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.