Localization of a position effect element that affects alcohol dehydrogenase transgene expression in Drosophila melanogaster

The molecular basis for the abnormal expression of an alcohol dehydrogenase (Adh) transgene, introduced into the 25 C-D region of an Adh negative strain of Drosophila melanogaster, was investigated using a transient expression assay. A 14-kb genomic clone from the transformed line, MM-50, containing the transgene was isolated. A position effect element was identified upstream of the Adh gene within a 1.7-kb EcoRI fragment. This element acts as a silencer, greatly reducing the Adh transcriptional activity. Sequence analysis reveals that the silencer element is 1.18 kb long. There are no long open reading frames present in the sequence, suggesting that it is derived from a noncoding region. A 20-bp sequence (17/20 nucleotides matching) containing a high T/C content is repeated within the silencer region. Within each of these homologous regions there is an internal repeat of the sequence CTCTC. The repeated sequences in MM-50 contain a consensus motif, CCTCTC, for silencer elements found in the rat collagen 11 gene and several other vertebrate genes, including the human beta-interferon (beta-IFN) gene. Comparison of the sequence of the silencer region to the D. melanogaster whole genome sequence shows that the silencer is located within a clone derived from the 25C5-25C10 region of chromosome 2L.