Participation of the piRNA pathway in recruiting a component of RNA polymerase I transcription initiation complex to germline cell nucleoli

Abstract

Proteins of the Piwi family and short Piwi-interacting RNAs (piRNAs) ensure the protection of the genome from transposable elements. We have previously shown that nuclear Piwi protein tends to concentrate in the nucleoli of the cells of Drosophila melanogaster ovaries. It could be hypothesized that the function of Piwi in the nucleolus is associated with the repression of R1 and R2 retrotransposons inserted into the rDNA cluster. Here, we show that Piwi participates in recruiting Udd protein to nucleoli. Udd is a component of the conserved Selectivity Factor I-like (SL1-like) complex, which is required for transcription initiation by RNA polymerase I. We found that Udd localization depends on Piwi in germline cells, but not in somatic cells of the ovaries. In contrast, knockdowns of the SL1-like components (Udd or TAF1b) do not disrupt Piwi localization. We also observed that the absence of Udd or TAF1b in germline cells, as well as the impairment of Piwi nuclear localization lead to the accumulation of late stage egg chambers in the ovaries, which could be explained by reduced rRNA transcription. These results allow us to propose for the first time a role for Piwi in the nucleolus that is not directly associated with transposable element repression.