In order to fully understand mechanisms resulting in effects of low dose, whole system rather than cells must be examined. Although not identical to mammalian systems, simple systems usually have many similarities and give direction for further study of more complex systems. We use the budding yeast, Saccharomyces cerevisiae, as a model system because it is easy to manipulate and its genome is simple and well characterized.

Examine mechanisms and effects of low dose radiation response for:

1. Genetic recombination mechanisms that lead to genomic instability
   
   
2. Genetic factors that affect individual susceptibility to low-dose radiation
   
   
3. The adaptive response

1. The definition of the recombination mechanisms that lead to genomic instability must include the study and identification of the multiple genes required to rejoin broken DNA ends. Recently, we identified a new gene, NEJ1, that regulates this DNA repair in diploid cells and determines the cellular localization of another component of DNA end-joining, Lif1p. We are using a collection of yeast strains, each of which lacks a single DNA rejoining gene, to define the multiple steps involved in DNA repair
   
   
2. Nearly all homologous DNA recombination in yeast depends on the radiation-sensitivity gene, RAD52. Such radiation sensitive genes may affect individual susceptibility to radiation exposure. However, in vertebrate cells homologous recombination is only weakly affected by the absence of such genes. This suggests other RAD52-independent DNA repair pathways exist in vertebrates. In budding yeast, we have now identified a similar RAD52 independent DNA repair pathway in both mitotic and meiotic cells. We will determine the proteins produced by the genes that are required to carry out recombination in the absence of RAD52.
   
   
3. "Adaptive responses" in DNA repair may help explain why human cells become more resistant to a high dose (>1 Gy) of ionizing radiation if they have been previously treated with a lower dose (<1 cGy). It is presumed that the initial treatment modifies the activity of DNA repair enzymes. We have created a model system to study adaptive responses in yeast. In this model system, each cell suffers a single double strand DNA break. This damage is an excellent model of the damage that occurs with low-level exposure to ionizing radiation. This lesion is efficiently repaired about 6 hr after induction. During the interval between the creation of the lesion and its repair, cells are challenged by high doses of either X-rays or by radio-mimetic drugs to assess their adaptive response. Results to date indicate a clear radio-protective "adaptive response" in cells that have been initially damaged. Genes whose expression is altered by this adaptive response will be determined using microarrays. These altered genes will be knocked out to test their role in the adaptive response

1. Link genomic instability to the molecular process of non-homologous end-joining of damaged DNA.
   
   
2. Identify the human genes that may be important in coping with low-level DNA damage.
   
   
3. Form a mechanistic basis for understanding three important biological processes induced by exposure to low doses of ionizing radiation, DNA repair, genetic susceptibility, and adaptive response.