A 1-CENTER-DOT-5 KB DIRECT REPEAT SEQUENCE FLANKS THE SUPPRESSOR OF FORKED GENE AT THE EUCHROMATIN-HETEROCHROMATIN BOUNDARY OF THE DROSOPHILA-MELANOGASTER X-CHROMOSOME

A 1.5 kilobasepair repeated DNA sequence is duplicated in direct orien tation so as to flank the suppressor of forked gene in the euchromatin -heterochromatin transition region on the X chromosome of Drosophila m elanogaster. These two copies are almost identical, but DNA blotting, analysis of cloned sequences and database searches show that elsewhere in the genome, homologous sequences are poorly conserved. They are of ten associated with other repeats, suggesting that they may belong to a scrambled and clustered middle repetitive DNA family. The sequences do not appear to be related to transposable elements and their locatio n in different strains is conserved. In situ hybridization to metaphas e chromosomes shows that homologous sequences are concentrated in the pericentric regions of the autosomes and the X chromosome. The sequenc es are not significantly under-represented in DNA from polytene tissue and must lie in the replicated regions of polytene chromosomes. The a lmost perfect conservation of the two repeats around suppressor of for ked in D. melanogaster suggests they arose by duplication or gene conv ersion. Suppression of recombination in this chromosomal region presum ably allows this unusual organization to be stably maintained. In the X-ray induced allele, suppressor of forked-L26, the sequence between t he repeats, including the gene, and one copy of the repeat have been d eleted.