Abstract
Drosophila melanogaster F elements are mobile, oligo(A)-terminated DNA sequences that likely propagate by the retrotranscription of RNA intermediates. Plasmids bearing DNA segments from the left-hand region of a full-length F element fused to the CAT gene were used as templates for transient expression assays in Drosophila Schneider II cultured cells. Protein and RNA analyses led to the identification of two promoters, Fin and Fout, that transcribe in opposite orientations. The Fin promoter drives the synthesis of transcripts that initiate around residue +6 and are directed toward the element. Fin, that probably controls the formation of F transposition RNA intermediates and gene products, is internal to the transcribed region. Sequences important for accumulation of Fin transcripts are included within the +1 to +30 interval; an additional regulatory element may coincide with a heptamer located downstream of this region also found in the 5' end regions of F-like Drosophila retrotransposons. Analysis of the template activity of 3' deletion derivatives indicates that the level of accumulation of Fin RNA is also dependent upon the presence of sequences located within the +175 to +218 interval. The Fout promoter drives transcription in the opposite orientation with respect to Fin. Fout transcripts initiate at nearby sites within the +92 to +102 interval. Sequences downstream of these multiple RNA start sites are not required for the activity of the Fout promoter. Deletions knocking out the Fin promoter do not impair Fout transcription; conversely, initiation at the Fin promoter still takes place in templates that lack the Fout promoter. At a low level, both promoters are active in cultured cells.