Investigation of the timing of secondary small RNA biogenesis during programmed DNA elimination in Tetrahymena

ABSTRACT: Signal amplification of the initial small RNA trigger is important to ensure the silencing of repetitive transposable elements (TEs). Curiously, secondary small RNA biogenesis occurs by various mechanisms that are coupled with distinct steps of TE silencing in different eukaryotes, such as nucleolytic cleavage of TE transcripts, recruitment of RNA-dependent RNA polymerase, and heterochromatin-directed transcription. How such a variety of small RNA amplification mechanisms has evolved has not been thoroughly elucidated to date. Ciliated protozoa perform small RNA-directed programmed DNA elimination of thousands of TE-related internal eliminated sequences (IESs) in the newly developed somatic nucleus. In the ciliate Paramecium, secondary small RNAs are produced after primary small RNAs induce the excision of IESs. To examine whether such post-excision production of secondary small RNAs is conserved, we investigate the causality between the excision of IESs and the biogenesis of secondary small RNAs in another ciliate, Tetrahymena. We show that secondary small RNAs accumulate at least a few hours before their derived IESs are excised and that DNA excision is dispensable for their biogenesis in this ciliate. Therefore, unlike the situation in Paramecium, small RNA amplification occurs prior to IES excision in Tetrahymena. This study reveals remarkable mechanistic diversity of secondary small RNA biogenesis mechanisms, even among ciliates showing similar DNA elimination processes, and thus raises the possibility that the evolution of TE-targeting small RNA amplification can be traced by investigating the DNA elimination mechanisms of ciliates.

ORGANISM(S): Tetrahymena thermophila

PROVIDER: GSE125297 | GEO | 2019/06/30

REPOSITORIES: GEO