| Summary: | The I factor is a Long Interspersed Nuclear Element of <i>Drosophila melanogaster. </i>Transposition of the I factor occurs via an RNA intermediate and is restricted to the germline of hybrid females that have received one half of their chromosome complement from a parent devoid of active I factors. If the I factor devoid chromosomes are of maternal origin, I factor transposition is associated with lethality of the F2 embryos and high frequency of lethal mutations in the F3 generation. In contrast to its restricted pattern of transposition, I factor expression is detectable in somatic issues of all flies that carry active I factors. Accumulation of I factor copies over the generations is associated with cessation of transposition and a decrease in expression in the female germline. As previously shown, the I factor 5' Untranslated Transcribed Region (UTR) can impose on a reporter gene a pattern of expression similar to that observed for complete I factors, i.e. high levels of expression in the germline of permissive females and lower yet significant levels in other tissues and in males. Furthermore, the I factor 5'UTR is sensitive to the presence of complete I factors and multiple copies of this sequence significantly reduce of the F2 lethality associated with I factor transposition. This work investigates the role of the 5' UTR in determining parental effects and in mediating processes of Polycomb-dependent I factor silencing. In addition, the possibility that the high Adenine-Thymine content of the 5'UTR might impose on this sequence an unusual conformation that could be responsible for some of the biological properties of the I factor is also investigated.
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