IDENTIFICATION OF A STRONG TRANSCRIPTIONAL ACTIVATOR FOR THE COPIA RETROTRANSPOSON RESPONSIBLE FOR ITS DIFFERENTIAL EXPRESSION IN DROSOPHILA-HYDEI AND MELANOGASTER CELL-LINES

We have characterized the regulatory properties of a 72bp sequence loc ated in the 5' untranslated domain of the Drosophila copia retrotransp oson, 3' to the left LTR, by transient transfection assays with cell l ines derived from either Drosophila hydei (DH33 cells) or Drosophila m elanogaster (Schneider II and Kc cells). Reporter plasmids were constr ucted which contained the lacZ gene under the control of either the en tire copia LTR with 5' untranslated domain, or a minimal heterologous promoter flanked with the identified copia regulatory sequences. Upon transfection into the copia-free DH33 cells, the presence of the 72bp sequence resulted for all reporter plasmids in a 100-700 fold increase in expression level -as well as in reporter gene RNA levels- whereas this sequence had no enhancing effect upon transfection of the same pl asmids into the copia-containing Schneider II or Kc cells. Moreover, m obility shift assays with the 72bp enhancer sequence disclosed two spe cific bands of retarded mobility with whole-cell extracts from DH33 ce lls, whereas no retarded band could be detected, under identical condi tions, with extracts from Schneider II cells. UV crosslinking experime nts between the enhancer sequence and DH33 extracts revealed a single protein species -of app. mel. wt. 50kD- for both retarded bands, thus strongly suggesting that they simply correspond to the sequential bind ing of two identical factor molecules to the enhancer sequence. These data demonstrate that the copia-free D. hydei cells express a strong t ranscriptional activator for the copia element and possible interpreta tions for the absence of this factor in the copia-containing D. melano gaster cells are discussed in terms of a possible ''adaptation'' of th e ''host'' (D. melanogaster) to an otherwise highly mutagenic ''parasi te'' (copia with its transcription factor). (C) 1994 Academic Press, I nc.