The role of tract stability in telomere maintenance of Saccharomyces cerevisiae
Description
The maintenance of the simple repetitive DNA at telomeres is essential to chromosome stability and integrity. We are interested in understanding the regulation of telomeric DNA size and stability. The focus of my dissertation work has been an investigation into processes that regulate small scale (rearrangements of simple repeat sequence) and large scale (overall length) telomere tract stability. Both processes have implications for normal telomere function. Small-scale tract stability refers to sequence rearrangements that involve changes <150 bp within the telomere tract. We hypothesized that in the S. cerevisiae model system telomeric DNA may be susceptible to small rearrangements in the basal region of the tract comparable to rearrangements that occur at non-telomeric simple repetitive DNA in yeast and other eukaryotes. To this end, we investigated mutations in the recombination and replication machinery that elevate small scale tract instability at non-telomeric repetitive arrays. Our results indicate that telomeres are relatively resistant to elevations in simple sequence rearrangements, suggesting differential regulation of telomeric and non-telomeric repetitive DNA. Even telomere deprotection does not cause elevation of rearrangements in the basal telomere tract. Surprisingly, we did uncover a novel pathway of telomere recombination in response to short, unstable telomere ends Large-scale tract stability in this study refers to changes that reduce the sizes of elongated telomeres to wild type lengths. This mechanism, termed telomeric rapid deletion, is a recombinational process that is most strongly regulated by the Mre11p subunit of the widely conserved Mre11p/Rad50p/Xrs2p complex. The Mre11p complex has multiple roles in DNA metabolism and telomere maintenance. To discover its function in TRD, we conducted a structure-function analysis of Mre11p by testing the TRD phenotypes of various mutant alleles. Our results exclude an active role of Mre11p in the processing of telomere ends that is pre-requisite for TRD. However, our results do suggest that Mre11p both positively and negatively regulates TRD in a manner that is dependent on formation of the complex