This project is designed to determine the relationship between DNA repair, chromosome structure and mutagenesis in Drosophila melanogaster. A mutation which increases the mutation frequency (a mutator) has been identified and characterized. This mutator greatly reduces the efficacy of a repair pathway for x- ray induced chromosome breaks, thereby allowing a previously undescribed repair pathway to be observed. By this newly identified repair pathway individual broken chromosome ends are "healed", allowing the recovery of terminal deletions. The distribution of chromosome breaks that can be healed has been examined at several levels. (1) When females are irradiated and crossed to males carrying a translocation between the X and Y chromosomes, long terminal deletions can be recovered, suggesting that, at a gross level, breaks can be recovered anywhere in the euchromatin, without regard to proximity to the nearest telomere. (2) The mutator increases the recovery of breaks in the heterochromatin, but the breaks are not recovered as terminal deletions. Rather, they are recovered as rearrangements similar to the rearrangements from wild-type females. Thus, while the mutator itself does not have differential effects on euchromatin and heterochromatin, the healing process may be specific to euchromatin. (3) At the DNA sequence level, the breakpoints of the terminal deletions are distributed randomly, suggesting that specific DNA sequences are not required for the healing process to occur. (4) The breakpoints associated with the terminal deletions move (i.e., DNA sequences are being lost from the deficient chromosomes), suggesting that the neotelomeres on the broken ends are not as effective as the original telomeres and that the process of replicating chromosome ends involves loss and replacement of DNA sequences.