Regulation of Telomere Length and its Maintenance in the Absence of Telomerase in Budding Yeast Saccharomyces Cerevisiae

Telomeres are ribo-nucleoprotein structures that cap the end of linear chromosomes protecting them from nucleolytic degradation. In budding yeast Saccharomyces cerevisiae, telomeres are maintained by telomerase and a network of over 300 telomere length maintenance genes. Loss of telomerase leads to progressive shortening of telomeres and eventually cellular senescence. However, a few cells can undergo RAD52 dependent recombination to elongate telomeres and become ‘survivors’. Survivors can be of two major types; Type I and Type II, characterized by amplification of the sub-telomeric Y’ repeats or the telomeric TG₁₋₃repeats respectively. In this study, I have demonstrated that in addition to the genetic components, initial telomere length and the timing of senescence are essential factors that influence telomerase negative survival. Longer initial telomere lengths favour Type II survivor pathway, lead to increased efficiency of recovery from crisis and higher overall frequency of survivor formation. Furthermore, longer initial telomeres increased the proportion of Type II and Type II-like survivors in the absence of RAD59, a gene generally required for Type II survival. Early senescence was induced from a single critically short telomere in a telomerase negative cell population and led to higher proportion of Type II survivors. Contrary to previous assumptions, the increased proportion of Type II survivors observed was independent to the telomere length at the time of senescence. Genetic variants that regulate telomere lengths were explored via inter-cross QTL (i-QTL) Multipool analysis using parents from two independent inbred natural populations of S. cerevisiae. Seven candidate genes with known telomere function were identified, with the most prominent QTLs being EST2 and STN1. Five intervals containing genes with no prior association to telomere length regulation were also identified. This study demonstrated the sensitivity of the i-QTL Multipool method in determining novel genetic variants regulating telomere length even with a small sample size.