NMR detection of slow conformational dynamics in an endonuclease toxin

The cytotoxic activity of the secreted bacterial toxin colicin E9 is due to a non-specific DNase housed in the C-terminus of the protein. Double-resonance and triple-resonance NMR studies of the 134-amino acid 15N- and 13C/15N-labelled DNase domain are presented. Extensive conformational heterogeneity was evident from the presence of far more resonances than expected based on the amino acid sequence of the DNase, and from the appearance of chemical exchange cross-peaks in TOCSY and NOESY spectra. EXSY spectra were recorded to confirm that slow chemical exchange was occurring. Unambiguous sequence-specific resonance assignments are presented for one region of the protein, Pro65-Asn72, which exists in two slowly exchanging conformers based on the identification of chemical exchange cross-peaks in 3D 1H-1H-15N EXSY-HSQC, NOESY-HSQC and TOCSY-HSQC spectra, together with C alpha and C beta chemical shifts measured in triple-resonance spectra and sequential NH NOEs. The rates of conformational exchange for backbone amide resonances in this stretch of amino acids, and for the indole NH of either Trp22 or Trp58, were determined from the intensity variation of the appropriate diagonal and chemical exchange cross-peaks recorded in 3D 1H-1H-15N NOESY-HSQC spectra. The data fitted a model in which this region of the DNase has two conformers, NA and NB, which interchange at 15 degrees C with a forward rate constant of 1.61 +/- 0.5 s-1 and a backward rate constant of 1.05 +/- 0.5 s-1. Demonstration of this conformational equilibrium has led to a reappraisal of a previously proposed kinetic scheme describing the interaction of E9 DNase with immunity proteins [Wallis et al. (1995) Biochemistry, 34, 13743-13750 and 13751-13759]. The revised scheme is consistent with the specific inhibitor protein for the E9 DNase, Im9, associating with both the NA and NB conformers of the DNase and with binding only to the NB conformer detected because the rate of dissociation of the complex of Im9 and the NA conformer, NAI. is extremely rapid. In this model stoichiometric amounts of Im9 convert, the E9 DNase is converted wholly into the NBI form. The possibility that cis-trans isomerisation of peptide bonds preceding proline residues is the cause of the conformational heterogeneity is discussed. E9 DNase contains 10 prolines, with two bracketing the stretch of amino acids that have allowed the NA [symbol: see text] NB interconversion to be identified, Pro65 and Pro73. The model assumes that one or both of these can exist in either the cis or trans form with strong Im9 binding possible to only one form.

Interpretations of healthy and unhealthy eating, and implications for dietary change

Central to many considerations of health-related dietary change is the interpretation given to 'healthy' and 'unhealthy' eating. Results are reported from two studies which investigated people's understanding of these concepts. The first study involved structured interviews with 48 respondents: a wide range of interpretations of healthy and unhealthy eating was found using content analysis techniques. The second study (n = 242) used questionnaires to assess the extent to which different interpretations were endorsed. The extent to which different eating behaviours were perceived as healthy or unhealthy varied greatly. A principal components analysis of perceptions of healthy and unhealthy eating produced five separate factors. Some differences in perceptions were found for people of different age, gender and educational level. Objectively assessed dietary intake and perceived intake were found to be two distinct quantities. 'Healthy' eaters were found to perceive 'healthy eating' slightly differently to 'unhealthy' eaters. The results suggest that lay understanding of healthy eating does generally conform with dietary guidelines, and therefore health promotion priorities should focus on physical and psychological constraints to healthy eating, rather than attempting to increase the public's knowledge as a whole.

Three patients with a 45,X/46,X,psu dic(Xp) karyotype

Few cases of isochromosomes for the short arm of the X have been reported and all are dicentric with variable portions of the long arms interposed between the two centromeres. This paper reports three cases of complete short arm duplication of one X chromosome in unrelated female patients. All patients also have a 45,X cell line and present with some characteristic features of Turner syndrome. We used conventional cytogenetics, in situ hybridisation, and molecular genetics to describe all three structurally abnormal chromosomes and the parental origin of two of them. We briefly discuss the "inactivation enhancement" theory; however, any genotype-phenotype correlation is complicated by the presence of the 45,X cell line.

Do shorter-acting neuromuscular blocking drugs or opioids associate with reduced intensive care unit or hospital lengths of stay after coronary artery bypass grafting? CABG Clinical Benchmarking Data Base Participants

BACKGROUND

The authors hypothesized that shorter-acting opioid and neuromuscular blocking drugs would be associated with reductions in duration of intubation, length of stay (LOS) in the intensive care unit (ICU) after tracheal extubation, or postoperative (exclusive of ICU) LOS, and that shorter durations of intubation would be associated with reduced ICU LOS after extubation and postoperative (exclusive of ICU) LOS.

METHODS

One-thousand ninety-four patients undergoing primary coronary artery bypass graft surgery at 40 academic health centers were studied. Multiple patient-related factors were included in multivariate models for hypothesis testing.

RESULTS

The duration of tracheal intubation, ICU LOS after extubation, and postoperative (exclusive of ICU) LOS all varied significantly by site. There was no difference between vecuronium and pancuronium in duration of intubation, ICU LOS after extubation, or postoperative (exclusive of ICU) LOS. Use of sufentanil rather than fentanyl was associated with a significant (P=0.045) reduction of 1.9 h (95% CI, 0.04 to 4.1 h) in duration of tracheal intubation but had no significant effect on ICU LOS after extubation, total ICU LOS, postoperative (exclusive of ICU) LOS, or total postoperative LOS. The authors' best model predicts a complex association between increasing duration of intubation and both ICU LOS after tracheal extubation and postoperative (exclusive of ICU) LOS, which was associated with an increase in those measures when duration of intubation exceeded 7.3 or 3 h, respectively.

CONCLUSIONS

The LOS measures varied considerably among the institutions. Use of shorter-acting opioid and neuromuscular blocking drugs had no association with ICU LOS after tracheal extubation or with postoperative (exclusive of ICU) LOS. Only when the duration of intubation exceeded threshold values was it associated with increased LOS measures.

Phase II study of docetaxel and cisplatin in advanced non-small-cell lung cancer

PURPOSE

Docetaxel (Taxotere, Rhone-Poulenc Rorer, Antony, France) and cisplatin are two of the most active single agents used in the treatment of non-small-cell lung cancer (NSCLC). A recently reported phase I study of the combination of docetaxel and cisplatin recommended a dose of 75 mg/m2 of both drugs every 3 weeks for subsequent phase II study.

PATIENTS AND METHODS

Eligible patients were aged 18 to 75 years with a World Health Organization (WHO) performance status < or = 2 and life expectancy > or = 12 weeks, with metastatic and/or locally advanced NSCLC proven histologically or cytologically. Patients were not permitted to have received prior chemotherapy, extensive radiotherapy, or any radiotherapy to the target lesion and must have had measurable disease. Concurrent treatment with colony-stimulating factors (CSFs) or prophylactic antibiotics was not permitted. Docetaxel (75 mg/m2) in 250 mL 5% dextrose was given intravenously (i.v.) over 1 hour immediately before cisplatin (75 mg/m2) in 500 mL normal saline given i.v. over 1 hour in 3-week cycles. Premedication included ondansetron, dexamethasone, promethazine, and standard hyperhydration with magnesium supplementation.

RESULTS

A total of 47 patients, two thirds of whom had metastatic disease, were entered onto this phase II study. The majority of patients were male (72%) and of good (WHO 0 to 1) performance status (85%). All 47 patients were assessable for toxicity and 36 were for response. Three patients were ineligible and eight (17%) discontinued treatment because of significant toxicity. In assessable patients, the overall objective response rate was 38.9% (95% confidence limits [CL], 23.1% to 56.5%), 36.1% had stable disease, and 25% progressive disease. On an intention-to-treat analysis, the objective response rate was 29.8%. Median survival was 9.6 months and estimated 1-year survival was 33%. Significant (grade 3/4) toxicities included nausea (26%), hypotension (15%), diarrhea (13%), and dyspnea mainly related to chest infection (13%). One patient experienced National Cancer Institute (NCI) grade 3 neurosensory toxicity after eight cycles. Grade 3/4 neutropenia was common and occurred in 87% of patients, but thrombocytopenia > or = grade 3 was rare (one patient). Significant (grade 3/4) abnormalities of magnesium levels were common (24%). Febrile neutropenia occurred in 13% of patients and neutropenic infection in 11%, contributing to two treatment-related deaths. No neutropenic enterocolitis or severe fluid retention was reported.

CONCLUSION

Compared with other active regimens used in this setting, the combination of docetaxel and cisplatin in advanced NSCLC is an active regimen with a similar toxicity profile to other combination regimens.

Scaling of muscle performance during escape responses in the fish myoxocephalus scorpius L

Fast-starts associated with escape responses were studied in short-horn sculpin (Myoxocephalus scorpius L.), ranging from 5.5 to 32 cm in total length (L). Electromyography and sonomicrometry were used simultaneously to measure muscle activation and length changes, respectively, in the superficial layers of fast muscle in rostral myotomes. Escape responses consisted of a half tailbeat to bend the body into a C-shape (C-bend), another half tailbeat (contralateral contraction), followed by one or two more tailbeats and/or a gliding phase. The scaling relationships for both muscle strain and shortening duration differed between the C-bend and the contralateral contraction. As a result, relative muscle shortening velocity (V/V0) scaled as -1.18L1.06 for the C-bend and as 1.23L-0. 66 for the contralateral contraction. Therefore, the scaling relationships for muscle shortening velocity varied throughout the time course of the escape response. Muscle power output was determined by using the work-loop technique to subject isolated muscle fibres to in vivo strain and stimulation patterns. Plots of the instantaneous muscle forces and velocities achieved during the contralateral contraction were found to deviate from the steady-state force-velocity relationship. Maximum instantaneous muscle power output was independent of body size, with mean maximum values of 307 and 222 W kg-1 wet muscle mass for the C-bend and the contralateral contraction, respectively. &lt;P&gt;

Farm and tractor-related fatalities in children in South Australia

OBJECTIVE

To analyse the circumstances surrounding farm and tractor-related childhood fatalities in South Australia, to identify potentially dangerous situations and to delineate prevention strategies.

METHODS

A retrospective search of files was conducted at the Women's and Children's Hospital Department of Histopathology from 1981 to 1996, and the State Coroner's Office from 1988 to 1996.

RESULTS

Fifteen fatalities were identified comprising 11 boys and four girls (age range, 2 years 11 months to 13 years; average, 6 years). Activities prior to death included riding on a tractor/trailer (n=8); playing near a field grain storage bin (n=2); playing near machinery (n=2); sleeping in a car near a burn off (n=1); walking around a dam (n=1) and riding as a passenger on a motorbike (n=1). Causes of death included multiple skull fractures and cerebral damage (n=11), asphyxia (n=2), drowning (n=1) and incineration (n=1).

CONCLUSIONS

The most dangerous activity involved children riding unrestrained on tractors; falls resulted in extensive injuries from tractor rear wheels or towed machinery. Allowing children on farms to ride on tractors or machinery only if there is a safe seat with a restraining harness would substantially reduce the number of fatal farm-related childhood injuries in South Australia.

Scaling of intrinsic contractile properties and myofibrillar protein composition of fast muscle in the fish myoxocephalus scorpius L

Contractile properties were determined for fast muscle fibres from short-horn sculpin (Myoxocephalus scorpius L.) ranging from 5 to 35 cm total body length (L) and from 2.0 to 758 g body mass. Maximum unloaded shortening velocity (V0) decreased with increasing body size, scaling as 19.5L-0.34, and isometric twitch activation and tetanus relaxation times became longer, scaling as 12.0L0.31 and 19. 5L0.42 respectively. Myofibrillar Mg2+/Ca2+-ATPase activity scaled as 2.51L-0.28. In order to investigate the mechanisms underlying the scaling of contractile properties, myofibrillar protein composition and I filament lengths were determined. One-dimensional SDS-PAGE and two-dimensional isoelectric focusing/non-equilibrium isoelectric focusing-PAGE revealed no differences in the myofibrillar protein isoforms of myosin light chains, actin, tropomyosin, troponin-T and troponin-C in fish of differing body size. Peptide maps of purified myosin heavy chains digested with eight different proteolytic enzymes were also similar in all fish examined. Three isoforms of troponin-I were present in fish less than 20 cm in total length with relative molecular masses of 17 (TnIf3), 22 (TnIf2) and 23 (TnIf1). The ratio of TnI isoforms varied with body length, and only TnIf3 was present in fish greater than 28 cm total length. The length of I filaments was independent of body length. Thus, although the expression of TnI isoforms changes during growth, the underlying mechanism responsible for increased shortening velocity with decreased body size remains unknown. &lt;P&gt;

Immunity proteins and their specificity for endonuclease colicins: telling right from wrong in protein-protein recognition

Immunity proteins inhibit colicins, protein toxins released by bacteria during times of environmental stress, by binding and inactivating their cytotoxic domains. This protects the producing organism as it attempts to kill off competing bacteria. The cytotoxic domains of related colicins share a high degree of sequence identity, as do their corresponding immunity proteins, yet specificity and affinity are also high, with little non-cognate biological cross-protection evident under physiological conditions. We review recent work on DNase-specific immunity proteins, which shows that, although both cognate and non-cognate proteins can bind a single toxin, their affinities can differ by as much as 12 orders of magnitude. We have termed this mode of binding dual recognition, because the DNase-binding surface of an immunity protein is made up of two components, one conserved and the other variable. The strength of the binding interaction is dominated by the conserved residues, while neighbouring variable residues control specificity. Similar dual recognition systems may exist in other biological contexts, particularly where a protein must discriminate the right binding partner from numerous, structurally homologous alternatives.

The health of Canadians with diabetes

OBJECTIVES

This article focuses on the prevalence of diabetes mellitus among Canadians, the health status of those with the disease, their socioeconomic characteristics, personal health behaviours, and use of health services.

DATA SOURCE

The data are from the 1994/95 National Population Health Survey.

ANALYTICAL TECHNIQUES

Weighted counts were used in computing the prevalence of diabetes and the proportions of people with the characteristics and health behaviours considered.

MAIN RESULTS

In 1994/95, 3% of Canadians aged 12 and older--over 722,000 people--reported having diabetes that had been diagnosed by a health care professional. The prevalence of diabetes increases with age and is associated with low income. A higher percentage of adults with diabetes were overweight compared to those without the disease.

Analysis of 1.9 Mb of contiguous sequence from chromosome 4 of Arabidopsis thaliana

The plant Arabidopsis thaliana (Arabidopsis) has become an important model species for the study of many aspects of plant biology. The relatively small size of the nuclear genome and the availability of extensive physical maps of the five chromosomes provide a feasible basis for initiating sequencing of the five chromosomes. The YAC (yeast artificial chromosome)-based physical map of chromosome 4 was used to construct a sequence-ready map of cosmid and BAC (bacterial artificial chromosome) clones covering a 1.9-megabase (Mb) contiguous region, and the sequence of this region is reported here. Analysis of the sequence revealed an average gene density of one gene every 4.8 kilobases (kb), and 54% of the predicted genes had significant similarity to known genes. Other interesting features were found, such as the sequence of a disease-resistance gene locus, the distribution of retroelements, the frequent occurrence of clustered gene families, and the sequence of several classes of genes not previously encountered in plants.

Specificity in protein-protein recognition: conserved Im9 residues are the major determinants of stability in the colicin E9 DNase-Im9 complex

The endonuclease group of E colicins are a family of bacterial toxins whose cytotoxic activity in a producing host is inactivated by a specific immunity protein. The DNase of colicin E9 can be bound and inhibited by both cognate and noncognate immunity proteins, the dissociation constants for which span a range of 12-orders of magnitude. DNase binding specificity of the immunity proteins is governed primarily by helix II, the sequence of which is variable in this family of proteins. Heteronuclear NMR experiments have identified helix III along with helix II as the likely DNase binding site, although other regions of Im9 also showed perturbations on binding the E9 DNase. In the present work, we have used the NMR experiments as a guide for alanine scanning mutagenesis of Im9. Our data show that helices II and III of Im9 are indeed the DNase binding site and in addition quantitate the relative binding energy associated with each helix. We find that the conserved residues of helix III make the largest relative contribution toward E9 DNase binding. In conjunction with previous studies, the data suggest that specificity in the colicin-immunity system is governed by a dual recognition mechanism in which highly stabilizing interactions emanating from the conserved regions of an immunity protein act as the binding site anchor and these are modulated by interactions from neighboring, nonconserved amino acid residues. This modulation is likely to take the form of both favorable and unfavorable interactions, the balance of which define the specificity of the protein-protein interaction. The generality of such a dual recognition mechanism in other systems is also discussed.

Turner syndrome: a cytogenetic and molecular study

Two hundred and eleven patients with a clinical diagnosis of Turner syndrome were studied. We report (i) the cytogenetic results, (ii) the frequency of cryptic mosaicism and (iii) the parental age and the parental origin of the abnormality. We scored 100 cells from blood cultures and found 97 patients to have a 45,X constitution, 15 to be 45,X/46,XX or 45,X/47,XXX mosaics, 86 to have a structurally abnormal X and 13 to have a structurally abnormal Y chromosome. Molecular methods were used to look for cryptic X and Y chromosome mosaicism in patients with a 45,X constitution. Two cryptic X but no cryptic Y mosaics were detected. In 74% of the 45,X patients the X was maternal in origin. The i(Xq)s were approximately equally likely to involve the paternal or maternal chromosome, while the majority of deletions and rings and virtually all the abnormal Y chromosomes were paternal in origin. We suggest that the preponderance of paternal errors in Turner syndrome may result from the absence of pairing along the greater part of the XY bivalent during paternal mei I, which may make the sex chromosomes particularly susceptible to both structural and non-disjunctional errors during male gametogenesis.

Induction of caspase-3 protease activity and apoptosis by butyrate and trichostatin A (inhibitors of histone deacetylase): dependence on protein synthesis and synergy with a mitochondrial/cytochrome c-dependent pathway

The induction of apoptosis of tumor cells by the colonic fermentation product butyrate is thought to be an important mechanism in protection against colorectal cancer. Because a major action of butyrate is to inhibit histone deacetylase (leading to chromatin relaxation and altered gene expression), butyrate may induce apoptosis by derepression of specific cell death genes. Here we show that butyrate and trichostatin A (a more selective inhibitor of histone deacetylase) induce the same program of apoptosis in Jurkat lymphoid and LIM 1215 colorectal cancer cell lines that is strictly dependent on new protein synthesis (within 10 h) and that leads to the conversion of the proenzyme form of caspase-3 to the catalytically active effector protease (within 16 h) and apoptotic death (within 24 h). Cells primed with a low concentration of butyrate that itself did not induce activation of caspase-3 or apoptosis were, nevertheless, rendered highly susceptible to induction of apoptosis by staurosporine (an agent that has recently been shown to act by causing mitochondrial release of cytochrome c). Synergy between butyrate and staurosporine was due to the presence of a factor in the cytosol of butyrate-primed cells which enhanced over 7-fold the activation of caspase-3 induced by the addition of cytochrome c and dATP to isolated cytosol. We propose that changes at the level of chromatin structure, induced by a physiological substance butyrate, lead to the expression of a protein that facilitates the pathway by which mitochondria activate caspase-3 and trigger apoptotic death of lymphoid and colorectal cancer cells.

Expression of superoxide dismutase following axotomy

Oxidative injury has been implicated in the pathophysiology of neuronal injury and neurodegenerative disease. Antioxidant proteins provide an endogenous defense against such oxidative injury and may yield important clues to mechanisms of cytoprotection and neuronal recovery. Axotomy is the simplest model of neuronal injury and lesioning the sciatic nerve allows concurrent study of both motor (spinal cord) and sensory (dorsal root ganglia, DRG) neurons affected by the same injury. This study was designed to examine the expression of superoxide dismutase (SOD), an essential antioxidant protein, in motor and sensory neurons following complete axotomy of peripheral nerve. Immunocytochemical, quantitative immunoblot, and enzymatic activity assay techniques are used. By 12 days after axotomy, immunocytochemical expression of Mn-SOD is markedly increased in affected DRG and spinal cord. A similar increase in Cu/Zn-SOD is not seen in DRG or spinal cord. This immunocytochemical staining is associated with a significant increase in specific activity and Mn-SOD protein content as measured on quantitative immunoblots. This report suggests, for the first time, that Mn-SOD and not Cu/Zn-SOD increases in sensory neurons of the DRG and motor neurons of the spinal cord following distal axotomy of the sciatic nerve. Quantitative measurements of Mn-SOD following axotomy reveals that the increase in immunocytochemical reactivity is associated with an approximately 30% increase in specific activity when comparing lesioned and contralateral spinal cord samples. These data suggest that Mn-SOD may have a more significant role in the pathophysiology of neuronal injury than Cu/Zn-SOD.

Protein-protein interaction specificity of Im9 for the endonuclease toxin colicin E9 defined by homologue-scanning mutagenesis

The colicin DNase-specific immunity proteins interact with the endonuclease domain of the bacterial toxin colicin E9 with dissociation constants that span the millimolar to femtomolar affinity range. Among the non-cognate interactions Im2 shows the strongest binding toward the E9 DNase domain with a Kd of 10(-8) M, 6 orders of magnitude weaker than that of the cognate immunity protein Im9. Based on a NMR structure of Im9 that shows it to be a 4-helix protein, we have conducted a mutagenic scan in which elements of Im9 secondary structure were substituted into Im2 to precisely delineate regions that define specificity. Eleven chimeras were constructed, and their biological cross-reactivity toward colicins E2 and E9 was evaluated. From this set of mutants seven proteins were purified, and the Kd for their interaction with the E9 DNase domain was measured by a combination of stopped-flow fluorescence and subunit exchange kinetics. Our results show that immunity specificity is dominated by residues on helix II, accounting for 5 orders of magnitude binding specificity relative to Im2, and that packing interactions of helix II with its neighbor helix I and the loop connecting helix III with helix IV play minor roles. The conformational stability of these chimeric proteins was also determined. Proteins displaying an Im9 phenotype were all more stable than the parent Im2 protein, and surprisingly some chimeras were significantly more stable than either Im2 or Im9.

Safety of surfaces and equipment for children in playgrounds

BACKGROUND

The safety of playgrounds is important to protect children from injury, but studies are mostly done mainly under laboratory conditions without epidemiological data. We investigated the safety of different playground surfaces, and types and heights of equipment in public playgrounds in the City of Cardiff, UK.

METHODS

We did a correlational study of 330 children aged between 0 and 14 years. All children were hurt when playing in playgrounds in Cardiff and presented to the Accident and Emergency Department in Cardiff Royal Infirmary during summer (April to September) 1992 and 1993, and the whole of 1994. We studied the children's hospital records to establish the type of injury and interviewed their parents to find out the playground and type of equipment involved. The main outcome measures were the number of children injured whilst playing, and injury rates per observed number of children on different surfaces, types, and heights of equipment.

FINDINGS

Children sustained significantly more injuries in playgrounds with concrete surfaces than in those with bark or rubberised surfaces (p < 0.001). Playgrounds with rubber surfaces had the lowest rate of injury, with a risk half that of bark and a fifth of that of concrete. Bark surfaces were not significantly more protective against arm fractures than concrete. Most injuries were equipment related. Injury risk due to falls from monkey bars (suspended parallel bars or rings between which children swing) was twice that for climbing-frames and seven times that for swings or slides. The height of the equipment correlated significantly with the number of fractures (p = 0.005) from falls.

INTERPRETATION

Rubber or bark surfacing is associated with a low rate of injuries and we support their use in all public playgrounds. Bark alone is insufficient, however, to prevent all injuries, particularly arm fractures. Rubberised impact-absorbing surfaces are safer than bark. We believe that playing on monkeys bars increases the risk of injury in playgrounds and that they should generally not be installed. Safety standards should be based on physical and epidemiological data. Our data suggest that the proposed raising of the maximum fall height from 2.5 m to 3.0 m in Europe is worrying.

An analysis of Michigan and California CO remote sensing measurements

Remote sensing measurements of CO emissions from onroad vehicles were made in California in 1991 and in Michigan in 1992. It was determined that both fleets had a small linear increase in the high emitter frequency (vehicles emitting more than 4% CO) as a function of vehicle age for 1986 and newer model vehicles. Although high emitting vehicles were only a small minority of the fleet, they had a dominant impact on the mean CO and total CO emitted by the fleet. In Michigan, the highest emitting 5% of passenger cars generated 45% of the CO from cars. In California, the highest emitting 5% of passenger cars generated 38% of the CO from cars. There was a high correlation between the mean CO emitted by each model year of vehicle and the frequency of high emitting vehicles within the model year for both the Michigan and California fleets. The frequency of high emitters within any model year had no obvious relation to that model year's certification standards. The high emitter frequencies for vehicles less than nine years old were very similar for the California and Michigan fleets. An increase in the high emitter frequency in the ten-year-old and older Michigan passenger car fleet (relative to the California passenger car fleet), suggests, but does not conclusively demonstrate, that the rate of high emitters in Michigan and California is reduced by the inspection and maintenance (I/M) programs.

Identification of critical residues in the colicin E9 DNase binding region of the Im9 protein

1H-15N NMR studies, in conjunction with mutagenesis experiments, have been used to delineate the DNase-binding surface of the colicin E9 inhibitor protein Im9 (where Im stands for immunity protein). Complexes were formed between the 15 kDa unlabelled E9 DNase domain and the 9.5 kDa Im9 protein uniformly labelled with 15N. Approx. 90% of the amide resonances of the bound Im9 were assigned and spectral parameters obtained from 1H-15N heteronuclear single quantum coherence (HSQC) spectra were compared with those for the free Im9 assigned previously. Many of the amide resonances were shifted on complex formation, some by more than 2 p.p.m. in the 15N dimension and more than 0.5 p.p.m. in the 1H dimension. Most of the strongly shifted amides are located on the surfaces of two of the four helices, helix II and helix III. Whereas helix II had already been identified through genetic and biochemical investigations as an important determinant of biological specificity, helix III had not previously been implicated in binding to the DNase. To test the robustness of the NMR-delineated DNase-binding site, a selection of Im9 alanine mutants were constructed and their dissociation rate constants from E9 DNase-immunity protein complexes quantified by radioactive subunit exchange kinetics. Their off-rates correlated well with the NMR perturbation analysis; for example, residues that were highly perturbed in HSQC experiments, such as residues 34 (helix II) and 54 (helix III), had a marked effect on the DNase-immunity protein dissociation rate when replaced by alanine. The NMR and mutagenesis data are consistent with a DNase-binding region on Im9 composed of invariant residues in helix III and variable residues in helix II. The relationship of this binding site model to the wide range of affinities (Kd values in the range 10(-4) to 10(-16)M) that have been measured for cognate and non-cognate colicin DNase-immunity protein interactions is discussed.

[3Fe-4S] <--> [4Fe-4S] cluster interconversion in Desulfovibrio africanus ferredoxin III: properties of an Asp14 --> Cys mutant

The 8Fe ferredoxin III from Desulfovibrio africanus is a monomeric protein which contains two [4Fe-4S]2+/1+ clusters, one of which is labile and can readily and reversibly lose one Fe under oxidative conditions to yield a [3Fe-4S]1+/0 cluster. This 4Fe cluster has an S = 3/2 ground sping state insteaed of S = 1/2 in the reduced +1 state [George, Armstrong, Hatchikian and Thomson (1989) Biochem. J. 264, 275-284]. The co-ordination to this cluster is unusual in that an aspartate (Asp14, D14, is found where a cysteine residue normally occurs. Using a mutant protein obtained from the overexpression in Escherichia coli of a synthetic gene in which Asp14, the putative ligand to the removable Fe, has been changed to Cys, we have studied the cluster interconversion properties of the labile cluster. Analysis by EPR and magnetic-circular-dichroism spectroscopies showed that the Asp14 --> Cys (D14C) mutant contains two [4Fe-4S]2+/1+ clusters, both with S = 1/2 in the reduced state. Also, unlike in native 8Fe D. africanus ferredoxin III, the 4Fe <--> 3Fe cluster interconversion reaction was found to be sluggish and did not go to completion. It is inferred that the reversibility of the reaction in the native protein is due to the presence of the aspartate residue at position 14 and that this residue might protect the [3Fe-4S] cluster from further degradation.

New non-peptide endothelin-A receptor antagonists: synthesis, biological properties, and structure-activity relationships of 5-(dimethylamino)-N-pyridyl-,-N-pyrimidinyl-,-N-pyridazinyl-, and -N-pyrazinyl-1-naphthalenesulfonamides

Use of automated synthesis led to the discovery of several 6-membered nitrogen heterocycles as replacements for the N-isoxazolyl substituent present in the 1-naphthalenesulfonamides endothelin-A (ETA) antagonist 5-(dimethylamino)-N-(3,4-dimethyl-5-isoxazolyl)-1-naphthalenesu lfo namides (BMS 182874). In each of these heterocycles, a small substituent such as halogen para to the position of attachment to the sulfonamide nitrogen atom was found to be advantageous for ETA receptor affinity. Of these heterocycles, 2-pyrazines offered the greatest scope for improving receptor affinity. Optimization of the substituents at the 3- and 5-positions in the pyrazine ring led to potent, ETA-selective compounds such as 5-(dimethylamino)-N-(5-chloro-3-methoxy-2-pyrazinyl)-1- naphthalenesulfonamides (7m, ETA pIC50 8.1). When dosed orally at 10 mg/kg to conscious, normotensive rats infused with big ET-1, compounds such as 7m showed significant inhibition of the pressor response with a duration of effect lasting for the 5-h course of the experiment.

Homeosis and intestinal tumours in Cdx2 mutant mice

In Drosophila, disturbing the expression of the homeobox gene caudal causes a severe disruption in body segmentation and global body patterning. There are three mouse homologues of Drosophila caudal: Cdx1 (ref. 2), Cdx2 (ref. 3) and Cdx4 (ref. 4). We have generated a null mutation of murine Cdx2 by homologous recombination. Cdx2 homozygote null mutants die between 3.5 and 5.5 days post coitum (d.p.c.). Cdx2 heterozygote mutants exhibit a variable phenotype, with many showing tail abnormalities or stunted growth. Skeletal analysis demonstrates a homeotic shift of vertebrae and compatible malformations of the ribs. Within the first three months of life, 90% of Cdx2 heterozygotes develop multiple intestinal adenomatous polyps, particularly in the proximal colon. These polyps occasionally contain areas of true metaplasia. In contrast to the surrounding intestinal epithelium, the neoplastic cells do not express Cdx2 from the remaining allele. These results suggest that Cdx2 mutation is the primary event in the genesis of some intestinal tumours.