1 transposase bound to a single transposon end. Titrations and mixing experiments with the native transposase and transposase fusions suggested that they contain different numbers of transposase monomers. However, the DNA protection footprints of the two most abundant single-end complexes are identical. This indicates that some transposase monomers may be bound to the transposon end solely by protein-protein interactions. This would mean that the Himar1 transposase can dimerize independently of the second transposon end and that the architecture of the synaptic complex has more in common with V(D)J recombination than with bacterial insertion elements. Like V(D)J recombination and in contrast to the case for bacterial elements, Himar1 catalysis does not appear to depend on synapsis of the transposon ends, and the single-end complexes are active for nicking and probably for cleavage. We discuss the role of this single-end activity in generating the mutations that inactivate the vast majority of mariner elements in eukaryotes.

Promiscuous target interactions in the mariner transposon Himar1

We have previously characterized the early intermediates of mariner transposition. Here we characterize the target interactions that occur later in the reaction. We find that, in contrast to the early transposition intermediates, the strand transfer complex is extremely stable and difficult to disassemble. Transposase is tightly bound to the transposon ends constraining rotation of the DNA at the single strand gaps in the target site flanking the element on either side. We also find that although the cleavage step requires Mg2+ or Mn2+ as cofactor, the strand transfer step is also supported by Ca2+, suggesting that the structure of the active site changes between cleavage and insertion. Finally, we show that, in contrast to the bacterial cut and paste transposons, mariner target interactions are promiscuous and can take place either before or after cleavage of the flanking DNA. This is similar to the behavior of the V(D)J system, which is believed to be derived from an ancestral eukaryotic transposon. We discuss the implications of promiscuous target interactions for promoting local transposition and whether this is an adaptation to facilitate the invasion of a genome following horizontal transfer to a new host species.