UC San Diego

Telomeres are the ends of eukaryotic chromosomes. They play a critical role in genome maintenance since impairment of telomeric homeostasis inevitably mediates genomic catastrophe, leading to cell death or, more dangerously, cancerous transformations. It is therefore imperative for every eukaryotic cell to maintain and preserve telomere integrity. In budding yeast, a telomeric dedicated RPA complex (the t-RPA complex), formed by the three essential proteins Cdc13, Stn1 and Ten1 sits at the central node of telomere homeostasis. According to the current model, the trimer, binding tightly to the telomeric single stranded repeats, ensures the elongation of the telomeres by recruiting the cellular retro- transcriptase telomerase. On the other hand, it also supposedly involved in protecting telomeres from uncontrolled resection through a still poorly understood mechanism. This dissertation describes my efforts in using the homology between the t-RPA and the original RPA complex, to decipher the overall structure of the t-RPA complex: first pinpointing the molecular details of the Ten1-Stn1 interaction and then identifying a domain of the t-RPA complex that specifically evolved to perform a new telomeric-specific function. Subsequently, the attentive review of a key temperature sensitive allele of Cdc13 together with a number of published observations, casted significant doubts on the current interpretation of the role of the t-RPA complex at telomeres, forcing the conception of a new model for telomere maintenance where the essential function of the t-RPA complex is not the protection of chromosome ends, but is actually the efficient replication of the telomere as a telomeric specific complex that becomes part of the replisome as the replication fork advances through the duplex telomeric DNA. The last sections of this manuscript describe the experimental approaches used successfully test the new hypothesis, leading to the confirmation of the t-RPA complex as a telomere dedicated replicative factor and fundamentally changing the paradigm of telomere homeostasis