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T S Stewart

Publications and source records attributed to T S Stewart.

At least 19 recordsLinked to original sources
PubMed2002

Student perceptions of tutor skills in problem-based learning tutorials.

OBJECTIVE: The problem-based learning (PBL) tutor plays a role that is different from the role of a teacher in a conventional teaching format. In the Faculty of Medicine and Health Sciences, United Arab Emirates, all students are Arab nationals and tutors are expatriates with different sociocultural backgrounds from the students. This study was designed to investigate how students evaluate tutors in PBL tutorials and whether student evaluations of tutors change with the progress of students in PBL tutorials. METHODS: Differences in tutor performance evaluation by male and female students were also analysed. The students evaluated 12 tutor skills in a scale of 1-3, 1 being 'below average' and 3, 'outstanding'. Student responses from a total of 314 (98.1%) completed forms collected over 2 academic years were analysed statistically. A total of 14 tutors participated in the PBL programme. RESULTS: The analysis revealed that tutors as a group were rated as having average to outstanding tutor skills in 10 items of the evaluation form. Students and faculty perceptions were different for the tutor skills of guiding students for information management. The students expected more support from tutors, whereas the tutors tried to emphasize self-learning in the PBL curriculum. Lower scores to the tutors in the 'problem' bringing sociocultural and religious issues for discussion showed that a gap in sociocultural/religious understanding between students and tutors might influence tutor skills. CONCLUSIONS: Differences in tutor evaluation by male and female students indicate necessity of adopting different strategies by tutors in a different sociocultural background. The results of the study have direct implications for faculty development.

Attitude↗
PubMed1999

Antimalarial antisense activity of hexitol nucleic acids.

Antisense oligonucleotides and ribozymes have shown promise both as antimalarial agents and as tools for identifying genes vital for parasite survival. This task is urgent due to the ineffectiveness of current drug regimes on the most virulent human malarial parasite, Plasmodium falciparum. The development of new ways to modify and/or protect conventional phosphodiester oligonucleotides to improve nuclease resistance is also important. We assessed the effect of antisense oligonucleotides containing phosphorylated anhydrohexitols in suppressing the growth of P. falciparum in culture. The modified oligonucleotides were able to inhibit parasite growth in a sequence-specific manner, but not as well as the phosphorothioated antisense oligonucleotides, which are effective antimalarials at submicromolar concentrations. Two reasons are suggested: the absence of RNase H activation and differences in membrane transport.

Animals↗
PubMed1998

Molecular cloning and characterization of the Plasmodium falciparum cytidine triphosphate synthetase gene.

Using degenerate oligonucleotides derived from conserved amino acid regions of cytidine triphosphate synthetase, a fragment of the gene from the malarial parasite, Plasmodium falciparum, was isolated by polymerase chain reaction (PCR). This fragment was used as a probe in the isolation of genomic clones containing the entire pfCTP synthetase coding region (2580 bp). The gene encodes the largest CTP synthetase found in any organism to date due to the presence of two additional sequences which are part of the continuous open reading frame and are not introns as their presence in the mRNA was confirmed by reverse transcriptase-PCR. These features distinguish the parasite enzyme from that of the host making it an attractive target for structure based drug design.

Amino Acid Sequence↗
PubMed1998

Cloning and expression of a prokaryotic enzyme, arginine deiminase, from a primitive eukaryote Giardia intestinalis.

Arginine deiminase (EC 3.5.3.6) catalyzes the irreversible catabolism of arginine to citrulline in the arginine dihydrolase pathway. This pathway has been regarded as restricted to prokaryotic organisms but is an important source of energy to the primitive protozoan Giardia intestinalis. In this paper we report the cloning and expression of the arginine deiminase gene from this parasite. Degenerate oligonucleotides based on amino acid sequences of tryptic peptides from the purified protein were used to amplify a portion of the arginine deiminase gene. This was then used as a probe to screen HindIII and PstI "mini" libraries to obtain two overlapping clones that contained the arginine deiminase gene. The open reading frame encoded 581 amino acids including all of the tryptic peptides that were sequenced and corresponded to a molecular mass of 67 kDa. Northern blot analysis identified a single 1.8-kilobase transcript in both trophozoites and encysting cells. Arginine deiminase was successfully expressed in Escherichia coli and purified to homogeneity. The recombinant protein was found to have characteristics comparable with those of the native enzyme.

Amino Acid Sequence↗
PubMed1997

Inhibition of Plasmodium falciparum proliferation in vitro by ribozymes.

Catalytic RNA (ribozymes) suppressed the growth of the human malarial parasite Plasmodium falciparum in vitro. The phosphorothioated hammerhead ribozymes targeted unique regions of the P. falciparum carbamoyl-phosphate synthetase II gene. The P. falciparum carbamoyl-phosphate synthetase II gene encodes the first and limiting enzyme in the pathway, and its mRNA transcript contains two large insert regions absent in other carbamoyl-phosphate synthetases, including that from humans. These inserts are ideal targets for nucleic acid therapy. Exogenous delivery of ribozymes to cultures reduced malarial viability up to 55% at 0.5 microM ribozyme concentrations, which is significantly greater than control levels (5-15% reduction), suggesting a sequence-specific inhibition. This inhibition was shown to be stage-specific, with optimal inhibitions being detected after 24 h, coincident with maximal production of the carbamoyl-phosphate synthetase enzyme in the course of the life cycle of the parasite. A decrease in total carbamoyl-phosphate synthetase activity was observed only in cultures treated with the ribozymes. The task of developing alternative therapeutic agents against malaria is urgent due to the evolution of drug-resistant strains of P. falciparum, the most virulent of all human malarial parasites. Another critical issue to be addressed is the possibility of eliminating or reducing any systemic toxicity to the host, which can potentially be provided by nucleic acid therapy. This work is the first reported assessment of the ability of ribozymes as antimalarials. Ribozyme inhibition assays can also aid in identifying important antimalarial loci for chemotherapy. The malarial parasite can, in turn, be a useful in vivo host to study the catalysis and function of new ribozyme designs.

Animals↗
PubMed1997

Long-term cultivation of Plasmodium falciparum in media with commercial non-serum supplements.

Continuous cultivation of both drug-sensitive and drug-resistant strains of Plasmodium falciparum was achieved using a mixture of commercially available serum substitutes. This provides easier standardisation of experimental data and minimises the risk of infection from handling of human serum. In addition, Nutridoma SR and AlbuMAXI are readily available and are biologically defined. Parasites grown in serum-free medium are morphologically and metabolically identical to those cultivated in medium supplemented with 10% human serum.

Animals↗
PubMed1996

Isolation, characterization and expression of the gene encoding cytidine triphosphate synthetase from Giardia intestinalis.

The cytidine triphosphate synthetase gene from Giardia intestinalis was cloned using a PCR-based strategy. A 519 bp PCR product was obtained from the amplification of genomic DNA using two oligonucleotides derived from the CTP synthetase amino acid consensus sequences DPYINVDPG and KTKPTQ. This product was used to probe restriction endonuclease digested genomic DNA and the respective plasmid mini-libraries. Two genomic clones were obtained one with a 3.6 kb HindIII DNA fragment, containing approximately three-quarters of the 5'-end of the synthetase gene and subsequently, a 5.8 kb PstI DNA fragment which contained the whole gene. The intronless gene has a 1863 bp open reading frame encoding 620 amino acids (M(r) of 68.3 kDa). A well conserved catalytic glutamine aminotransferase (GAT) domain was identified. In addition, three insert sequences were found which are not present in CTP synthetase from other species. Alignment and comparison of the deduced amino acid sequence relative to CTP synthetases from other species revealed a high degree of identity (34%) with a greater resemblance to prokaryotes than eukaryotes. The gene is located on chromosome 6 and the messenger RNA encoding it is estimated to be 1.9 kb. The coding region of G. intestinalis CTP synthetase was generated by PCR and subsequently cloned into the pQE30 vector for expression in E. coli. This construct yielded a soluble and enzymatically active recombinant protein which was purified by a Ni-NTA affinity column. The purified recombinant protein had a subunit molecular weight of 69.5 kDa and a native molecular weight of approximately 274 kDa. Kinetic studies of the partially purified recombinant G. intestinalis CTP synthetase gave apparent K(m) values of 0.1 mM and approximately 0.5 mM for the substrates UTP and L-glutamine respectively in accord with previously reported values for the native enzyme.

Amino Acid Sequence↗
PubMed1994

A cytidine triphosphate synthetase gene in Plasmodium falciparum.

The malarial parasite Plasmodium falciparum is dependent on de novo synthesis for its pyrimidine nucleotide requirements. However, the activity of the key enzyme in cytidine nucleotide synthesis, CTP synthetase (EC 6.3.4.2), has not been reported. We present evidence for a CTP synthetase gene in P. falciparum, having isolated a PCR product obtained using 2 primers derived from the CTP synthetase amino acid consensus sequences DPYINVDPG and GICLGMQ. The amplified DNA segment encodes an amino acid sequence with considerable homology to CTP synthetases from several other species including human and yeast.

Amino Acid Sequence↗
PubMed1994

Optimal contemporary group structure to maximize genetic progress through genetic evaluation of swine.

Accuracies of sire EPD were calculated for a typical growth trait and a typical maternal trait for alternative contemporary group structures. For a given family size (number of progeny tested for a sire in a contemporary group), accuracy increased as the number of sires increased and as the number of contemporary groups increased. An exponential equation was found to best predict accuracy from the number of sires and groups. Partial derivatives can be used to determine the optimal number of sires and groups for a given economic situation (fixed number of animals tested per group). It is recommended that progeny of at least two sires be represented in each contemporary group, but having more than five sires does not effectively increase accuracy further. Accuracy increases with a larger herd size, as more sires and more groups of pigs are tested. When the number of litters that can be tested is at its limit, accuracy is maximized with a small number of sires, and progeny divided among several contemporary groups. However, accuracy is only part of the herd's genetic improvement. Selection decisions must be made to avoid inbreeding, which can be a problem, particularly in small herds.

Animals↗
PubMed1991

Subfamilies of serine tRNA genes in the bovine genome.

A bovine tRNA gene cluster has been characterized and the sequences of four tDNAs determined. Two of the tDNAs could encode tRNA(SerIGA), one tRNA(SerUGA), and the fourth tRNA(GlnCUG). The three serine tDNAs representing the UCN codon isoacceptor family are almost identical. However, the sequence of the tDNA(SerTGA) differs from a previously sequenced bovine tDNA(SerTGA) at 12 positions (ca. 14%). This finding suggests that in the bovine genome, two subfamilies of genes might encode tRNA(SerUGA). It also raises the possibility that new genes for a specific UCN isoacceptor might arise from the genes of a different isoacceptor, and could explain previously observed differences between species in the anticodons of coevolving pairs of tRNAs(SerUCN). The gene cluster also contains complete and partial copies, and fragments, of the BCS (bovine consensus sequence) SINE (short interspersed nuclear element) family, six examples of which were sequenced. Some of these elements occur in close proximity to two of the serine tDNAs.

Amino Acid Sequence↗
PubMed1991

A simple method for the purification of mitochondrial DNA from Plasmodium falciparum.

Mitochondrial DNA (mtDNA) from Plasmodium falciparum was isolated by conventional differential centrifugation in an SS34 rotor, a simpler method than CsC1 centrifugation of total DNA as employed by other workers. The nature of the sample was verified by sequencing a polymerase chain reaction (PCR) product obtained using oligonucleotide primers derived from known malarial mtDNA sequence.

Animals↗
PubMed1991

Efficacy of alternative multivariate best linear unbiased prediction models for genetic evaluation of swine.

A comparison of the accuracy of alternative BLUP evaluations of swine performance data is reported. A simulated data set of performance for days to market, backfat, number born alive, number weaned, and litter weight in six herds was used for the evaluation. The data structure was derived by using the performance records of six herds sampled from the American Yorkshire Club's STAGES (Swine Testing and Genetic Evaluation System) database. For growth traits, 10,360 pig records from 129 sires and 897 dams were recorded. For maternal traits, records on 2,598 litters of 1,209 sows from 147 sires and 585 dams were included. The actual observed performance of each record was removed and replaced with simulated performance. These simulated data were analyzed by within- and across-herd BLUP models using STAGES and Animal Model (AM) procedures. Results indicate that the alternative BLUP procedures produced similar estimates. Correlations between STAGES and AM EPD ranged from .84 to .95. Correlations between STAGES EPD and true genetic value (G) ranged from .41 to .74, and correlations between AM EPD and G ranged from .40 to .74. On average, AM EPD had a .04 larger correlation with G than did STAGES EPD, although the difference in the correlations was not significant (P greater than .05). Trends in EPD for sires, dams, and pigs or sows were the same. Likewise, standard errors of prediction for AM EPD averaged 4% smaller than those for STAGES EPD. Computationally, the AM procedures used 15 to 20 times as much processing time as did STAGES procedures.

Animals↗
PubMed1991

Genetic improvement programs in livestock: swine testing and genetic evaluation system (stages).

Genetic evaluations for the U.S. swine industry are conducted by the eight purebred associations of the National Association of Swine Records. Within-herd evaluations of the growth traits (days to 105 kg [market] and backfat depth) were first reported in 1986. Analyses of the maternal traits (litter size at birth and weaning, and litter 21-d weight) were inaugurated in 1987. Expected progeny differences (EPD) are reported for all traits and for general, paternal, and maternal bioeconomic indexes. A sow productivity index combining only maternal traits is available. All records are adjusted according to National Swine Improvement Federation (NSIF) guidelines for effects such as number of pigs transferred at crossfostering and age at recorded observation prior to the BLUP evaluation. Within-herd analyses of individual contemporary groups are conducted immediately on receipt of performance records at each breed association office. All parents in the herd and the young pigs in the current group are evaluated. A report is returned to the breeder for use in herd selection and the EPD are placed in the pedigree file. The genetic base of each herd is defined as the first n tested pigs or litters, where n is the number of pigs registered annually within the herd. Change in mean EPD between groups is indicative of genetic trend. Periodic across-herd analyses are used to update interim within-herd analyses and a national sire summary is published.

Animals↗
PubMed1990

Saturation mutagenesis of the Drosophila tRNA(Arg) gene B-Box intragenic promoter element: requirements for transcription activation and stable complex formation.

Transcription of eukaryotic tRNA genes is dependent on the A- and B-Box internal control regions (ICRs) and the upstream transcription modulatory region. The B-Box ICR spans nucleotides 52 to 62 and directs the primary binding of transcription factor C as the first step in the formation of a transcription complex. The conservation of the sequence of the B-Box in all tRNA species reflects its importance in both the expression of the gene and the processing, structure and function of the gene product. In order to identify the nucleotides essential to the promoter function of the B-Box ICR, site-directed mutagenesis was used to generate all the possible single point mutations at positions 52 to 58, 61 and 62 of a Drosophila melanogaster tRNA(Arg) gene. The effect of these mutations on gene transcription was evaluated using in vitro transcription and template exclusion competition assays. Optimal activity was displayed by the wild type tDNA(Arg) B-Box sequence but several other sequences supported in vitro transcription at wild type levels. The majority of mutants, however, showed lower efficiency in the in vitro transcription assay. Of the single point mutations, those at positions 53, 55, and 56 had a critical effect on gene function in Drosophila and HeLa transcription extracts and transcription factor interaction most likely requires base contacts at these positions. Since the effect of several of the point mutations cannot be explained in terms of possible major or minor groove contributions the possibility is raised that local DNA geometry also is an important determinant in specifying B-Box function.

Animals↗
PubMed1990

Effects of body composition, pre- and postpartum energy intake and stage of production of energy utilization by beef cows.

Mature Charolais x Angus cows (n = 128) were adjusted to a body condition score (BCS) of 3 (1 = very thin, 3 = moderate, 5 = very fat) between 130 and 190 d of gestation. When cows averaged 190 d of gestation, they were assigned randomly to a maintenance energy diet (ME) or to a diet very low in energy (LE). Cows were allotted within these prepartum (PRP) diets to a high-energy (HE) or LE diet at parturition. At 30 d postpartum (PP), cows were allotted to four treatments: a) slaughter at 0 (n = 32) or b) 48 h after calf removal (n = 32), c) cows whose calves were early weaned at 30 d PP (n = 32) or d) normally weaned at 205 d PP (n = 32). Low energy PRP intake reduced (P less than .01) BCS, cow weight, total empty body lipid (TEBL), body energy (BE) and daily predicted maintenance energy (PME)/W.75 by parturition. Pre- and postpartum energy intake combined to affect (P less than .10) these same factors in a similar manner at 30 d PP. From 30 to 60 d PP, early weaning reduced (P less than .05) PME/W.75. Cow BCS at 190 d of gestation had little effect (P greater than .10) on PME/W.75 or PME/Mcal of BE. Body condition score at parturition, however, affected (P less than .05) delta BCS, delta BE and PME/Mcal of BE by 30 and 60 d PP, with thinner cows losing less condition and requiring more PME/Mcal of BE. Cow BCS had a similar effect from 30 to 60 d PP. Total daily PME was greater for the fat cows during both the PRP and PP periods. Body composition, PRP and PP energy intake and early weaning affect energy requirements and energetic efficiency of beef cows.

Analysis of Variance↗