Escherichia coli photolyase is a DNA repair enzyme which catalyzes the photo monomerization of pyrimidine dimers by near UV (300-500 nm). The enzyme is composed of protein and RNA components. We have previously mapped and clonned the structural gene for the apo-enzyme. We propose to map and clone the gene for the RNA co-factor and to determine the nucleotide sequence of both the apo-enzyme and co-factor genes. We will study the regulation of these genes in vivo by measuring enzyme and specific RNA levels in various genetic backgrounds known to affects photolyase activity or that of other DNA repair genes. Transcriptional control of the apo-enzyme and co-factor genes will also be studies in vitro. Photolyase will be purified in mg quantities and the structure of the active stie will be probed by protein-DNA and protein-RNA cross-linking experiments. These studies will help us understand the mechanism by which hotolyase maintains the integrity of the genetic material.