Telomeric silencing, nucleotide excision repair and chromatin organisation in Saccharomyces cerevisiae

In this thesis, the influence of subtelomeric silencing on chromatin structure and Nucleotide excision repair (NER) efficiency in Saccharomyces cerevisiae is addressed. To assess the chromatin organisation, NER efficiency and acetylation levels yeast strains containing the URA3 marker gene at two subtelomeres with distinct levels of silencing were used. All the analyses were undertaken at the coding region of the URA3 sequence. Overall, the results presented in this thesis showed that at subtelomeres with high levels of silencing, the DNA is less accessible to the NER machinery therefore, the repair is slower. This is probably due to the variations in chromatin organisation since the chromatin structure is more compact at highly silenced subtelomeres. When telomeric silencing is abrogated by the deletion of the SIR2 gene, the NER efficiency and the chromatin organisation became similar at both subtelomeres, enforcing the idea that the differences in NER and chromatin organisation are due to variations in silencing levels. Furthermore, there was a considerable difference at the highly silenced subtelomere between the SIR2+ and sir2A strains. However, the differences at the non-highly silenced telomeres between SIR2+ and sir2A were not so dramatic. This work also concentrated on the inhibitory role of Sir2p on the acetylation response after UV irradiation. The results showed an incremental increase in acetylation levels after UV treatment when the SIR2 gene was deleted at the highly repressive end. However, the acetylation levels remained the same after and before the UV treatment when SIR2 was not deleted. In contrast, there were not significant differences in acetylation levels between the SIR2+ and the sir2A strains after UV irradiation at the non-repressive end. These findings imply that silencing influence NER efficiency and DNA accessibility. Furthermore, SIR2 suppresses any UV induced response in acetylation at highly silenced subtelomeres.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Telomeric silencing, nucleotide excision repair and chromatin organisation in Saccharomyces cerevisiae

In this thesis, the influence of subtelomeric silencing on chromatin structure and Nucleotide excision repair (NER) efficiency in Saccharomyces cerevisiae is addressed. To assess the chromatin organisation, NER efficiency and acetylation levels yeast strains containing the URA3 marker gene at two subtelomeres with distinct levels of silencing were used. All the analyses were undertaken at the coding region of the URA3 sequence. Overall, the results presented in this thesis showed that at subtelomeres with high levels of silencing, the DNA is less accessible to the NER machinery therefore, the repair is slower. This is probably due to the variations in chromatin organisation since the chromatin structure is more compact at highly silenced subtelomeres. When telomeric silencing is abrogated by the deletion of the SIR2 gene, the NER efficiency and the chromatin organisation became similar at both subtelomeres, enforcing the idea that the differences in NER and chromatin organisation are due to variations in silencing levels. Furthermore, there was a considerable difference at the highly silenced subtelomere between the SIR2+ and sir2A strains. However, the differences at the non-highly silenced telomeres between SIR2+ and sir2A were not so dramatic. This work also concentrated on the inhibitory role of Sir2p on the acetylation response after UV irradiation. The results showed an incremental increase in acetylation levels after UV treatment when the SIR2 gene was deleted at the highly repressive end. However, the acetylation levels remained the same after and before the UV treatment when SIR2 was not deleted. In contrast, there were not significant differences in acetylation levels between the SIR2+ and the sir2A strains after UV irradiation at the non-repressive end. These findings imply that silencing influence NER efficiency and DNA accessibility. Furthermore, SIR2 suppresses any UV induced response in acetylation at highly silenced subtelomeres

Intragenic repeat expansion in the cell wall protein gene HPF1 controls yeast chronological aging

Aging varies among individuals due to both genetics and environment, but the underlying molecular mechanisms remain largely unknown. Using a highly recombined Saccharomyces cerevisiae population, we found 30 distinct quantitative trait loci (QTLs) that control chronological life span (CLS) in calorie-rich and calorie-restricted environments and under rapamycin exposure. Calorie restriction and rapamycin extended life span in virtually all genotypes but through different genetic variants. We tracked the two major QTLs to the cell wall glycoprotein genes FLO11 and HPF1. We found that massive expansion of intragenic tandem repeats within the N-terminal domain of HPF1 was sufficient to cause pronounced life span shortening. Life span impairment by HPF1 was buffered by rapamycin but not by calorie restriction. The HPF1 repeat expansion shifted yeast cells from a sedentary to a buoyant state, thereby increasing their exposure to surrounding oxygen. The higher oxygenation altered methionine, lipid, and purine metabolism, and inhibited quiescence, which explains the life span shortening. We conclude that fast-evolving intragenic repeat expansions can fundamentally change the relationship between cells and their environment with profound effects on cellular lifestyle and longevity

¿En el proceso de aprendizaje, qué elementos favorecen el desarrollo de la competencia aprender a aprender? : Construcción de una escala para el profesorado universitario

En el marco actual de la docencia universitaria, contar con instrumentos científicos de medida de los contextos facilitadores de enseñanza-aprendizaje en el desarrollo de la competencia de aprender a aprender es de vital importancia. En este trabajo se presenta el proceso seguido en la construcción de los ítems y la dimensionalidad de un instrumento de nueva creación. Se han utilizado diferentes técnicas siguiendo los estándares internacionales que rigen la creación de pruebas psicométricas.In the current context of university teaching, counting on measuring instruments to assess teaching-learning facilitating contexts in the development of learning to learn competence is very important. This work contains the procedure followed to elaborate the items for a newly created instrument. We used different methods according to the international standards that rule the establishment of psychometric tests

A yeast living ancestor reveals the origin of genomic introgressions

Genome\ua0introgressions\ua0drive evolution across the animal1, plant2\ua0and fungal3\ua0kingdoms.\ua0Introgressions\ua0initiate from archaic admixtures followed by repeated backcrossing to one parental species. However, how\ua0introgressions\ua0arise in reproductively isolated species, such as\ua0yeast4, has remained unclear. Here we identify\ua0a\ua0clonal descendant of the ancestral\ua0yeast\ua0hybrid that founded the extant Saccharomyces cerevisiae Alpechin lineage5, which carries abundant Saccharomyces paradoxus\ua0introgressions. We show that this clonal descendant, hereafter defined as\ua0a\ua0‘living\ua0ancestor’, retained the ancestral genome structure of the first-generation hybrid with contiguous S. cerevisiae and S. paradoxus subgenomes. The ancestral first-generation hybrid underwent catastrophic\ua0genomic\ua0instability through more than\ua0a\ua0hundred mitotic recombination events, mainly manifesting as homozygous genome blocks generated by loss of heterozygosity. These homozygous sequence blocks rescue hybrid fertility by restoring meiotic recombination and are the direct origins of the\ua0introgressions\ua0present in the Alpechin lineage. We suggest\ua0a\ua0plausible route for\ua0introgression\ua0evolution through the reconstruction of extinct stages and propose that genome instability allows hybrids to overcome reproductive isolation and enables\ua0introgressions\ua0to emerge

Domestication reprogrammed the budding yeast life cycle

Domestication of plants and animals is the foundation for feeding the world human population but can profoundly alter the biology of the domesticated species. Here we investigated the effect of domestication on one of our prime model organisms, the yeast\ua0Saccharomyces cerevisiae, at a species-wide level. We tracked the capacity for sexual and asexual reproduction and the chronological life span across a global collection of 1,011 genome-sequenced yeast isolates and found a remarkable dichotomy between domesticated and wild strains. Domestication had systematically enhanced fermentative and reduced respiratory asexual growth, altered the tolerance to many stresses and abolished or impaired the sexual life cycle. The chronological life span remained largely unaffected by domestication and was instead dictated by clade-specific evolution. We traced the genetic origins of the yeast domestication syndrome using genome-wide association analysis and genetic engineering and disclosed causative effects of aneuploidy, gene presence/absence variations, copy number variations and single-nucleotide polymorphisms. Overall, we propose domestication to be the most dramatic event in budding yeast evolution, raising questions about how much domestication has distorted our understanding of the natural biology of this key model species

Genome-wide Control of Heterochromatin Replication by the Telomere Capping Protein TRF2

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Aborting meiosis allows recombination in sterile diploid yeast hybrids

International audienceHybrids between diverged lineages contain novel genetic combinations but an impaired meiosis often makes them evolutionary dead ends. Here, we explore to what extent an aborted meiosis followed by a return-to-growth (RTG) promotes recombination across a panel of 20 Saccharomyces cerevisiae and S. paradoxus diploid hybrids with different genomic structures and levels of sterility. Genome analyses of 275 clones reveal that RTG promotes recombination and generates extensive regions of loss-of-heterozygosity in sterile hybrids with either a defective meiosis or a heavily rearranged karyotype, whereas RTG recombination is reduced by high sequence divergence between parental subgenomes. The RTG recombination preferentially arises in regions with low local heterozygosity and near meiotic recombination hotspots. The loss-of-heterozygosity has a profound impact on sexual and asexual fitness, and enables genetic mapping of phenotypic differences in sterile lineages where linkage analysis would fail. We propose that RTG gives sterile yeast hybrids access to a natural route for genome recombination and adaptation