Measuring the rates of transcriptional elongation in the female Drosophila melanogaster germ line by nuclear run-on

Global Run-On Sequencing (GRO-seq) Library Preparation from Drosophila Ovaries

In the past decade, deep-sequencing approaches have greatly improved our knowledge of the genome's potential and have become a crucial milestone for new discoveries in genomics. Transcription is the first step of gene expression; therefore, the detection and measurement of transcription rates is of great interest. Here, a detailed protocol for global run-on sequencing (GRO-seq) library preparation from Drosophila ovaries is described. The method relies on rapid isolation of nuclei with halted transcription, then restarting transcription in physiological conditions in the presence of a labeled nucleotide. The newly transcribed nascent RNA is then isolated and cloned using a small RNA cloning protocol. Although it is time-consuming, the global run-on method allows the user to profile the position, orientation and amount of transcriptionally engaged RNA polymerases across the genome, therefore providing a snapshot of genome-wide transcription.

Targeting heterochromatin formation to transposable elements in Drosophila: potential roles of the piRNA system

Successful heterochromatin formation is critical for genome stability in eukaryotes, both to maintain structures needed for mitosis and meiosis and to silence potentially harmful transposable elements. Conversely, inappropriate heterochromatin assembly can lead to inappropriate silencing and other deleterious effects. Hence targeting heterochromatin assembly to appropriate regions of the genome is of utmost importance. Here we focus on heterochromatin assembly in Drosophila melanogaster, the model organism in which variegation, or cell-to-cell variable gene expression resulting from heterochromatin formation, was first described. In particular, we review the potential role of transposable elements as genetic determinants of the chromatin state and examine how small RNA pathways may participate in the process of targeted heterochromatin formation.

Small RNA-directed silencing: the fly finds its inner fission yeast?

Several recent studies demonstrate that piRNAs guide Piwi protein to repress transposon transcription in fly ovaries, much as fission yeast use siRNAs to silence repeat sequences. Still mysterious though is how Piwi targets euchromatic transposons for silencing, but not the specialized heterochromatic loci that produce piRNA precursors.

Mechanism of the piRNA-mediated silencing of Drosophila telomeric retrotransposons

In the Drosophila germline, retrotransposons are silenced by the PIWI-interacting RNA (piRNA) pathway. Telomeric retroelements HeT-A, TART and TAHRE, which are involved in telomere maintenance in Drosophila, are also the targets of piRNA-mediated silencing. We have demonstrated that expression of reporter genes driven by the HeT-A promoter is under the control of the piRNA silencing pathway independent of the transgene location. In order to test directly whether piRNAs affect the transcriptional state of retrotransposons we performed a nuclear run-on (NRO) assay and revealed increased density of the active RNA polymerase complexes at the sequences of endogenous HeT-A and TART telomeric retroelements as well as HeT-A-containing constructs in the ovaries of spn-E mutants and in flies with piwi knockdown. This strongly correlates with enrichment of two histone H3 modifications (dimethylation of lysine 79 and dimethylation of lysine 4), which mark transcriptionally active chromatin, on the same sequences in the piRNA pathway mutants. spn-E mutation and piwi knockdown results in transcriptional activation of some other non-telomeric retrotransposons in the ovaries, such as I-element and HMS Beagle. Therefore piRNA-mediated transcriptional mode of silencing is involved in the control of retrotransposon expression in the Drosophila germline.

Evolutionary dynamics of transposable elements in a small RNA world

Transposable elements (TEs) are selfish elements that cause harmful mutations, contribute to the structure of regulatory networks and shape the architecture of genomes. Natural selection against their harmful effects has long been considered the dominant force limiting their spread. It is now clear that a genome defense system of RNA-mediated silencing also plays a crucial role in limiting TE proliferation. A full understanding of TE evolutionary dynamics must consider how these forces jointly determine their proliferation within genomes. Here I consider these forces from two perspectives - dynamics within populations and evolutionary games within the germline. The analysis of TE dynamics from these two perspectives promises to provide new insight into their role in evolution.

[Regulation of fertility by the small RNA pathway that defends the genome against transposons]

Small RNAs can silence transposons at transcriptional or post-transcriptional level. Newly identified small RNAs (e.g., piRNAs and endo-siRNAs) can repress the activity of transposons. Drosophila piRNAs, mouse piRNAs, mouse endo-siRNAs, and their related proteins are expressed primarily within the germline. This suggests that the small RNA pathway plays an important role in transposon silencing of the germline. Recent studies revealed the connection between small RNAs, transposon silencing, and the regulation of fertility, but the mechanism has not been clarified in detail. In this review, the advances in the control of genomic transposon activity and the regulation of fertility by the small RNA pathway were summarized and discussed.

Small RNAs as guardians of the genome

Transposons populate the landscape of all eukaryotic genomes. Often considered purely genomic parasites, transposons can also benefit their hosts, playing roles in gene regulation and in genome organization and evolution. Peaceful coexistence with mobile elements depends upon adaptive control mechanisms, since unchecked transposon activity can impact long-term fitness and acutely reduce the fertility of progeny. Here, we review the conserved roles played by small RNAs in the adaptation of eukaryotes to coexist with their genomic colonists. An understanding of transposon-defense pathways has uncovered recurring themes in the mechanisms by which genomes distinguish "self" from "non-self" and selectively silence the latter.

Interactions between transposable elements and Argonautes have (probably) been shaping the Drosophila genome throughout evolution

Transposable elements (TEs) are powerful mutagenic agents responsible for generating variation in the host genome. As TEs can be overtly deleterious, a variety of different mechanisms have evolved to keep their activities in check. In plants, fungi, and animals, RNA silencing has been implicated as a major defense against repetitive element transposition. This nucleic acid-based defense mechanism also appears to be directed at inherited silencing of TEs without altering the underlying DNA sequence. Complex interactions between TEs and RNA silencing machineries have been co-opted to regulate cellular genes.