Abstract
The goal of this research project is to ascertain the role of factors in the initiation of PIWI-directed silencing of transposable elements (TEs). There have been genetic studies on the factors involved in the PIWI/piRNA pathway, but the initiation of TE silencing is not fully understood. To study this question, we applied a Piwi-interacting RNA (piRNA)-targeted reporter assay to Drosophila OSS cells. We found that piRNA biogenesis factors and PIWI-associated factors have direct roles in the initiation of PIWI silencing and that chromatin-modifying factors may act downstream. Our findings also reveal that RNA Polymerase II associated proteins PAF1 and RTF1 oppose PIWI-directed silencing. When the piRNA targeted transcript region is close to the promoter, PAF1 and RTF1 knockdowns enhance PIWI silencing of the reporters. Our results show that the role of PAF1 in the AGO1/siRNA-directed silencing of fission yeast may be conserved with the role of PAF1 in PIWI/piRNA-directed silencing in Drosophila. Our reporter assay was also successfully applied for PIWI knockdown in WRR-1 cells that have a male genotype, confirming that PIWI has a direct role in TE silencing in this cell line. This study indicates that our reporter assay can be used to test other factors in the initiation of PIWI-directed silencing. We hope to apply the assay to OSC-delta-I(3)mbt cell that expresses the secondary piRNA ping-pong pathway, but transfection conditions need to be optimized for the assay to work.