Insertion of unique and repetitive DNA fragments into the aprt locus of hamster cells.

Two insertion mutations occurring at the aprt locus of Chinese hamster ovary cells were analyzed at the nucleotide level by cloning and sequencing the mutant genes. The insertions are similar with respect to events at the target site in that both are accompanied by small deletions. The nature of the DNA introduced, on the other hand, is very different, a unique fragment in a spontaneously occurring mutant and a highly dispersed, repetitive fragment in a gamma radiation-induced strain. The inserts are small (285 and 58 base-pairs) and have none of the structural features or sequences related to putative mobile sequences in mammalian cells. The mechanism of transposition was further examined by cloning the unique donor fragment of the spontaneous mutant. These analyses revealed that the insert in the mutant gene was a precise duplicate of the donor DNA fragment.