Convergent Transcription Initiates from Oppositely Oriented Promoters within the 5′ End Regions of Drosophila melanogaster F Elements

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, F_in and F_out, that transcribe in opposite orientations. The F_in promoter drives the synthesis of transcripts that initiate around residue +6 and are directed toward the element. F_in, that probably controls the formation of F transposition RNA intermediates and gene products, is internal to the transcribed region. Sequences important for accumulation of F_in 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 F_in RNA is also dependent upon the presence of sequences located within the +175 to +218 interval. The F_out promoter drives transcription in the opposite orientation with respect to F_in. F_out 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 F_out promoter. Deletions knocking out the F_in promoter do not impair F_out transcription; conversely, initiation at the F_in promoter still takes place in templates that lack the F_out promoter. At a low level, both promoters are active in cultured cells.