Using Experimental and Computational Strategies to Understand the Biogenesis of microRNAs and piRNAs: A Dissertation

Abstract

Small RNAs are single-stranded, 18–36 nucleotide RNAs that can be categorized as miRNA, siRNA, and piRNA. miRNA are expressed ubiquitously in tissues and at particular developmental stages. They fine-tune gene expression by regulating the stability and translation of mRNAs. piRNAs are mainly expressed in the animal gonads and their major function is repressing transposable elements to ensure the faithful transfer of genetic information from generation to generation. My thesis research focused on the biogenesis of miRNAs and piRNAs using both experimental and computational strategies. The biogenesis of miRNAs involves sequential processing of their precursors by the RNase III enzymes Drosha and Dicer to generate miRNA/miRNA* duplexes, which are subsequently loaded into Argonaute proteins to form the RNA-induced silencing complex (RISC). We discovered that, after assembled into Ago1, more than a quarter of Drosophila miRNAs undergo 3′ end trimming by the 3′-to-5′ exoribonuclease Nibbler. Such trimming occurs after removal of the miRNA* strand from pre-RISC and may be the final step in RISC assembly, ultimately enhancing target messenger RNA repression. Moreover, by developing a specialized Burrow-Wheeler Transform based short reads aligner, we discovered that in the absence of Nibbler a subgroup of miRNAs undergoes increased tailing—non-templated nucleotide addition to their 3′ ends, which are usually associated with miRNA degradation. Therefore, the 3′ trimming by Nibbler might increase miRNA stability by protecting them from degradation. In Drosophila germ line, piRNAs associate with three PIWI-clade Argonaute proteins, Piwi, Aub, and Ago3. piRNAs bound by Aub and Ago3 are generated by reciprocal cleavages of sense and antisense transposon transcripts (a.k.a., the “Ping-Pong” cycle), which amplifies piRNA abundance and degrades transposon transcripts in the cytoplasm. On the other hand, Piwi and its associated piRNA repress the transcription of transposons in the nucleus. We discovered that Aub- and Ago3-mediated transposon RNA cleavage not only generates piRNAs bound to each other, but also produces substrates for the endonuclease Zucchini, which processively cleaves those substrates in a periodicity of ~26 nt and generates piRNAs that predominantly load into Piwi. Without Aub or Ago3, the abundance of Piwi-bound piRNAs drops and transcriptional silencing is compromised. Our discovery revises the current model of piRNA biogenesis.