Background

Telomeres, the specialized protein-DNA structures present at the chromosomal terminus serve as 'caps' to prevent deleterious processes that would result in loss of linear chromosomes. The maintenance of an appropriate length of telomeric simple sequences to provide cap function is therefore essential for cell viability. Indeed, defects in telomere size control have been linked with both aging and oncogenesis. We have been studying the mechanism of size control in the yeast Saccharomyces cerevisiae as a model system. The basic outline of telomere size control is remarkably similar in yeast and vertebrates. Hence, the construction of a model system that can be easily manipulated gives us the opportunity to provide the framework for higher eukaryotic functional homologs.

Our previous studies have uncovered a process termed telomeric rapid deletion (TRD) that excises over-elongated telomere tracts to wild type sizes by a recombination mechanism. Two components regulate this process: the mechanism for the deletion process, and the mechanism of sizing among telomeres that governs the precision of deletion.

Our major goal is to understand the mechanism of TRD. Three general directions will be followed. The first goal is the characterization of the cell cycle control of TRD and the characterization of meiotic TRD that displays dramatically increased rates of TRD. The only protein known to be essential for TRD is the recombination protein Mre11. Our second goal is to study the essential role of Mre11 in TRD through the extensive characterization of a battery of missense alleles falling within unique domains of the protein The third goal examines an unusual allele of Mre11, A470T, that confers two major phenotypes, a loss of TRD precision and an ability to bypass senescence conferred by a loss of telomerase. Proteins that interact with the motif surrounding A470T will be sought through the use of novel methods of isolating soluble native complexes.

Telomeres also confer transcriptionally silent states onto subtelomeric sequences, a likely reflection of the effects of telomere structure on adjacent sequences.. We have become particularly interested in the role of histone H2A in the structure of the telomere during silencing. Our initial data suggest a direct role in heterochromatin formation. We will test this hypothesis by applying ChIP and other physical assays to measure the effects of mutants of H2A in telomeric heterochromatin.