Molecular organization of the Drosophila melanogaster Pig-1 gene.

The Pre-intermoult gene-1 (Pig-1) of Drosophila melanogaster maps on the X chromosome, at polytene bands 3C11-12, and is nested within the 79 kb intron of the dunce gene. Pig-1 has so far been characterized only preliminarily and its function is still unknown. We analysed the molecular organization of the gene by cDNA clone isolation and sequencing as well as S1 mapping and primer extension analyses. The results obtained reveal that the gene is colinear with its genomic sequence and define the usage of both 5' and 3' alternative sites for Pig-1 transcription; two continuous open reading frames (ORFs) are fully contained within the Pig-1 transcribed region, although several lines of evidence suggest that only the longer ORF is likely to be translated. We also report that the level of Pig-1 transcript is nearly fourfold reduced in a variant strain carrying a deletion within the Pig-1 upstream sequence, thus identifying a regulatory element required for high level gene expression.