We have found that defects in DNA repair genes result in the accumulation of spontaneous DNA damage that partially or completely arrests cells during or after DNA replication. This block can be overcome by mutating DNA damage checkpoint genes. We believe the DNA damage that accumulates spontaneously leads to a replication block and is the result of an attempt to repair collapsed replication forks or other replication blocks through homologous recombination. Our studies point to MEC1 (ATR) as a key molecule that signals the presence of unrepaired DNA damage. We are currently looking at the effect of the Mec1 signal on DNA replication in our mutants. Although loss of the Mec1 kinase permits cells with unrepaired damage to grow better, the cost to the cell is an greatly increased rate of chromosome loss. We are studying this process as a model for the spontaneous loss of heterozygosity seen in many tumor cells.

Genome Instability and Reduced Life Span

A second project in the lab is focused on a novel RNA polymerase II complex component. We initially identified the HPR1 gene through a mutant screen for strains that showed increased instability of repeated DNA sequences. The HPR1 gene turned out to encode a topoisomerase-like gene that is part of a RNA polymerase II complex. When cells are deficient in this gene they become unstable for repeated sequences and this has been shown to be the direct result of transcription elongation defects. These cells also have a reduced life span, linking transcription to aging. We have identified suppressors the of genome instability of hpr1 mutants. These suppressors also suppress the life span reduction phenotype of the hpr1 mutants.