Evidence for a multistep control in transposition of I factor in Drosophila melanogaster

Distribution, evolution, and diversity of retrotransposons at the flamenco locus reflect the regulatory properties of piRNA clusters

Most of our understanding of Drosophila heterochromatin structure and evolution has come from the annotation of heterochromatin from the isogenic y; cn bw sp strain. However, almost nothing is known about the heterochromatin's structural dynamics and evolution. Here, we focus on a 180-kb heterochromatic locus producing Piwi-interacting RNAs (piRNA cluster), the flamenco (flam) locus, known to be responsible for the control of at least three transposable elements (TEs). We report its detailed structure in three different Drosophila lines chosen according to their capacity to repress or not to repress the expression of two retrotransposons named ZAM and Idefix, and we show that they display high structural diversity. Numerous rearrangements due to homologous and nonhomologous recombination, deletions and segmental duplications, and loss and gain of TEs are diverse sources of active genomic variation at this locus. Notably, we evidence a correlation between the presence of ZAM and Idefix in this piRNA cluster and their silencing. They are absent from flam in the strain where they are derepressed. We show that, unexpectedly, more than half of the flam locus results from recent TE insertions and that most of the elements concerned are prone to horizontal transfer between species of the melanogaster subgroup. We build a model showing how such high and constant dynamics of a piRNA master locus open the way to continual emergence of new patterns of piRNA biogenesis leading to changes in the level of transposition control.

Environmental factors affecting transcription of the human L1 retrotransposon. II. Stressors

Retrotransposons have clearly molded the structure of the human genome. The reverse transcriptase coded for by long interspersed nuclear elements (LINEs) accounts for 35% of the human genome, with 8-9 x 10(5) copies of the most common human LINE element, L1Hs. Retrotransposons cycle through an RNA intermediate with transcription as the rate limiting step. Because various retrotransposons have been demonstrated to be induced by environmental stimuli, we investigated the response of the L1Hs promoter to various agents. L1Hs promoter activity was analyzed by transfecting an L1Hs-expressing cell line with plasmids containing one of two L1Hs promoters fused to the LacZ reporter gene. L1Hs promoter activity was then monitored with a beta-galactosidase assay. Treatment with UV light and heat shock resulted in a small increase in beta-galactosidase activity from one promoter, while treatment with tetradecanoylphorbol 13-acetate resulted in small increases in beta-galactosidase activity from both promoters. No increase in beta-galactosidase activity was observed after exposure to X-rays or hydrogen peroxide.

Tandem UAA repeats at the 3'-end of the transcript are essential for the precise initiation of reverse transcription of the I factor in Drosophila melanogaster

Non-long terminal repeat retrotransposons, widespread among eukaryotic genomes, transpose by reverse transcription of an RNA intermediate. Some of them, like L1 in the human, terminate at the 3'-end with a poly(dA) stretch whereas others, like the I factor in Drosophila melanogaster, have instead a short sequence repeated in tandem. This suggests different requirements for the initiation of reverse transcription. Here, we have used an RNA circularization/reverse transcription-PCR technique to analyze the 5'- and 3'-ends of the full-length transcripts produced by the I factor at the time of active retrotransposition. These transcripts are capped and polyadenylated similar to conventional messenger RNAs. We have analyzed the 3'-ends of transcripts and transposed copies produced by I elements mutated at the 3'-ends. Transcripts devoid of tandem UAA repeats, although capable of building the components of the retrotransposition machinery, are inefficiently used as retrotransposition intermediates. Such transcripts produce rare new integrated copies issued from the inaccurate initiation of reverse transcription near the 3'-end of the element. The tandem UAA repeats at the 3'-end of the transcripts of I are required for the efficient and precise initiation of reverse transcription. This strong specificity of the I factor reverse transcriptase for its own transcript has implications for the impact of I factor retrotransposition on the host genome.

Artificial and epigenetic regulation of the I factor, a nonviral retrotransposon of Drosophila melanogaster

The I factor (IF) is a LINE-like transposable element from Drosophila melanogaster. IF is silenced in most strains, but under special circumstances its transposition can be induced and correlates with the appearance of a syndrome of female sterility called hybrid dysgenesis. To elucidate the relationship between IF expression and female sterility, different transgenic antisense and/or sense RNAs homologous to the IF ORF1 have been expressed. Increasing the transgene copy number decreases both the expression of an IF-lacZ fusion and the intensity of the female sterile phenotype, demonstrating that IF expression is correlated with sterility. Some transgenes, however, exert their repressive abilities not only through a copy number-dependent zygotic effect, but also through additional maternal and paternal effects that may be induced at the DNA and/or RNA level. Properties of the maternal effect have been detailed: (1) it represses hybrid dysgenesis more efficiently than does the paternal effect; (2) its efficacy increases with both the transgene copy number and the aging of sterile females; (3) it accumulates slowly over generations after the transgene has been established; and (4) it is maintained for at least two generations after transgene removal. Conversely, the paternal effect increases only with female aging. The last two properties of the maternal effect and the genuine existence of a paternal effect argue for the occurrence, in the IF regulation pathway, of a cellular memory transmitted through mitosis, as well as through male and female meiosis, and akin to epigenetic phenomena.