EXPRESSION OF DROSOPHILA-MELANOGASTER F-ELEMENTS IN-VIVO

Drosophila melanogaster F elements are mobile, oligo(A)-terminated DNA sequences probably propagate by the retrotranscription of RNA interme diates. Polyadenylated transcripts corresponding in size to full-lengt h (4.7 kb) family members were detected in the Drosophila melanogaster Canton-S strain from 2nd larval instar to the adult stage. RNA accumu lation reached a maximum in pupae. In the adult, F elements are transc ribed in both sexes. F expression is directed in vivo by the intrageni c promoter (F-in) located at the 5' end of F. Whole-mount hybridizatio ns were carried out to define the site of synthesis of full-length tra nscripts found in the ovary. Selective RNA accumulation was not detect ed in the cytoplasm of any specific cell type. Stained nuclear dots we re observed in nurse cells from stage 2-3 to the end of oogenesis. RNa se treatment of egg chambers prior to the addition of the probe led to disappearance of the nuclear dots and appearance of a cytoplasmic hyb ridization signal suggesting leakage of nuclear transcripts. Transgeni c lines harbouring the chloramphenicol acetyltransferase (CAT) gene un der the control of the F-in promoter were obtained. In independent lin es, CAT enzyme levels mirror the ontogenetic profile of F expression d rawn from Northern RNA blotting data. An antisense promoter (F-out) th at is located downstream from the F-in promoter and transcribes toward s the 5' end of F seems to be constitutively expressed in the fly.