The 5′ Untranslated Region and Gag product of Idefix, a Long Terminal Repeat-Retrotransposon from Drosophila melanogaster, Act Together To Initiate a Switch between Translated and Untranslated States of the Genomic mRNA

Abstract

Idefix is a long terminal repeat (LTR)-retrotransposon present in Drosophila melanogaster which shares similarities with vertebrates retroviruses both in its genomic arrangement and in the mechanism of transposition. Like in retroviruses, its two LTRs flank a long 5′ untranslated region (5′UTR) and three open reading frames referred to as the gag, pol, and env genes. Here we report that its 5′UTR, located upstream of the gag gene, can fold into highly structured domains that are known to be incompatible with efficient translation by ribosome scanning. Using dicistronic plasmids analyzed by both (i) in vitro transcription and translation in rabbit reticulocyte or wheat germ lysates and (ii) in vivo expression in transgenic flies, we show that the 5′UTR of Idefix exhibits an internal ribosome entry site (IRES) activity that is able to promote translation of a downstream cistron in a cap-independent manner. The functional state of this novel IRES depends on eukaryotic factors that are independent of their host origin. However, in vivo, its function can be down-regulated by trans-acting factors specific to tissues or developmental stages of its host. We identify one of these trans-acting factors as the Gag protein encoded by Idefix itself. Our data support a model in which nascent Gag is able to block translation initiated from the viral mRNA and thus its own translation. These data highlight the fact that LTR-retrotransposons may autoregulate their replication cycle through their Gag production.

ACKNOWLEDGMENTS

We are grateful to A.C. Prats for comments on a draft of the manuscript, J. L. Darlix for general discussions, A. Vincent for providing clones, and M. Galloni and the Bloomington Stock Center for flies.

This work was supported by grants from the Association de Recherche contre le Cancer (ARC 5585), CNRS (GDR 2157), INSERM (U384), and Université Franco-Italienne (UFI) through the “Vinci 2002” program. C.M. received a graduate grant from MESR and FRM, and F.A. received a graduate grant from MESR.