Amplification of the X-linked Drosophila chorion gene cluster requires a region upstream from the s38 chorion gene.

Genomic sequences controlling follicle cell-specific amplification of the X-linked Drosophila chorion gene cluster were mapped by P element-mediated transformation. Several DNA fragments containing the s38 gene and flanking sequences induced tissue-specific amplification, although replication levels were subject to position effects. Deletion analysis identified a 467-bp region upstream from the s38 transcription start site that contained sequences essential in cis for amplification. The essential region shared 32 bp of imperfect sequence homology with a previously identified region necessary for third chromosome chorion gene cluster amplification. This homologous segment contained a repetitive motif consisting of perfect and imperfect AATAC repeats; it was localized near the boundary of the essential domain since most, but not all, the repeats could be deleted without eliminating transposon-induced amplification. The repetitive region was not required for developmentally regulated s38 transcription, therefore our results identified at least one element required for amplification but not for chorion gene transcription. The homologous repetitive sequences within the amplification-essential regions may constitute part of the replication origins used to differentially replicate the two chorion domains during oogenesis.