A survey of the DNA sequences surrounding the Bari1 repeats in the pericentromeric h39 region of Drosophila melanogaster

In Drosophila melanogaster, clustered copies of the Bari1 transposon are only present in the pericentromeric h39 region of the second chromosome, where other clusters of repetitive elements, either found organized in large tandem arrays only in the h39 region (Responder, PortoI), or both in the h39 region and in other heterochromatic regions (Hoppel), are also observed. The topological relationship among the repetitive sequences of the h39 region and the nature of the sequences separating its large repeat clusters are at present largely unknown. To get new insights on the sequence composition of the heterochromatin and on the forces governing its origin and maintenance, we have cloned and analyzed part of the DNA sequences flanking the h39 Bari1 repeats. In a region spanning 3 and 9 kb, respectively, from the ends of a Bari1 array we found only single copies of the PortoI and Hoppel transposable elements, and five copies of a variant form of the Responder repeats. No large tandem arrays of any repeated element were present. In addition, a highly conserved 596 bp sequence, that may have a functional role, is present on both sides of the Bari1 repeats. We suggest that the current organization of the h39 heterochromatin implies some topological or functional constraint that prevents the formation of further arrays of repetitive elements in the region.