Regulation of I-Transposon Activity in Drosophila: Evidence for Cosuppression of Nonhomologous Transgenes and Possible Role of Ancestral I-Related Pericentromeric Elements

Corresponding author: Thierry Heidmann, Institut Gustave Roussy, 94805 Villejuif Cedex, France., heidmann{at}igr.fr (E-mail)

Communicating editor: J. A. BIRCHLER

We have previously shown that the activity of functional I retrotransposons (I factors) introduced into Drosophila devoid of such elements can be repressed by transgenes containing an internal fragment of the I factor itself and that this repressing effect presents the characteristic features of homology-dependent gene silencing or cosuppression. Here we show that the same transgenes can induce silencing of a nonhomologous reporter gene containing as the sole I-factor sequence its 100-bp promoter fragment. Silencing of the nonhomologous reporter gene shows strong similarities to I-factor cosuppression: It does not require any translation product from the regulating transgenes, sense and antisense constructs are equally potent, and the silencing effect is only maternally transmitted and fully reversible. A search for genomic I-like sequences containing domains with similarities to those of both the regulating and the reporter transgenes led to the identification of four such elements, which therefore could act as intermediates—or relays—in the cosuppression machinery. These results strongly suggest that ancestral transposition-defective I-related elements, which are naturally present in the Drosophila genome, may participate per se in the natural conditions of I-factor silencing.

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