Inverted duplication-transposition event in mammalian cells at an illegitimate recombination join

Illegitimate recombination events in mammalian cells often contain extraneous nucleotides or filler DNA at the recombinant joins. The polyomavirus-transformed cell line 7axB has previously been found to contain 37 base pairs (bp) of filler DNA at one virus-host join of the single insert of integrated viral DNA (A. Hayday, H. E. Ruley, and M. Fried, J. Virol. 44:67-77, 1982). By using a synthetic oligomer of these 37 bp as a probe, we demonstrated that this filler DNA is an inverted duplication of a single-copy rat sequence found 650 bp upstream from this virus-host join. The other virus-host join appears to be the result of a simple illegitimate recombination event between viral and host sequences. This is the first identification of filler DNA as a transposed copy of a chromosomal sequence. The relevance of the recombination events studied to cellular rearrangements and viral integration is discussed.