OpenUCT :: Browsing by Author "Thomson, Jennifer Ann"

Browsing by Author "Thomson, Jennifer Ann"

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Open Access
The analysis of an enzyme (Ce1A) and a gene system (abg) involved in the utilization of lignocellulose in the rumen
(1996) Brown, Gordon Douglas; Thomson, Jennifer Ann
As lignocellulose represents an abundant renewable resource, research is in progress to obtain a better understanding of the natural mechanisms whereby this resource is utilised. Of particular interest is the degradation of forage in the rumen and one research goal is to ultimately increase animal productivity through an improvement in lignocellulose utilisation. However, although the mechanisms behind lignocellulose utilisation are reasonably well understood, relatively little is known about the mechanisms which occur in the rumen. Thus, the aim of this thesis was to gain more insight into the mechanisms of lignocellulose utilisation which occur in the rumen. Initially this research was focused on the poorly characterised exo-acting cellulases from rumen bacteria. Preliminary enzymology studies on one cellulase from Ruminococcus flavefaciens FD-1, previously isolated in this laboratory, indicated that an exo-acting cellodextrinase, CelA, had been isolated. In this report, the enzyme was purified and biochemically characterised and was shown to be an exo-acting cellodextrinase.
Open Access
Analysis of genes and enzymes involved in the degradation of hemicellulose and cellulose by Butyrivibrio fibrisolvens H17c
(1992) Lin, Long-Liu; Thomson, Jennifer Ann
B. fibrisolvens H17c is a Gram-positive obligate anaerobe which has been found in the rumen of most ruminants. Strains of B. fibrisol vens have been reported to exhibit activity toward cellulosic and hemicellulosic substrates. The aim of this thesis was to screen a genebank of B. fibrisolvens H17c DNA and to isolate genes expressing cellulase and xylanase activity. Two genes encoding β-1-4- glucosidase (BglA) and endo-β-1-4-xylanase (XynB) were cloned in E. coli.
Open Access
Biological control of a plant pathogen and pest by expression of a cloned Serratia marcescens chiA gene and Bacillus thuringiensis cryIA(c) gene in endophytic bacteria
(1997) Downing, Katrina Jo; Thomson, Jennifer Ann
Protection of plants from pathogens and pests by introduced microorganisms provides an alternative to environmentally hazardous chemical pesticides and fungicides. Plant associated, free living, non-pathogenic bacteria found in the rhizosphere and phyllo plane have been the emphasis of research on biological control· agents. These regions however pose problems of competition between introduced microorganisms and native microflora and environmental extremes. Endophytic bacteria, present in the interior regions of healthy plants, offer a solution to these problems. Since little work has been done to date with endophytes, the work reported in this thesis comprises a novel approach to achieving biological control of a plant pest and pathogens .An endophytic strain of Pseudomonas fluorescens was isolated from micro propagated apple plantlets and shown to be present in the roots of beans at a level of 1.2 x 10⁵ CFU/g fresh weight 10 days after introduction. Generation of spontaneous rifampicin resistant mutants of this strain resulted in P. fluorescens Rifl. Isolates of P. fluorescens and Aeromonas caviae were recovered from the interior regions of surface sterilized bean seeds. In addition, two sugarcane endophytes, Acetobacter diazotrophicus and Herbaspirillum seropedicae, which has also been isolated from rice, maize and sorghum, as well as an endophytic Citrobacter sp. of pine seeds and bean plants were used in this work. The gene coding for the major chitinase of S. marcescens, chiA, was cloned under the control of the tac promoter by PCR amplification. The gene contained the endogenous Shine Dalgarno sequence 7 bases upstream of the ATG start codon in the plasmid pTC33. The plasmid ptacchiA had previously been constructed with a distance of 20 bases between the ribosome binding site of this vector and the A TG start codon of the gene. Gene expression of chiA carried on pTC33 was shown to be approximately 16-foldhigher than that of ptacchiA. This clearly illustrated that the distance between the Shine Dalgarno sequence and ATG start codon was critical and that the shorter distance of 7 bases significantly increased the translation efficiency of chiA. The first and second generation tacchiA cassettes from ptacchiA and pTC33 respectively were cloned into the broad host range plasmids pKT240, pDER405 and pML122 and into the integration vector pJfF350.These plasmids were introduced into the endophytes P.fluorescens Rift, H. seropedicae and Citrobacter sp.AJl by conjugative transfer and electroporation and were shown to express the gene at varying levels. pKTCl and pMTC33 carrying the 1 and 2nd generation tacchiA cassettes on pKT240 and pML122respectively were not stably maintained in P. fluorescens Rift or H. seropedicae.
Open Access
Characterisation of XvPrx2 : a type II peroxiredoxin isolated from the resurrection plant Xerophyta viscosa (Baker)
(2006) Govender, Kershini; Mundree, Sagadevan G; Thomson, Jennifer Ann
Knowledge of the biochemical and molecular mechanisms by which plants tolerate environmental stresses is necessary for genetic engineering approaches to improve crop performance. A unique feature of resurrection plants, such as Xerophyta viscosa, is their ability to cope with severe water loss of greater than 90%. A full-length cDNA library was synthesised from a cold stressed X viscosa plant. Sequencing and BLAST analysis revealed the identity of sixty genes. A type 2 peroxiredoxin (XvPrx2) was selected for further analyses as it was observed, by northern analyses, to be stress-inducible. The XvPrx2 protein was confirmed to be involved in the stress response by Western analyses. The XvPrx2 gene, which displays highest identity to a rice orthologue, has an open reading frame of 162 amino acids, and codes for a hydrophilic polypeptide of 162 residues with a predicted molecular weight of 17.5 kDa. The XvPrx2 polypeptide displays significant identity with other plant type II Prxs, with an absolutely conserved amino acid sequence proposed to constitute the active site of the enzyme (PGAFTPTCS). The XvPrx2 protein has a single cataly1ic cysteine residue at position 51 similar to Prxs from Oryza sativa and Candida boidinii. A mutated protein (XvV76C) was generated by converting the valine at position 76 to a cysteine resulting in a conformational change as determined by limited proteolysis. An in vitro DNA protection assay showed that, in the presence of either XvPrx2 or XvV76C, DNA protection occurred. In addition, an in vivo assay showed that increased protection was conferred on cell lines over-expressing either XvPrx2 or XvV76C. Several upstream promoter regions were identified for the XvPrx2 gene using the splinkerette method. Southern and two dimensional gel analyses revealed that multiple XvPrx2 homologues exist within the X viscosa genome. These homologues have similar pI values to Arabidopsis orthologues. Immuno-cytochemical data revealed that XvPrx2 is localised to the chloroplast, however, this could be attributed to cross reactivity with a chloroplastic homologue. Using YFP technology, the protein was observed to be expressed in the cytosol, and this location is supported by the absence of an upstream targeting signal in the XvPrx2 sequence. The XvPrx2 activity was maximal with DTT as electron donor and HzOz as substrate with t-BOOH being the next preferred. Using Trx£. coli a 2-15 fold lower enzyme activity was observed. The XvPrx2 activity with GSH was significantly lower and Grx had no measurable effect on this reaction. The XvV76C protein displayed significantly lower activity compared to XvPrx2 for all substrates assessed. Enzymatic kinetic parameter values determined for XvPrx2 using DTT as electron donor and HzOz as substrate were: Km = 45 IlM, V max = 278 Ilmol min-I.mg-I protein, kcat 6.173 x 103 s-1 and kcaJKm = 0.136 X 103 IlM-1.s-l. Based on knowledge-based models of XvPrx2 and XvV76C no structural differences were observed between the two molecules.
Open Access
Characterization of endoglucanase cela from the rumen bacterium Clostridium longisporum
(1994) Mittendorf, Volker; Thomson, Jennifer Ann
Cellulose is the most abundant organic compound on earth and offers great potential as a source of renewable energy and other chemicals. Cellulases are being studied to elucidate the enzymatic degradation of cellulose. We are interested in the molecular mechanisms of microbial degradation of cellulose in the rumen. The long-term aim is the potential genetic modification of lignocellulolytic activities in the rumen. · Clostridium longisporum was obtained from Varel (1989) because oxygen-resistant endospores might be suitable "vectors" for the introduction of genetically modified enzyme systems into the rumen via animal feeds. It is a sporadically occurring rumen bacterium and its role in ruminal cellulolysis is unclear. The aim of this project was the initial characterization of cellulases produced by C. longisporum. The celA gene was obtained by screening a library of C. longisporum genomic DNA in Escherichia colifor clones expressing CMCase activity. Approximately 38 CMCase-positive clones were obtained and the plasmid pCM4 was isolated from the clone expressing the highest activity. Southern analysis indicated that another plasmid, pCM64, contained a larger insert including the insert of pCM4. A total of 3620 bp were sequenced and a 1548-bp open reading frame, termed celA, was found. This gene showed homology with other endo-B-1,4-glucanases from family 5 (Henrissat & Bairoch, 1993). Plasmid pCM64 was found to contain the whole celA gene encoding endoglucanase CelA, while pCM4 has a 5'-truncated gene, termed celM5', which encodes a fusion protein, CelMN', that was initiated from an ATG codon in the vector. Sequence analysis of celA revealed the presence of a type I cellulose-binding domain (Beguin & Aubert, 1994) at the COOH-terminus of CelA.
Open Access
Cloning and characterisation of a bZIP transcription factor from a resurrection grass, Eragrostis nindensis
(2004) Brocklehurst, David; Farrant, Jill M; Mundree, Sagadevan G; Thomson, Jennifer Ann
The G-box is a plant DNA cis-acting element involved in the regulation of gene expression in response to a range of environmental signals including anaerobiosis, dehydration and light as well as by abscisic acid (ABA). Basic leucine zipper (bZIP) transcription factors have been shown to specifically bind and activate transcription from G-boxes in a dimerized form. A 1.5 kb cDNA for a bZIP class transcription factor, designated EnGBF1, was cloned from a desiccation-tolerant grass, Eragrostis nindensis by degenerate RT-PCR.
Open Access
Cloning and expression of a modified oryzacystatin inhibitor gene and an investigation of its inhibitory capabilities
(1997) Haworth, Caroline Joanne; Thomson, Jennifer Ann; Maeder, Dennis
Cysteine proteinase inhibitors have shown potential as biocontrol agents for the protection of plants against insect and pathogen attack. With the advent of protein and genetic engineering such inhibitors can now be modified in order to improve their effectiveness. Because cystatins have already been isolated from plants. they provide a good starting point for developing modifications which may improve their function as biocontrol agents. The purpose of this project, therefore, was to design a potentially improved analogue of the rice cysteine proteinase inhibitor, oryzacystatin I, through molecular modelling studies. The gene sequence for this modified protein was then synthesised and expressed for kinetic analysis and insect trial assays. A prediction of the oryzacystatin I (OC I) tertiary structure was made using Biograf software on an Evans and Sutherland workstation. This structure was based on the known structures of stefin B and chicken cystatin ho se co-ordinates are published in the Brookhaven data files. Chicken cystatin is one of the most potent inhibitors of papain in the cystatin superfamily. This is believed to be due, in part, to an increased binding of the cystatin to papain through its amino-terminal region with the residues Leu7 to Gly9 playing a particularly important role.
Open Access
The development of transgenic plants resistant to cucumber mosaic virus and tobacco necrosis virus
(1994) Hackland, Andrew F; Thomson, Jennifer Ann; Rybicki, Edward P
Cucumber mosaic virus (CMV) and tobacco necrosis virus (TN V) often occur in mixed virus infections in South Africa. Both viruses are of economic importance because of their world-wide distribution, extensive host range and their effects on yields of agriculturally important crop plants. The complete cDNA sequences of CMV-Wemmershoek (CMV-Wem) coat protein (CP) and TNV-F5P CP genes were cloned and subjected to sequence analysis. CMV-Wem is closely related to CMV-WL and CMV-Q, and therefore falls into CMV subgroup II. Similar analysis showed that TNV-F5P is closely related to TNV-A. By characterizing and sequencing these clones the authenticity of the CMV and TNV CP genes was also determined, prior to sub cloning into the appropriate vectors for expression in E. coli and tobacco. Constructs containing both the full-length CP genes of CMV-Wem and TNV-F5P were subcloned in frame with the malE gene, encoding the maltose binding protein (MBP), in the IPTG-inducible pMALTM vector system, and expressed in E. coli. Through immunological detection the authenticity of both CPs was confirmed. The CMV CP translation product expressed in E.coli was used as an antigen to raise antiserum free from contaminating plant host-specific antibodies. The CP genes of both viruses were individually cloned in both orientations (sense and antisense) in Agrobacterium tumefaciens Ti-plasmid-based binary and cointegrate vectors. The study was then extended to include engineering doubly transgenic plants. In order to determine whether the full-length CP is required to mediate virus resistance, a truncated form of the TNV CP was generated by deleting 83 amino acids from the C-terminus. Transgenic Nicotiana tabacum cv Petit Havana SRl plants containing one of a number of different forms of CMV and TNV CP nucleotide sequence were generated. In whole plant studies, mechanical inoculation of Ro lines with CMV-Wem resulted in more than 50% of the CMV CP-sense (CP+) and CP-antisense plants not developing visible systemic disease symptoms. In both the CMV CP+ and doubly transgenic plants CMV-Wem accumulation was delayed, but virus was found to accumulate in the inoculated leaves over time. The CMV CP+ lines showed excellent protection against CMV-Q, but showed only a delay in symptom production when inoculated with CMV -Y, from subgroup I.
Open Access
Elucidation of the osmoregulatory locus, ompRZ, in Erwinia chrysanthemi
(1999) Adams, Craig Hadley; Qhobela, Molapo; Thomson, Jennifer Ann
Bacteria are constantly faced with harsh environmental conditions to which they have to adapt. These adaptive mechanism generally involve the use of two-component sensory systems, comprising of sensor proteins interacting with their cognate response regulator proteins. To survive fluctuating environments such as osmotic conditions, certain bacterial species employ the ompR-envZ (ompB) two-component system to monitor and respond to the osmotic cue. The EnvZ protein functions as the sensor and relays information regarding changes to the external environment, to the response regulator, OmpR. OmpR, in turn, regulates the porins, OmpF and OmpC in a reciprocal manner, so that one porin predominates over the other, depending on osmotic conditions. Erwinia chrysanthemi, which causes "soft rot" in a wide range of economically important crops, has been demonstrated to contain porin-like proteins similar to OmpF and OmpC. The expression of these porins was regulated in a similar manner to OmpF and OmpC with respect to medium osmolarity. Furthermore, preliminary studies have shown that changes in osmolarity affect the expression of pathogenecity genes. Evidence for an osmoregulatory system analagous to the ompB system of Escherichia coli was, therefore, sought. Primers specific for conserved regions in ompR were designed and used to PCR amplify a 631 bp fragment from E. coli. This fragment was cloned into the vector, pBluescriptSk, and end-sequenced to confirm its authenticity. The same strategy was followed, using envZ-specific primers to generate an E. coli envZ clone. Southern hybridisation analyses, using an ompR probe, confirmed the presence of an ompR homologue in E. chrysanthemi. An E. chrysanthemi genomic library was thus constructed and screened and a clone homologous to the ompR probe was isolated. The resulting plasmid, pRZ69, was partially characterised and determined to have both envZ and ompR homologues resident. Southern hybridisation analyses were employed to localise the ompR and envZ genes on the plasmid. A 1200 bp EcoRV-Pst1 fragment containing the ompR homologue and a 2000 bp EcoRV-EcoRV fragment containing the envZ homologue, were subcloned into pBluescriptSk, generating the plasmids, pRS1 and pZS2 respectively.
Open Access
Functional Analysis of the Novel Stress- Inducible XVPSAP promoter isolated from Xerophya Viscosa
(2009) Oduor, Okoth Richard; Thomson, Jennifer Ann
Open Access
Gene expression associated with drought tolerance in Xerophyta viscosa Baker
(2000) Ndima, Tozama Beauty; Mundree, Sagadevan G; Farrant, Jill M; Thomson, Jennifer Ann
Herophyta viscosa (Baker) is a monocytyledonous resurrection plant that can tolerate extremes of dessication. Upon rewatering, it rehydrates completely and assumes its full physiological activities. Studies on changes in gene expression associated with dehydration stress tolerance were conducted. A cDNA library constructed from m RNA isolated from dehydrated (85%, 37% and 5% relative water content) X. viscosa leaves, was differently screened. Of the 192 randomly selected cDNAs screened, 30 showed higher expression levels when X. viscosa was dehydrated while 20 showed lower expession. XVLEA, XVDH and XVLEC represent three cDNAs that were upregulated during dehydration stress. XVLEA showed the highest identity at the amino acid level with a late embryogenesis abundant protein, LEA29G, from Gossipium hirsutum (30%) and LEA D-29 from cotton (50%). XVDH exhibited significant identity to dehydrin proteins from Arabidopsis thaliana (45%) and Pisum sativum (43%) at the amino acid level. It encodes a glycine-rich protein (27kDa) which is largely hydrophilic and contains a hydrophobic segment at the C-terminus. XVLEC showed 28% identity and 50% similarity to a lectin-like protein from Arabidopsis thaliana. Southern blot analysis confirmed the presence of the three cDNAs in the X.viscosa genome. Both XVLEA and XVDH transcripts were highly expressed during dehydration- (37% RWC) and rehydration (4%, 32%, 72% RWC) treatment of the plant ͌ 1.0kb was observed. However, with XVDH a transcript of ͌ 1.0 kb and 1.09 kb were observed. XVDH transcripts accumulated in X. viscosa plants in response to low temperature, heat and dehydration stresses, as well as to exogenous supply of abscisic acid, ethylene and methyl jasmonate. Localization studies of the XVDH encoded protein showed that XVDH is located in the plasma membrane-cell wall region.
Open Access
Genetic effects of prolonged UV-B exposure in a Namaqualand daisy - Dimorphotheca sinuata
(2001) Mpoloka, Sununguko Wata; Thomson, Jennifer Ann; Abratt, Valerie Rose; Mundree, Sagadevan G; Riddoch, Bruce
This thesis describes investigations into the genetic effects of long term UV-B exposure in Namaqualand daisies (Dimorphotheea sinuata) grown for several generations under ambient and enhanced UV-B levels. Enhanced UV-B radiation was found to have a major effect on the biochemical composition of the chloroplast accompanied by impairment of photosynthetic function, involving a down-regulation of photosynthetic genes and an up-regulation of flavonoid biosynthesis.
Open Access
The investigation of a novel proteinase inhibitor as a means to transfer insect resistance to plants
(1996) Tasker, Jacqueline Ruth; Thomson, Jennifer Ann; Maeder, Dennis; Botes, Dawie
A viable IPM programme involves a clear understanding of and the use of a number of components, the most important being the bionomics of insect pests; monitoring systems to establish the prevalence and seasonal occurrence of insect pests; calculation of economic thresholds; the biology of parasites and predators; utilisation of insect resistant plant varieties; and methods to maximise the advantages of pesticides and minimise their disadvantages. Insect-resistant crop varieties form an important part of IPM. There is usually no extra cost to the farmer once the resistant variety has been obtained and it is easily available to him thereafter. In the context of an IPM strategy, plant resistance should improve the impact on a pest population when both biological and chemical control methods are used.
Open Access
Investigation of Streptomyces promoters
(1995) Bourn, William Richard; Thomson, Jennifer Ann; Kirby, Ralph
[The work described here had multiple aims: to create a promoter probe that was suitable for the isolation of developmentally regulated Strepcomyces promoters, to isolate such promoters, to develop a computer assisted analysis system whereby potential promoter sequences could be determined and to use this in the analysis of the cloned promoters. Initially the suitability of the Streptomyces antibioticus me1C operon for use as a reporter system in Streptomyces was investigated. It was established that late-expressed promoters could be identified and that it was possible to use the me1C2 gene alone for this purpose. However, it was shown that the use of both me1C1 and me1C2 resulted in a more sensitive reporter system. High copy number promoter probe vectors were constructed and tested. A low copy number promoter probe (which used the Streptomyces penemefaciens pSPN1 origin of replication) was also constructed. The characteristics (copy number, stability and mobility) of the probe were established. The conditions in which sporulation was induced by phosphate limitation were identified. Under such conditions late expressing, phosphate dependent promoters were isolated, using the promoter probes previously developed. The expression of these promoters was tested in Streptomyces coelicolor bldA mutants, and the bldA dependent promoters identified. These were sequenced. Computer assisted analysis of DNA sequence bias was conducted, with the intention of using bias patterns to identify potential regulatory regions. The initial approach of using the sequence bias of protein coding regions (based on the premise that regulatory sites are likely to be under represented in these regions) was unsuccessful. Further analysis in which the positional preference of sequences that were over represented in regulatory regions was conducted. Based on this the known promoters of Streptomyces were partially classified. The sequence bias of protein coding DNA regions was used to develop a novel method to identify the protein coding regions of Streptomyces DNA. The computer programs were then used to identify protein coding and potential regulatory regions.
Open Access
An investigation of the region of DNA required for Streptocymes Penemafaciens plasmid pSPN1 replication
(1991) Smith, Anthony; Thomson, Jennifer Ann
Plasmid pSPNl is a 26.5kb cryptic plasmid, originally isolated from Streptomyces penemafaciens ATCC 31599. A 12.5kb BglII fragment of pSPNl was cloned into the vector pLR2, and this conferred on pLR2 which lacks a Streptomyces origin of replication, the ability to replicate in a number of Streptomyces species. A vector pBlue was constructed by inserting a streptomycin resistance gene from plasmid pIJ4642 into the ampicillin resistance gene of the vector Bluescript. The resistance gene was able to function in both E.coli and Streptomyces species and thus pBlue could serve as a vector for shortening and sequencing in E. coli as well as a origin-probe vector in Streptomyces. The origin-containing BglII fragment of pSPNl was cloned into pBlue to create pFull, which was able to be selected for and replicate in Streptomyces. The conditions affecting selection of pFull in Streptomyces were investigated and optimized. The copy number of pFull was found to be 0.2 per chromosome. Attempts were made to clone origin-containing fragments smaller than the 12.5kb BglII fragment. Initially a Sau3A partial library was made of the origin-containing fragment, this however did not produce any replicating plasmids. As an alternative approach, pFull was extensively mapped and a series of deletion derivatives were constructed. The derivatives were tested for the ability to replicate in Streptomyces. Judging from the deletions that were and were not able to replicate it is apparent that at least 5.5kb of DNA is required for pFull and hence for pSPNl to replicate.
Open Access
Isolation and characterisation of an Hsp90 homologue from the resurrection plant Xerophyta viscosa
(2002) Walford, Sally-Ann; Mundree, Sagadevan G; Thomson, Jennifer Ann; Farrant, Jill M
Prior to this study, a eDNA library of dehydrated Xerophyta viscosa was differentially screened and several genes were found to be upregulated during dehydration. One of these cDNAs was found to share a high degree of sequence identity with the ER-Iocated Hsp90 or Grp94 family of proteins (hereafter referred to as XVGrp94) and forms the basis of this work. The XVGrp94 eDNA was found to be truncated at the 5· terminus and a full length eDNA was isolated using SMART-RACE™ (§witching Mechanism gt 5' end of RNA Iranscript- Random ~mplification of Complementary .!;rids). This eDNA was sequenced and appeared to be a representative of the Hsp90 family of genes. The putative gene contained an ORF (Open Reading frame) potentially coding for an 812 amino acid protein with a calculated size of 92.83 kDa. It shares 85% homology with other Hsp90s from plants and it contains several characteristic features of these proteins. Additionally, it contains the ER (endoplasmic reticulum) targeting and retention signals. Southern blot analysis confirmed the presence of the gene in the X. viscosa genome possibly as a member of a family of closely related genes. Northern blot analysis revealed a transcript size of 2.8 kb, however, expression patterns of the transcript could not be established. Western blot analysis showed that the XVGrp94 concentration increased significantly in response to heat and dehydration, and a slight increase was observed in response to conditions of high salt, but no response was seen in response to high light, cold or exogenous ABA (abscisic acid) application. The XVGrp94 open reading frame was cloned into the pProEX HTa expression vector and expressed in E. coli, but purification of the recombinant protein was not successful.
Open Access
Isolation and molecular characterisation of two Pseudomonas sp. ACC deaminase genes
(1995) Campbell, Bridget Genevieve; Thomson, Jennifer Ann; Brand Reon
The phytohormone ethylene is essential to many plant developmental processes, of which the control of climacteric fruit ripening is among the best characterised. However this hormone eventually causes fruit rotting which results in a non-marketable product. One approach to reduce ethylene synthesis in plants is metabolism of its immediate precursor, 1-aminocyclopropane-1carboxylic acid (ACC). This can be achieved through degradation of ACC by the enzyme ACC deaminase to form α-ketobutyric acid and ammonia. ACC degrading soil microorganisms were identified by their ability to grow on ACC as a sole nitrogen source. Enzyme assays indicated that Pseudomonas had high ACC deaminase gene-specific primers and probes respectively revealed that only one bacterium, Pseudomonas fluerescens strain 17, had a gene with homology to previously sequenced ACC deaminase genes.
Open Access
Isolation of the aldose reductase gene (XvAld1) from the resurrection plant Xerophyta viscosa, and characterisation of the gene product and transgenic plants expressing the gene
(2007) Maredza, Alice T; Thomson, Jennifer Ann; Farrant, Jill M; Mundree, Sagadevan G
The Xerophyta viscosa aldose reductase cDNA (XvAld1) was isolated from a dehydration library. Gene transcripts that are upregulated during stress are normally involved in protection and/ or adaptation, leading to stress tolerance. The genomic organisation of XvAld1 was characterised using Southern blot analysis and DNA sequencing. The results revealed more than one copy of the gene with a complex banding pattern that was partially resolved by sequencing. The sequencing of PCRamplified genomic clones showed that the gene is organised into nine exons and eight introns spanning ~2.9 kb. The observed nucleotide differences between the sequenced clones could reflect polymorphisms between different copies of the gene. An 870-bp clone of the 5′ untranslated region, matching the 5′ leader sequence on the XvAld1 cDNA was analysed for cis-acting response elements. Many of the sequence motifs matched those for hormonal regulation, organ specific expression, dehydration, high or low temperature responses, light and phytochrome responsiveness, wounding, as well as G-box, CAAT and TATA-boxes.
Open Access
Maize streak virus (MSV) diversity in Uganda and the assessment of gene silencing as a tool for development of resistance to MSV
(2008) Owor, Betty Elizabeth; Thomson, Jennifer Ann; Shepherd, Dionne
Maize streak virus (MSV: Family Geminiviridae, Genus Mastrevirus) is the causal agent of maize streak disease (MSD) that contributes significantly to low maize yields in Africa, thereby threatening food security of sub-Saharan Africa’s poorest people. In Uganda, MSD has been identified as one of the most important constraints to maize production. In order to have a better understanding of the disease in that country, this thesis set out to establish MSD levels in farmers’ fields; develop a new sampling and virus isolation method; assess the diversity of MSVs throughout Uganda; and, through the cloning of sampled virus genomes, to determine the genetic characteristics of different isolates. In addition, this study also included an assessment of RNA silencing as a resistance strategy against MSV.
Open Access
Molecular analysis of two cellulase genes from Ruminococcus flavefaciens FD-1 and their transcriptional regulation
(1993) Wang, Wenyen; Thomson, Jennifer Ann
The mesophilic Ruminococcus flavefaciens FD-1 (NCDO 2215) is a Gram-positive obligate anaerobic bacterium. The aim of this thesis was to clone, sequence and analyze a cellodextrinase gene (celA) and a carboxymethylcellulase gene (celE), and study their regulation and induction at the transcriptional level. The sequence of the celA gene from FD-1 was determined and the amino acid sequence of the CelA enzyme (336 amino acid residues) deduced. It showed 40% identity with endoglucanase C of Clostridium thermocellum and 27.4% identity with endoglucanase 3 of Fibrobacter succinogenes. These three enzymes are grouped into subfamily "A3". The ATG start codon of celA is preceded by a GAGG sequence, predicted to be a ribosome binding site. The derived amino acid sequence corresponded to a protein of Mᵣ 38686. SDS-PAGE analysis of in vitro and in vivo translational products showed that CelA has a molecular mass of ca 39 kDa and was secreted into the Escherichia coli periplasmic space. Although CelA has activity on carboxymethylcellulose, further study on the enzyme showed that it degraded cellopentaose and other cellodextrins to predominantly cellobiose. Thus CelA is a cellodextrinase. It also has high activity against p-nitrophenyl-β-D-cellobioside. A gene, expressing a protein with both carboxymethyl cellulase and xylanase activity, was cloned from R. flavefaciens FD-1 using an E. coli/Bacillus subtilis shuttle vector, pEBl. The 3.6 kb DNA insert on the plasmid pWFl, which carried the gene, celE, contained an open reading frame of 963 bp encoding 320 amino acid residues with a caculated Mᵣ of 35937. Homology analysis showed 11.6% identity and 55.3% similarity with the N-terminal catalytic region of the cellulase gene of alkalophilic Bacillus sp. strain 1139. In order to obtain expression in E. coli, the gene had to be transcribed from the lambda Pᵣ promoter. To determine whether cellulase genes of R. flavefaciens FD-1 were regulated at the level of transcription, celA and celE were used as probes against RNA isolated from R. flavefaciens FD-1 grown on cellobiose, cellulose or cellotriose. Transcription of both genes was induced when cellulose was added to cells growing in cellobiose. This induction continued after cellulose depletion and after cell division had ceased. Transcription of both genes was also induced by cellotriose although not to the same extent as by cellulose. This suggests that cellotriose and possibly ether dextrins may act as key inducers to trlgger celA and eels gene expression in R. flavefaciens FD-1.