The I factor is a functional non-viral retrotransposon, or LINE, from Drosophila melanogaster. Its mobility is associated with the I-R hybrid dysgenesis. In order to study the expression pattern of this LINE in vivo, a translational fusion between the first ORF of the I factor and the lacZ gene of Escherichia coli has been carried out and introduced in the genome of reactive (R) flies. Homozygous transgenic Drosophila lines have been established and analysed. ORF1 expression is limited to germ-line cells (nurse cells and oocyte) between stage 2 and 10 of oogenesis. No somatic expression is found. Position effects may limit the level of expression of a given transgene but do not modify its basic pattern of expression during the development of the fly. This reproducible control demonstrates both that I factor is driven by its own promoter, probably the internal one suggested by Mizrokhi et al. (Mizrokhi, L.J., Georgevia, S.G. and Ilying, Y.V. (1988). Cell 54, 685–691), and that tissue-specific regulatory sequences are present in the 5′ untranslated part of the I factor. The nuclear localization of the fusion protein reveals the presence of nuclear localization signals (NLS) in the ORF1-encoded protein correlating with the possible structural and/or regulatory role of this protein. This expression is restricted to dysgenic and reactive females, and is similar in the two conditions. All the results obtained in this work suggest that I factor transposition occurs as a meiotic event, between stage 2 and 10 of the oogenesis and is regulated at the transcriptional level. It also appears that our transgene is an efficient marker to follow I factor expression.

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