Many foreign agents form covalent adducts with DNA. These lesions are removed by nucleotide excision repair systems such as the UvrABC endonuclease system of E. coli. The mechanism and efficiency of repair may depend on the DNA structure at the site of the lesion. Using homogeneous substrates specifically modified with a psoralen monoadduct or crosslink, DNA sequences and helix conformation will be related to the in vitro efficiency of DNA repair. Protein DNA complexes and intermediate products will be analyzed in order to formulate biochemical mechanisms of repair. It is proposed to test whether the rate of repair of a crosslink depends on the DNA conformation; whether fast repair is due to rapid incision by UvrABC endonuclease and whether slow repair may form a high affinity UvrAB-lesion nucleoprotein complex which promotes subsequent slower steps such as recombinational repair. Nucleotide Excision DNA repair is a natural cellular defense mechanism by which a cell, using multienzyme complexes rids itself of a variety of lesions in its DNA. A feature in common among these DNA lesions may be a distortion of the local DNA conformation which becomes attractive to the nucleotide excision repair enzymes, allowing this enzyme system to exhibit a broad range of substrate specificity. In more complex DNA lesions repair may also require the participation of DNA recombinant enzymes. Dr. Yeung will study the repair of psoralen-DNA monoadducts and crosslinks in purified system using a number of repair enzymes and defined DNA substrates in order to better understand the mechanism of this important cellular process.
