published_or_final_version / abstract / Biological Sciences / Doctoral / Doctor of Philosophy

A high-resolution computer-based survey for transposable elements performed on 910 Kb of rice genomic DNA sequences revealed the presence of both class I and class II transposable elements. Elements from most major families of plant transposable elements were identified, and new groups were reported for these families. Miniature inverted-repeat transposable elements (MITES) are clearly the predominant type of transposable element in the rice sequences examined. Phylogenetic analysis of the putative transposases of several transposable elements indicated that Tourist-like miniature inverted-repeat transposable elements (MITES) are closely related to the bacterial insertion sequence 5 (IS5) family of transposable elements, while Emigrant-like and Stowaway-like MITES are both related to members of the IS630/TcI/mariner superfamily of elements. Finally, the nucleotide sequences of MITES, Ac-like, Mutator-like elements (MULE), short interspersed nuclear elements (SINEs) and other unclassified elements, as well as their insertion polymorphism data have been used to reconstruct the relationships between rice species in the AA genome. The use of a combination of transposable element data sets generated the most reliable cladograms.

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Receptor-like protein kinases (RLKs) containing an extracellular leucine-rich-repeat (eLRR) domain, a transmembrane domain, and a cytoplasmic kinase domain play important roles in plant disease resistance. Simple eLRR domain proteins structurally resembling the extracellular portion of the RLKs may also participate in signaling transduction and plant defense response. Yet the molecular mechanisms and subcellular localization in regulating plant disease resistance of these simple eLRR domain proteins are still largely unclear. We provided the first experimental evidence to demonstrate the endosomal localization and trafficking of a novel simple eLRR domain protein (OsLRR1) in the endosomal pathway, using both confocal and electron microscopy. Yeast 2-hybrid and in vitro pull-down assays show that OsLRR1 interacts with the rice hypersensitive induced response protein 1 (OsHIR1) which is localized on plasma membrane. The interaction between LRR1 and HIR1 homologs was shown to be highly conserved among different plant species, suggesting a close functional relationship between the two proteins. The function of OsLRR1 in plant defense response was examined by gain-of-function tests using transgenic Arabidopsis thaliana. The protective effects of OsLRR1 against bacterial pathogen infection were shown by the alleviating of disease symptoms, lowering of pathogen titers, and higher expression of defense marker genes. / Zhou, Liang. / Adviser: Hon Ming Lam. / Source: Dissertation Abstracts International, Volume: 73-01, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 90-107). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Transposons, also known as transposable elements (TEs), are genetic elements capable of changing their location in the genome and amplifying in number. Because of their ability to cause mutations in the host genome, often with detrimental consequences to the host, yet avoid being eliminated by natural selection, transposons have been labeled selfish elements or genomic parasites. However, the advent of genomics has allowed the identification of numerous instances where transposons have played a crucial role in host genome evolution. In this thesis, I evaluate the extent to which transposons have influenced the genomes of their hosts, with an emphasis on plant genomes. I review the present knowledge of different mechanisms by which this is achieved and provide examples to illustrate them. Next, I tackle the problem of annotating transposons in the completed genomic sequence of domestic rice by comparing RepeatMasker, the standard approach used in transposon annotation, with an alternative approach employing hidden Markov models. In addition, I perform a genome-wide analysis of gene fragment capture by rice Mutator-like transposons. I conclude that, while this is a widespread phenomenon in rice, it is unlikely to represent a major force in generating novel protein-coding genes. Nevertheless, the duplicated gene fragments that are transcribed may playa role in the regulation of host genes they arose from via an RNAi-like mechanism. Finally, I conduct an in silico analysis of a gene family derived from a domesticated Mutator-like transposase, called MUSTANG (MUG), in conjunction with an experimental characterization of the MUG family in Arabidopsis. The results of the study indicate that the MUG family arose in a common ancestor of flowering plants and that the Arabidopsis genes AtMUG1 and/or AtMUG2 may act as global regulators of mitochondrial function. I conclude that our appreciation of the role of transposons in host function and evolution will undoubtedly continue to grow as our understanding of these processes deepens.

Despite the ubiquity of transposons in eukaryotic genomes, their evolutionary role remains controversial. The discovery of several domesticated genes has suggested that transposons can gain host functions, and thus contribute to the evolution of their host. Here, I present the results of a genome-wide screen for transposon-derived host genes, which was based on the idea that, once domesticated, the open reading frame of such elements would be maintained, while terminal structures necessary for transposition would be lost. Eight-hundred-and-sixty-three such transposon-dissociated elements were mined from the genome of Arabidopsis thaliana var. Columbia-0, of which less than 10% are associated with expression data. Phylogenetic analysis of Mutator superfamily genes in the genomes of Oryza sativa ssp. japonica (cv Nipponbare) and Arabidopsis, including 121 Mutator-derived transposon-dissociated elements, found that only two gene families are taxonomically widespread. MUSTANG1, a member of one of these families, appears to be under purifying selection. Thus, despite the dearth of taxonomically widespread and/or expressed transposon-dissociated elements, MUSTANG1, as well as three transposon-dissociated elements that may be associated with mutant phenotypes, might be newly discovered transposon-derived host genes.

The sequencing of eukaryotic model organisms has provided us an unprecedented opportunity for a genome-wide characterization of Transposable Elements (TEs) and the study of TE-host relationships. By developing methodologies on database mining, we explored the existence of MUtator (Mu)-Like Elements (MULEs) in Arabidopsis thaliana and Oryza sativa. Mu elements were first discovered in Zea mays; so far, a dozen of the elements have been identified in the genome. We identified a total of 1392 MULEs from the sequenced Arabidopsis genome. They represent one of the most abundant, diversified, yet still mobile DNA transposon families in eukaryotes. The Arabidopsis MULEs are composed of not only the elements showing the typical Mu-family-specific terminal structure (that is the long Terminal Inverted Repeat, TIR), but also a novel type of non-TIR MULEs. Some of this latter type of elements was found to be active both transcriptionally and transpositionally. To understand host-mediated genome-wide regulation(s) on the MULE system in Arabidopsis, we characterized 235 MULE mobility-specific genes (or mudrA-like genes) by mapping them on the sequenced Arabidopsis chromosomes and performing a genome-wide expression assay utilizing Arabidopsis METHYLTRANSFERSE1 (MET1) mutant (met2) plants, we showed that MET1-mediated global CpG methylation can only repress a portion of the gene family; its efficiency depends largely on the gene locations within the context of Arabidopsis chromatin remodeling: stronger in heterochromatic regions but weaker in euchromatic ones. This finding suggests that the Arabidopsis heterochromatic regions are not just a graveyard for the accumulation of defective elements; rather, they may have been playing an important role on the repression of TE activity via, at least in part, exerting MET1-mediated silencing effect. Our expression analysis also suggested that a TIR structure is not necessarily required for the MET1-mediated si

A genome sequence can be conceptualized as a 'book' written with four nucleotide 'letters' in oligonucleotide (oligo) 'words'. These words can be used in genomics, bioinformatics and the development of molecular markers. The whole-genome sequence for rice (Oryza sativa L.) is almost finished and has been assembled into pseudomolecules. For barley ( Hordeum vulgare L.) expressed sequence tags (ESTs) have been assembled into 21,981 tentative consensus sequences (TCs). The availability of such sequence information provides opportunities to investigate oligo usage within and between genomes. For the first of three studies reported in this thesis, a C++ program was written to automatically design oligos that are conserved between two sets of sequence information. In silico mapping between rice coding sequences (CDS) and barley TCs indicated that oligos between 18 and 24 bp provide good specificity and sensitivity (83% and 86%, respectively, for 20mers). Conserved oligos used as PCR primers had a high (91%) success rate on barley lines. Sequencing of PCR products revealed conservation in exon sequence, size and order between barley and rice. Introns were not conserved in sequence but were relatively stable in size. Map locations of eight new markers in barley revealed both genome colinearity and rearrangements between barley and rice. The second study reported in this thesis examined word frequency within the rice genome. A non-random landscape composed of high-frequency and low-frequency zones was observed. Interestingly, high-frequency words seemed to be rice specific while single-copy words were gene specific and conserved across species. As in the first study, oligos of 12 bp or less were not specific, and 18 bp seemed to be a critical length for the specificity of oligos. The third study reported in this thesis involved the development of molecular markers for known genes using public sequence information. Six new polymorphic markers were d

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