We have examined the distribution of sequences homologous to the type I and type II rDNA insertions of Drosophila melanogaster in its sibling species. Each of the six species we have examined has sequences homologous to the type I insertion, which have undergone extensive divergence by the criterion of their EcoRI, BstI and HindIII restriction patterns. We have isolated cosmid clones containing type I sequences from D. simulans and D. mauritiana, the two species most closely related to D. melanogaster. Southern hybridisation analysis of these clones indicates that, as in D. melanogaster, the type I sequences can exist independently of rDNA and can also dissociate to give sub‐components homologous to the right hand segment of the D. melanogaster type I insertion. The type II sequences, on the other hand are present in five out of the six species, but their restriction endonuclease cleavage profile is highly conserved. The differences in the degree of conservation of the two types of insertion sequence are discussed.
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).
The advent of 33P nucleotide triphosphates has widened the options available for labelling nucleic acids. Choosing the most appropriate label requires a careful consideration of experimental needs and circumstances.
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