The role of TDP1 from budding yeast in the repair of DNA damage.

The TDP1 gene encodes a protein that can hydrolyze certain types of 3'-terminal phosphodiesters, but the relevance of these catalytic activities to gene function has not been previously tested. In this work we engineered a point mutation in TDP1 and present evidence that, as per design, it severely diminishes tyrosyl-DNA phosphodiesterase enzyme activity without affecting protein folding. The phenotypes of yeast strains that express this mutant show that the contribution of TDP1 to the repair of two kinds of damaged termini-induced, respectively, by camptothecin (CPT) and by bleomycin-strongly depends on enzyme activity. In routine assays of cell survival and growth the contribution of this activity is often overshadowed by other repair pathways. However, the value of TDP1 in the economy of the cell is highlighted by our discovery of several phenotypes that are evident even without deliberate inactivation of parallel pathways. These non-redundant mutant phenotypes include increased spontaneous mutation rate, transient accumulation of cells in a mid-anaphase checkpoint after exposure to camptothecin and, in cells that overexpress topoisomerase I (Top1), decreased survival of camptothecin-induced damage. The relationship between the role of TDP1 in Saccharomyces and its role in metazoans is discussed.