PIWI-interacting RNAs (piRNA) are a group of 23–35 nucleotide (nt) short RNAs that protect animal gonads from transposon activities. In Drosophila germ line, piRNAs can be categorized into two different categories— primary and secondary piRNAs— based on their origins. Primary piRNAs, generated from transcripts of specific genomic regions called piRNA clusters, which are enriched in transposon fragments that are unlikely to retain transposition activity. The transcription and maturation of primary piRNAs from those cluster transcripts are poorly understood. After being produced, a group of primary piRNAs associates Piwi proteins and directs them to repress transposons at the transcriptional level in the nucleus. Other than their direct role in repressing transposons, primary piRNAs can also initiate the production of secondary piRNA. piRNAs with such function are loaded in a second PIWI protein named Aubergine (Aub). Similar to Piwi, Aub is guided by piRNAs to identify its targets through base-pairing. Differently, Aub functions in the cytoplasm by cleaving transposon mRNAs. The 5' cleavage products are not degraded but loaded into the third PIWI protein Argonaute3 (Ago3). It is believed that an unidentified nuclease trims the 3' ends of those cleavage products to 23–29 nt, becoming mature piRNAs remained in Ago3. Such piRNAs whose 5' ends are generated by another PIWI protein are named secondary piRNAs. Intriguingly, secondary piRNAs loaded into Ago3 also cleave transposon mRNA or piRNA cluster transcripts and produce more secondary piRNAs loaded into Aub. This reciprocal feed-forward loop, named the “Ping-Pong cycle”, amplified piRNA abundance.
By dissecting and analyzing data from large-scale deep sequencing of piRNAs and transposon transcripts, my dissertation research elucidates the biogenesis of germline piRNAs in Drosophila.
How primary piRNAs are processed into mature piRNAs remains enigmatic. I discover that primary piRNA signal on the genome display a fixed periodicity of ~26 nt. Such phasing depends on Zucchini, Armitage and some other primary piRNA pathway components. Further analysis suggests that secondary piRNAs bound to Ago3 can initiate phased primary piRNA production from cleaved transposon RNAs. The first ~26 nt becomes a secondary piRNA that bind Aub while the subsequent piRNAs bind Piwi, allowing piRNAs to spread beyond the site of RNA cleavage. This discovery adds sequence diversity to the piRNA pool, allowing adaptation to changes in transposon sequence. We further find that most Piwi-associated piRNAs are generated from the cleavage products of Ago3, instead of being processed from piRNA cluster transcripts as the previous model suggests. The cardinal function of Ago3 is to produce antisense piRNAs that direct transcriptional silencing by Piwi, rather to make piRNAs that guide post-transcriptional silencing by Aub. Although Ago3 slicing is required to efficiently trigger phased piRNA production, an alternative, slicing-independent pathway suffices to generate Piwi-bound piRNAs that repress transcription of a subset of transposon families. The alternative pathway may help flies silence newly acquired transposons for which they lack extensively complementary piRNAs.
The Ping-Pong model depicts that first ten nucleotides of Aub-bound piRNAs are complementary to the first ten nt of Ago3-bound piRNAs. Supporting this view, piRNAs bound to Aub typically begin with Uridine (1U), while piRNAs bound to Ago3 often have adenine at position 10 (10A). Furthermore, the majority of Ping-Pong piRNAs form this 1U:10A pair. The Ping-Pong model proposes that the 10A is a consequence of 1U. By statistically quantifying those target piRNAs not paired to g1U, we discover that 10A is not directly caused by 1U. Instead, fly Aub as well as its homologs, Siwi in silkmoth and MILI in mice, have an intrinsic preference for adenine at the t1 position of their target RNAs. On the other hand, this t1A (and g10A after loading) piRNA directly give rise to 1U piRNA in the next Ping-Pong cycle, maximizing the affinity between piRNAs and PIWI proteins.
| Identifer | oai:union.ndltd.org:umassmed.edu/oai:escholarship.umassmed.edu:gsbs_diss-1804 |
| Date | 01 October 2015 |
| Creators | Wang, Wei |
| Publisher | eScholarship@UMassChan |
| Source Sets | University of Massachusetts Medical School |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Morningside Graduate School of Biomedical Sciences Dissertations and Theses |
| Rights | Copyright is held by the author, with all rights reserved. |
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