Site specific insertion of a type I rDNA element into a unique sequence in the Drosophila melanogaster genome.

We describe a cloned segment of unique DNA from the Oregon R strain of Drosophila melanogaster that contains a short type I insertion of the kind principally found within rDNA. The predominant type I rDNA insertion is 5kb in length, but there are also a co-terminal sub-set of shorter type I elements that share a common right hand junction with the rDNA. The insertion that we now describe is another member of this sub-set. The right hand junction of the type I sequence with the unique DNA is identical to the right hand junction of the type I sequences with rDNA. There is no significant feature within the insertion sequence that could have determined the position of the left junction with the sequence into which it is inserted. Like the corresponding short type I insertions in rDNA, the insertion into the unique DNA is flanked on both sides by a duplicated sequence, which in this case is 10 base pairs long. The cloning of a sequence corresponding to the uninterrupted unique location was facilitated by the observation that the Karsnas strain of D. melanogaster contains only uninterrupted sequences of this kind. The duplicated sequence at the target site for the insertion is only present as a single copy in the uninterrupted DNA. The sequence of the target site for the insertion (ACTGTTCT) in the unique segment shows a striking homology to the target in rDNA (ACTGTCCC).