Genetic Factors Linking Nucleolar Stress with Retrotransposon Expression in .

retrotransposons reside exclusively within the regions of 10-20% of all rDNA genes comprising the nucleolar organizer loci on the X and Y chromosomes of . These -inserted genes are normally silent and heterochromatic. When expressed, however, the transcript is co-transcribed with the rRNA. Self-cleavage releases a 3.6 kb mature transcript that encodes a single protein with endonuclease and reverse transcriptase activities that facilitate element transposition by target-primed reverse transcription. While we know the molecular details of transposition, we know little about the genetic mechanisms that initiate transcription. Here, we examine expression in wild type versus mutant backgrounds. expression in stage 1-4 wild type egg chambers was variable depending on the stock. expression was silent in wild type stages 5-10 but was consistently active during nurse cell nuclear breakdown in stages 12-13 regardless of the genetic background. Massive expression occurred in stages 5-10 upon loss of Udd, an RNA Pol I transcription factor. Similarly, loss of Nopp140, an early ribosome assembly factor, induced expression more so in somatic tissues. Interestingly, over-expression of the Nopp140-RGG isoform but not the Nopp140-True isoform induced expression in larval somatic tissues, suggesting Nopp140-RGG could potentially affect rDNA chromatin structure. Conversely, mutations in genes encoding ribosomal proteins had minor positive effects on expression. We conclude that expression is largely controlled by factors regulating RNA Pol I transcription and early ribosome assembly.