Abstract
Drosophila melanogaster F elements are members of the super-family of LINEs, mobile repeated DNA sequences that lack LTRs and propagate by the reverse transcription of unit-length RNA intermediates. The F 5' end region harbours two promoters (F(in) and F(out)) that transcribe in a convergent manner. Each promoter has been functionally dissected by assaying in D. melanogaster cultured cells templates carrying base substitutions and/or deletions across the +1 to +245 region of the element F12. F(in), that likely controls the synthesis of gene products and transposition intermediates, is internal to the transcribed region. Two elements play a major role in F-sense transcription. The proximal element spans the interval +6 to +14 and includes a major RNA start site. Heterologous DNA featuring a nearly identical purine-pyrimidine sequence can functionally replace the initiator-like module only when properly spaced from downstream F sequences. The distal element is within the interval +18 to +46 and may correspond to a motif (AGACGTTT, +34 to +41) conserved in other Drosophila LINEs. F(out) is a TATA-less promoter that directs transcription predominantly from three nearby sites (a to c). F(out) expression is influenced by multiple elements located upstream of residue -68 relative to site a as +1 within a region (alpha) shown to stimulate the D. melanogaster hsp70 promoter in an orientation and position-independent fashion. Changes within the -43 to +24 interval may suppress or stimulate transcription from sites a and c. Initiation from a site approximately 30 nucleotides upstream of site a is enhanced by alterations of the interval -43 to -5. The expression of the two F promoters, determined by the interaction of the transcriptional machinery with distinct DNA sequences, is influenced by a common element within the alpha region.
