Graduate Thesis Or Dissertation
 

Studies on the centromere-specific histone, CenH3, of Neurospora crassa and related ascomycetes

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  • In eukaryotes, the defined loci on each chromosome, the centromeres, accomplish the critical task of correct cell division. In some organisms, centromeres are composed of a euchromatic central core region embedded in a stretch of heterochromatin and the inheritance and maintenance of centromeres are controlled by dynamic epigenetic phenomena. Although the size of centromeres differs between organisms, its organization, and the placement of euchromatic and heterochromatic regions is conserved from the fission yeast, Schizosaccharomyces pombe, to humans, Homo sapiens. However, relatively little is known about centromeres in the filamentous fungi from the Ascomycota, representing the largest group of fungi and fungal pathogens. Further, studies from humans, flies, yeast and plants have shown that the inheritance of centromeres is not strictly guided by centromeric DNA content, which is highly AT-rich, repetitive and constantly evolving. Therefore, it is difficult to align ans assemble the sequenced contigs of centromeric regions of higher eukaryotes, including most filamentous fungi. A genetic technique, tetrad (or octad) analysis has helped to map the centromeres of the filamentous fungus Neurospora crassa early on. The research presented in this dissertation used N. crassa as a model to focus on characterizing different features of centromeres with an emphasis on the centromere-specific histone H3 (CenH3) protein. Data included here represent the first study on centromere-specific proteins in Neurospora, and demonstrate that the central core of the centromeres are heterochromatic, showing enrichment of silent histone marks, which is in contrast to the centromere arrangement in fission yeast. The CenH3 protein, whose deposition on the genome licenses formation or maintenance of centromeres, shows highly divergent N-terminal regions and a conserved histone fold domain (HFD) in all eukaryotes. This bipartite nature of CenH3 is also observed in the Ascomycota, which provides an opportunity for functional complementation assays by replacing Neurospora CenH3 (NcCenH3) with CenH3 genes from other species within the Ascomycota. The results from this experimental approach provide good measures for (1) determining the specific regions of CenH3 required for the assembly of centromeres during meiotic and mitotic cell divisions and (2) analyzing the resistance to changes in the organization of centromeres in N. crassa. The genetic analysis showed that the divergent N-terminal region is essential for the proper assembly of centromeres, and that the conserved carboxy-terminus of CenH3 is important for the process of meiosis but not mitotic cell division. ChIP-seq analyses suggest that the observed loss of Podospora anserina CenH3 (PaCenH3- GFP) from certain N. crassa centromeres does not result in obvious phenotypic defects, e.g. diminished growth or evidence for aneuploidy. Further, the low enrichment of PaCenH3-GFP at certain centromeres is possibly predetermined during meiosis, which results in irreversible and progressive decreases in enrichment. It remains to be determined if this process is random as far as selection of centromeres is concerned. Together the results presented here suggest that during meiosis more stringent structural requirements for centromere assembly apply and that these are dependent on CenH3, and that depletion of CenH3 from centromeres does not critically affect mitosis in the asynchronously dividing nuclei of Neurospora hyphae.
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