Patterns of Onchocerca volvulus recombinant antigen recognition in a bovine model of onchocerciasis

The antibody responses of 8 cattle experimentally infected with Onchocerca ochengi to 18 recombinant O. volvulus antigens were measured by ELISA. In addition to establishing antigenic cross-reactivity between the species, the dynamics of antigen-specific responses were examined to assess how the recognition of the antigens compared to the known stage-specificity of expression. Six cattle responded to all of the antigens and 2 animals responded to all but 1. The dynamics of the recognition of 4 antigens (B20, MOv-2, MOv-14 and OvNHR2 02E1) were characterized by rapid seroconversion following infection. Antibody levels to 2 antigens (Ov7 and OvALT-1) increased gradually over the course of infection. Antibody levels to 4 antigens (OvTPX-2, OvL3Chitinase, Ov103 and Ov9m) reached maximum levels coincident with the onset of patency. The levels to 3 antigens (OvProalf C50, OvAldolase, Ov39) varied little over the course of infection. Responses to antigens with functional similarities (OvSOD1, OvSOD2 and OvSOD3 or OvGST1 and OvGST2) showed comparable temporal profiles. This study demonstrates the high degree of immunological cross-reactivity between the antigens of O. volvulus and O. ochengi. The immunogenicity of antigens varied over the course of infection in an antigen-specific manner, which not always reflected developmentally regulated expression of the corresponding gene, possibly owing to cross-reactive epitopes on distinct parasite products.

Diagnosis of Wuchereria bancrofti infection by the polymerase chain reaction employing patients' sputum

A preliminary evaluation of the diagnostic potential of a polymerase chain reaction (PCR) assay using diurnally collected sputum from bancroftian filariasis patients is described. A new set of PCR primers amplifying a 254-bp-long sequence termed AccI, derived from a long dispersed repeated sequence and SspI primers previously employed for PCR-based diagnosis were employed in this study with similar results. Of the 34 sputum samples from patients, 32 (94%) were PCR positive. Of the 18 patients with low to high microfilaremia (21-1560 microfilariae/ml), 16 (88.8%) were PCR positive. Of the remaining 16 patients, 6 with very low microfilaremia (2-6 microfilariae/ml) and 10 without microfilaremia, all (100%) were PCR positive. Two PCR-positive cases among the 13 endemic normal individuals tested (15.4%) may represent cases of occult filariasis. PCR amplification was also demonstrated with one PCR-positive sputum aliquot when mixed with 14 sputum aliquots from uninfected (PCR-negative) individuals. The potential diagnostic merits of the sputum-PCR assay are discussed.

Helminth genome analysis: the current status of the filarial and schistosome genome projects. Filarial Genome Project. Schistosome Genome Project

Genome projects for the parasitic helminths Brugia malayi (a representative filarial nematode) and Schistosoma were initiated in 1995 by the World Health Organization with the ultimate objectives of identifying new vaccine candidates and drug targets and of developing low resolution genome maps. Because no genetic maps are available, and very few genes have been characterized from either parasite group, the first goal of both Initiatives has been to catalogue new genes for future placement on chromosome and physical maps. These genes have been identified by the expressed sequence tag (EST) approach, utilising cDNA libraries constructed from diverse life cycle stages. To date, the Initiatives have deposited over 16,000 Brugia ESTs and nearly 8000 Schistosoma ESTs in Genbank's dbEST database, corresponding to 6000 and over 3600 genes respectively (33% of Brugia's estimated gene compliment, 18-24% of that of Schistosoma). Large fragment, genomic libraries have been constructed in BAC and YAC vectors for studies of genomic organization and for physical and chromosome mapping, and public, hypertext genomic databases have been established to facilitate data access. We present a summary of progress within the helminth genome initiatives and give several examples of important gene discoveries and future applications of these data.

Cumulative effect of anatomical risk factors for schizophrenia: an MRI study

BACKGROUND

Although schizophrenic and control subjects differ on a variety of neuroanatomical measures, the specificity and sensitivity of any one measure for differentiating between groups are low. This study investigated the cumulative effect of deviant brain structure on diagnosis.

METHODS

Hemisphere and third ventricle volume and the normalized (Talairach) location of three association cortex sulcal landmarks were measured on high-resolution MRI scans in 37 male patients with schizophrenia and 33 male control subjects matched on age, handedness, and parental socioeconomic status.

RESULTS

While there were few group differences on individual anatomical measures, the 10 variables reliably discriminated between the two groups when used in concert in a discriminant function analysis (F[10.59] = 3.6, p < .0009) with 77% of the subjects correctly classified. Five of the measures (left posterior cingulate, left inferior frontal sulcus, right sylvian fissure, and left and right halves of the third ventricle) correlated significantly with the discriminant function (p < .005).

CONCLUSIONS

This is the first study to demonstrate that schizophrenics can be distinguished from matched controls on the basis of brain anatomy alone. The risk of schizophrenia may depend on the total amount of neural deviance, rather than on anomalies in a single structure or circuit.

Chemiluminescent detection of sequential DNA hybridizations to high-density, filter-arrayed cDNA libraries: a subtraction method for novel gene discovery

A chemiluminescent approach for sequential DNA hybridizations to high-density filter arrays of cDNAs, using a biotin-based random priming method followed by a streptavidin/alkaline phosphatase/CDP-Star detection protocol, is presented. The method has been applied to the Brugia malayi genome project, wherein cDNA libraries, cosmid and bacterial artificial chromosome (BAC) libraries have been gridded at high density onto nylon filters for subsequent analysis by hybridization. Individual probes and pools of rRNA probes, ribosomal protein probes and expressed sequence tag probes show correct specificity and high signal-to-noise ratios even after ten rounds of hybridization, detection, stripping of the probes from the membranes and rehybridization with additional probe sets. This approach provides a subtraction method that leads to a reduction in redundant DNA sequencing, thus increasing the rate of novel gene discovery. The method is also applicable for detecting target sequences, which are present in one or only a few copies per cell; it has proven useful for physical mapping of BAC and cosmid high-density filter arrays, wherein multiple probes have been hybridized at one time (multiplexed) and subsequently "deplexed" into individual components for specific probe localizations.

Specificity from the synapsis of DNA elements by the Sfi I endonuclease

The synapsis of DNA sites is a prerequisite for the reactions of many proteins that act at specific DNA sequences. The requirement for synapsis was investigated by analysing the reactions of Sfi I, a tetrameric restriction enzyme that cleaves DNA only after interacting with two recognition sites. In the presence of Mg2+, oligonucleotide duplexes with the cognate recognition sequence were cleaved rapidly, with cooperative kinetics, while non-cognate duplexes were not cleaved. In the absence of Mg2+, the primary complex formed by Sfi I with cognate DNA contained two duplexes synapsed by the tetramer: a secondary complex containing one duplex was seen only at elevated Sfi I concentrations. In contrast, the principal complex with non-cognate DNA contained one duplex bound to Sfi I. Pairs of non-cognate duplexes, or one cognate and one non-cognate duplex, generally failed to form synaptic complexes. On adding Mg2+to complexes with cognate DNA, cleavage occurred much more rapidly in the synaptic complex than in the secondary complex. DNA synapsis thus acts to enhance the specificity of Sfi I for its recognition sequence, by demanding two cognate sites for a catalytically active complex and by excluding non-cognate sites from the synaptic complex.

Deep within the filarial genome: progress of the filarial genome project

Four years ago, a WHO/United Nations Development Programme/World Bank-sponsored genome project to study the filarial lymphatic nematode parasite Brugia malayi was initiated. The project took as its aims gene discovery for drug target and vaccine candidate identification, genome mapping, dissemination of genomic data to the world community and training of endemic country partners in genomic research. In this article, the principal investigators in the laboratories behind the project describe the background to the project, the data now emerging and goals for the future. Open access to filarial genome data is emphasized.

Medical emergencies in general dental practice in Great Britain. Part 3: Perceptions of training and competence of GDPs in their management

OBJECTIVES

To ascertain the training received in management of medical emergencies recalled by general dental practitioners (GDPs), as undergraduates and since graduation, to examine how well prepared they felt to manage such events and how their readiness might be improved.

DESIGN

Postal questionnaire survey of a random sample of GDPs in Great Britain.

SUBJECTS

1500 GDPs, 1000 in England & Wales and 500 in Scotland.

RESULTS

There was a 74% response. Among respondents, 75% had received training as undergraduates in the management of medical emergencies, 95% had subsequently received training. The proportion considering themselves 'well' or 'fairly well' prepared to manage emergencies rose from 30% at graduation to 80% currently. The 20% who felt themselves 'not very well' or 'not at all' prepared were less likely to possess the drugs and equipment to manage an emergency. Nevertheless, a need for further training was expressed by 96%: only 3% felt no need.

CONCLUSIONS

Virtually all GDPs (99.2%), had received training in the management of medical emergencies at some time, principally since graduation. The 20% who feel inadequately prepared are less likely to possess the necessary drugs and equipment. There is a perceived need for further training among the majority of respondents.

Dental beliefs, knowledge and behaviour of Chinese people in the United Kingdom

OBJECTIVE

This study explored oral health beliefs, knowledge and behaviour among a sample of United Kingdom Chinese.

DESIGN

A quota sample of Chinese people, stratified by age and gender, were interviewed by trained and standardised Chinese interviewers using a piloted, validated semi-structured questionnaire.

SUBJECTS

One hundred and fifty-six Chinese people--with similar number of teenagers, younger adults and older people--resident in the North East of England.

SETTING

Chinese communities.

OUTCOME MEASURE

Reported dental knowledge, beliefs and behaviours.

RESULTS

Regardless of gender and age, the majority of respondents believed that it was natural for people to lose all their teeth in old age. Less than half were convinced that they would be able to keep their own teeth for life. The majority of the sample considered that they were susceptible to dental diseases, the consequences of which were thought to be serious. Approximately half presumed that dental diseases were preventable, although the aetiology of dental caries, periodontal disease and tooth loss was poorly understood. While 94% claimed to brush their teeth as part of routine dental care, dental visiting and dietary restriction of sugar intake were reported only in 61% and 30% of the sample respectively. Inter-generational differences were marked; older people tended to have a fatalistic attitude and were least likely to attend the dentist.

CONCLUSIONS

A low level of dental awareness was found among the UK Chinese. In order to facilitate effective health promotion and treatment services, the extent of Chinese people's traditional oral health beliefs and behaviour must be taken into account.

Development of a quantitative, competitive polymerase chain reaction--enzyme-linked immunosorbent assay for the detection of Wuchereria bancrofti DNA

A quantitative, competitive polymerase chain reaction (QC-PCR) assay for the sensitive detection of Wuchereria bancrofti DNA was developed. A competitor sequence was constructed by an exchange of nucleotides in the Wuchereria-specific Ssp I repeat. The PCR products were hybridized to specific DNA probes and their amounts, determined by an enzyme-linked immunosorbent assay (ELISA). In laboratory-prepared samples the QC-PCR-ELISA assay was capable of detecting the amount of DNA equivalent to 0.1 microfilaria (mf) added to 200 microl of blood lysate. The assay was also tested on 78 blood samples collected in endemic areas in Egypt. All 28 samples that were positive both for mf and for circulating antigen were also QC-PCR-ELISA-positive. In addition, one mf-negative but antigen-positive sample was also positive as determined by QC-PCR-ELISA. A positive correlation of mf density with the QC-PCR-ELISA was observed. Samples containing 10 or fewer mf/ml had a mean relative amount of Ssp I PCR product of 19.7 units, whereas samples with 11-100 mf/ml had a mean of 36.3 units and those with more than 100 mf/ml had a mean of 84.6 units. Because of the high standard deviation within each group, estimates of worm burdens in infected individuals using the QC-PCR-ELISA are not recommended. However, we present data indicating that the W. bancrofti QC-PCR-ELISA is a powerful new tool for evaluation of parasitic loads for community-based diagnosis of bancroftian filariasis.

Medical emergencies in general dental practice in Great Britain. Part 2: Drugs and equipment possessed by GDPs and used in the management of emergencies

OBJECTIVE

To ascertain the emergency drugs and equipment possessed by general dental practitioners (GDPs), the treatment provided and drugs used in management of the medical emergency events they reported.

DESIGN

Postal questionnaire survey of a random sample of GDPs in Great Britain.

SUBJECTS

1500 GDPs, 1000 in England & Wales and 500 in Scotland.

RESULTS

There was a 74% response. An aspirator, an airway, oxygen, adrenaline and an injectable steroid were possessed by about 90% of respondents; glucose, glyceryl trinitrate and a salbutamol inhaler by about 80%. Glucose was used in management of nearly one in ten of the events reported, an inhaler, glyceryl trinitrate and oxygen were the next most commonly used. Cardiopulmonary resuscitation (CPR) was carried out in management of 1.1-1.4% of events not associated with general anaesthesia (GA) and in 4.7-16% of events associated with GA, an average of once in 250 years of practice.

CONCLUSIONS

Most respondents possessed drugs and equipment necessary to manage a medical emergency. Half the drugs recommended by the 'Poswillo report' to be available in every dental practice were not used in more than 8000 years of practice.

Medical emergencies in general dental practice in Great Britain. Part 1: Their prevalence over a 10-year period

OBJECTIVE

To assess the prevalence, nature and outcome of medical emergencies experienced by general dental practitioners (GDPs) over a 10-year period.

DESIGN

Postal questionnaire survey of a random sample of GDPs in Great Britain.

SUBJECTS

1500 GDPs, 1000 in England & Wales and 500 in Scotland.

RESULTS

There was a 74% response. Emergency events were reported by 70.2%: the number reported by a single individual ranged from none to 33. The most commonly experienced events, including those associated with general anaesthesia, were (as a percentage of the total) for England & Wales and Scotland, respectively: fits and seizures (31.0%, 36.3%); swallowed foreign bodies (15.7%, 18.1%); attacks of asthma (13.8%, 11.1%); chest pain associated with angina pectoris (10.1%, 11.0%) and diabetic events (10.6%, 9.0%): none of these resulted in any serious sequelae. More events were reported in Scotland. Overall, there were 20 deaths resulting from medical emergencies reported in the survey, 4 affecting passers-by and none associated with general anaesthesia. 8849 years of practice experience were represented (by 94% of respondents), from which an estimate of the frequency of events was made.

CONCLUSIONS

An emergency event was reported, on average, for every 4.5 practice years in England & Wales and 3.6 years in Scotland and death associated with general dental practice, on average, once in 758 and 464 years, respectively.

Attitudes of Chinese people toward obtaining dental care in the UK

OBJECTIVE

To explore the attitudes of Chinese people regarding dentists and toward obtaining dental care in the UK.

INTRODUCTION

Injudicious use of general health services has been reported among Chinese people in the UK. In relation to oral healthcare, the extent to which cultural beliefs and values affect use remains uncertain.

SETTING

Chinese communities in the north east of England.

SUBJECTS AND METHODS

A quota sample of 156 Chinese people was recruited in 1996. 50 elderly people, 56 adults, 50 teenagers were interviewed by 5 trained and standardised multilingual Chinese interviewers using a validated structured questionnaire.

RESULTS

Access varied substantially according to age, with the younger generations more likely to obtain dental care without difficulty. Overall, the sample perceived that dentists possessed adequate technical competence, but their patient management skills lacked sensitivity. Older Chinese people placed less trust in dentists. Many elderly people reported problems in finding a UK dentist and would have preferred one of Chinese origin. Potential problems in follow-up prosthetic dental care were also apparent among this age group. Cost, communication problems, anxiety and cultural beliefs were identified as major barriers to dental care.

CONCLUSIONS

There is a clear need to raise cultural awareness among dental professionals as well as to help Chinese people's positive attitudes toward dentists and dental care.

Quality and care: patients' perceptions

Patients and health care professionals view quality nursing care from different perspectives. Health care professionals view competent nursing care as quality nursing care. Patients perceive quality nursing care as caring, interpersonal interactions. Institutions measure quality care through satisfaction surveys that exclude components of nurse caring behaviors. In three studies utilizing the Holistic Caring Inventory (HCI), patients perceived nurse caring behaviors and attitudes that indicated quality nursing care. The way to bridge the gap between institutions' and patients' perceptions of quality care lies in valuing the interactions that patients consider quality care and including these interactions in measures of quality care.

PCR-ELISA for the detection of Brugia malayi infection using finger-prick blood

A polymerase chain reaction assay based on the enzyme-linked immunosorbent assay (PCR-ELISA) has been developed to detect Brugia malayi infection in an area of low endemicity in Malaysia. Blood samples from 239 subjects were tested: 192 amicrofilaraemic individuals, 14 microfilaraemic persons and 3 chronic elephantiasis cases from endemic areas and 30 city-dwellers (non-endemic controls). PCR products were examined by ELISA and Southern hybridization. In the PCR-ELISA, digoxigenin-labelled PCR products were hybridized to a biotin-labelled probe. This was followed by incubation in streptavidin-coated microtitre wells and detection using anti-digoxigenin-peroxidase and ABTS [2,2'-azinobis(3-ethylbenzthiazoline-6-sulphonic acid)]. All microfilaraemic samples were positive by PCR-ELISA and Southern hybridization and all samples from non-endemic subjects and chronic elephantiasis patients were negative. The PCR-ELISA detected 12 times as many B. malayi infections as did thick blood film examination. Nineteen of the 194 samples from the endemic area gave positive results by both PCR-ELISA and Southern hybridization, and an additional 5 samples were positive by PCR-ELISA only. The PCR-ELISA was specific and sensitive, detected more infections, and was more reproducible than Southern hybridization.

Phospholipase A2-mediated activation of phospholipase D in rat heart sarcolemma

The effect of phospholipase A2 (PLA2)-dependent release of unsaturated fatty acids (FA) on phospholipase D (PLD) function was examined in purified sarcolemmal (SL) membranes isolated from rat heart. PLD hydrolytic activity was determined by measuring either [14C] phosphatidic acid formation from exogenous [14C] phosphatidylcholine (PtdCho) or [3H] choline release from prelabelled SL Ptd[3H]choline. SL membranes with endogenous [3H] PtdCho that were prelabelled with [3H] myristic acid were used for testing PLD transphosphatidylation activity. Exogenous cis-unsaturated FA, arachidonate and oleate, significantly enhanced the [3H] choline formation at 50 and 100 microM, respectively; their effect was maximal at 250 microM and declined at higher concentrations. Use of melittin (which stimulates membrane-bound PLA2, thus releasing FA) or exogenous PLA2 reproduced the stimulatory effect of added arachidonate and oleate. Under melittin, PLA2-dependent FA release was strongly correlated (r = 0.99) to the PLD-dependent phosphatidic acid formation. Arachidonate- or melittin-enhanced PLD transphosphatidylation activity confirmed the augmented catalytic rate of PLD by these agents. Melittin-evoked PLD activation was completely blocked by 1 microM E-6-(bromomethylene) tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one, a selective inhibitor of Ca(2+)-independent v Ca(2-)-dependent PLA2, thus indicating that PLD stimulation under melittin occurred via PLA2. Activity measurement and Western blotting studies revealed the presence of a Ca(2+)-independent, high molecular weight (110 kDa) PLA2 in the SL membrane, and its immunoprecipitation by monoclonal antibodies significantly reduced the melittin-related PLD stimulation. These results suggest that Ca(2+)-independent PLA2 and subsequent endogenous mobilization of sn-2 unsaturated FA modulate PLD activity in heart SL membranes. This event may occur in physiological conditions via hormonal stimulation of membranal PLA2 as well as in heart diseases characterized by PLA2 pathological dysfunction.

Detection of the filarial parasite Mansonella streptocerca in skin biopsies by a nested polymerase chain reaction-based assay

To differentiate the skin-dwelling filariae Mansonella streptocerca and Onchocerca volvulus, a nested polymerase chain reaction (PCR) assay was developed from small amounts of parasite material present in skin biopsies. One nonspecific and one specific pair of primers were used to amplify the 5S rDNA spacer region of M. streptocerca. Biopsies with different microfilaria densities obtained from 104 Ugandans living in an area endemic for M. streptocerca were tested using both the nested PCR assay and standard parasitologic assessment of microfilariae. All 82 samples from microfilaria carriers were positive when tested using the nested PCR assay. In addition, M. streptocerca DNA could be detected in 16 samples thought to be microfilaria negative. Furthermore, six days following ivermectin treatment, M. streptocerca DNA was found in 12 of 14 microfilaria-negative biopsies. Control skin samples from patients infected with O. volvulus were all negative in the nested PCR assay. This assay improves the diagnosis of M. streptocerca and will facilitate further epidemiologic studies.

The immunodominant Brugia malayi paramyosin as a marker of current infection with Wuchereria bancrofti adult worms

The full-length cDNA sequence encoding Brugia malayi L3 paramyosin has been isolated by immunoscreening a cDNA library with a mouse antiserum raised against Wuchereria bancrofti L3 infective larvae. A recombinant truncated form of paramyosin was expressed as a glutathione S-transferase fusion protein and used to evaluate humoral responses of adults from a W. bancrofti-endemic area in French Polynesia according to their parasitological status. Immunoglobulin G4 (IgG4) preferentially bound to paramyosin in W. bancrofti-parasitized individuals, in contrast to unparasitized individuals, who harbored neither microfilaria nor Og4C3 adult worm circulating antigen. Reduction of the anti-paramyosin IgG4 titer following combined chemotherapy with diethylcarbamazine and ivermectin was significantly correlated with a reduction in the adult worm burden. This indicates that the presence of paramyosin-reactive IgG4 is associated with the presence of parasites and that reduction can be used as an immunological marker for W. bancrofti clearance.

The reliability of interview data for age at which infants' toothcleaning begins

A number of studies have reported on the age at which toothbrushing started and drawn important conclusions. Such studies often relied on parental recall of a particular event that occurred some years previously, assuming the quality of retrospectively reported data. The present study aimed to investigate consistency of reporting the age at which toothcleaning began. Mothers from two different cultural backgrounds, 100 Caucasian and 150 Asian living in a deprived inner city area, were interviewed in their homes on two separate occasions, when the sample child was 6-24 months old and again at 3-4 years. Of the 39 Caucasian and 31 Asian mothers who specified an age at which toothbrushing commenced at both interviews, complete agreement occurred among 4 (10%) Caucasian and 2 (6%) Asian mothers, with a variation of +/- 2 months among 12 (31%) and 5 (16%) respectively. Intraclass correlation coefficients were -0.01 for Caucasian and -0.46 for Asian mothers. Kappa values were 0 and -0.09 respectively. Of the 139 mothers who stated that their children's teeth were not cleaned at the time of the first interview, 17 claimed an earlier age than this at the second interview. It was concluded that there was little agreement between the two interviews, although the extent of the variation differed between the two cultural groups. Although these findings represent relatively deprived population groups, caution is recommended in unquestioningly accepting the accuracy of retrospective reporting of oral hygiene practices in dental surveys until evidence of better reliability is available.

Sedation in primary dental care: an investigation in two districts of northern England

OBJECTIVE

To determine the current provision of sedation in primary dental care and investigate the knowledge and attitudes of dental practitioners and others on the use of sedation.

DESIGN

Qualitative interviews and postal questionnaire survey.

SETTING

Health districts of Bradford (West Yorkshire) and South Durham, UK in 1996.

SUBJECTS AND MATERIALS

15 key individuals associated with NHS primary dental services were interviewed. Questionnaires were sent to all 260 NHS general dental practitioners and community dental service clinicians.

RESULTS

208 questionnaires (80%) were returned. 42% of respondents reported current sedation use, with oral administration the favoured technique (26%). Significant differences were found between districts for intravenous sedation use (7% Bradford, 41% South Durham, P < 0.001). Almost all participants agreed the value of sedation in dental care for adults and children, for nervous, phobic patients or in association with unpleasant forms of treatment and 45% of dentists felt that provision should be expanded. Training, availability of referral services, finance and patient demand were seen as encouraging factors.

CONCLUSIONS

Substantial variations in sedation provision between the two districts were associated with teaching at the nearest dental schools. Innovative use of resources could help the expansion of provision supported in this study.

Phosphorothioate substrates for the SfiI restriction endonuclease

Oligodeoxynucleotides carrying the recognition sequence for the SfiI endonuclease were synthesised with phosphorothioates at the cleavage site. The Rp and Sp diastereoisomers of the oligonucleotides were separated by HPLC using a mobile phase containing L-cysteine. The duplex with Rp phosphorothioates was cleaved very slowly in the presence of Mg2+, though virtually complete cleavage was obtained with Mn2+. No significant cleavage of the duplex with Sp phosphorothioates occurred with either Mg2+ or Mn2+. When added to a plasmid with one SfiI site, the duplexes with either Rp or Sp phosphorothioates inhibited the rate at which SfiI cleaved the plasmid: a control duplex with oxyester linkages enhanced the rate of plasmid cleavage. In contrast to type IIe nucleases such as EcoRII and NaeI, which can be activated by non-hydrolysable analogues of their substrates, SfiI reactions require four susceptible phosphodiester bonds.