MERIT Award Recipient: Satya Prakash, Ph.D.
Repair of UV Irradiated DNA: Excision Genes of Yeast
Cellular DNA is subjected to damage by external environmental agents such as UV light from the sun and chemical pollutants, and by endogenous factors such as reactive oxygen species. Nucleotide excision repair (NER) is a highly versatile DNA repair process that functions in the removal of UV-induced DNA lesions, as well as a large array of other DNA lesions. Because of the absolute requirement for NER in the removal of UV lesions, a defect in this repair process in humans results in the disease "xeroderma pigmentosum" (XP). XP patients are extremely sensitive to sunlight, and the frequencies of basal cell carcinoma, squamous cell carcinoma, and melanoma are increased 1,000-fold or more in these patients. XP patients also suffer from an increase in the incidence of cancers in internal organs.
Dr. Prakash and coworkers have been using the yeast Saccharomyces cerevisiae to decipher the mechanism of NER in eukaryotes. Because of the high degree of conservation of NER from yeast to human, these studies with yeast have made important contributions to the understanding of this DNA repair process in humans. Work from this laboratory was instrumental in identifying the biochemical activities of various NER protein factors and in defining their roles in this repair process. In NER in both yeast and human, DNA is incised on both sides of the lesion, leading to the removal and subsequent replacement of a fragment approximately 25-30 nucleotides long.
NER is a multi-step process, in which the damage is recognized first, followed by DNA unwinding, dual incision, and finally DNA re-synthesis to fill the gap correctly. The DNA unwinding and incision processes are relatively well understood; however, none of the proteins that are involved in these two steps, or those inferred to act at the initial damage recognition step, bind damaged DNA with a high enough selectivity to explain the high specificity of NER for damaged (and not undamaged) DNA. This disparity suggests that the first step, in which damaged DNA is initially recognized, is quite complex and likely dependent on a multiplicity of protein factors. An ongoing focus of the Prakash laboratory's research program is to elucidate the roles of protein factors that contribute to the specificity of damage recognition on naked DNA and on nucleosomal DNA.