Lesions induced in DNA may be repaired by "error-proof" or "error-prone" repair mechanisms. Error-prone systems repair the lethal damage of the lesion, but in doing so may fail to restore the original nucleotide sequence, producing mutations. This error-prone process is called misrepair mutagenesis. Bacteriophage T4 has its own misrepair pathway, mediated by at least three genes, x, y and w. Mutations in two of these genes, x and y, suppress mutations of gene 49, a gene involved in DNA packaging. This allows the selection of alleles of x and y. ts alleles of x and y have been selected. These are temperature sensitive both for suppression of gene 49 amber mutants and for UV sensitivity. Amber alleles of x and y are being sought. Using these and other x and y alleles, the genes x and y will be mapped. The ts alleles will be used to study the time course of mutagenesis in T4. Finally the enzymology of the T4 misrepair system will be studied by genetic and biochemical approaches.