A survey of C. elegans small RNAs from 2006 (Ruby et al) first reported the discovery of a large class of 21nt RNAs with 5’ uridines – 21U-RNAs. The majority of these RNAs mapped to two distinct regions of the C. elegans genome located on chromosome IV. Generally, they were found between genes or in introns, and showed no sense or antisense bias (when within introns). Each 21U-RNA genomic locus shared an upstream sequence motif. This 34bp motif centred on an 8bp core consensus located 38bp upstream from the start of the 21U-coding sequence. Ruby et al had mapped 5,302 unique 21U-RNAs, but using the upstream consensus they identified 10,807 putative 21U-RNA generating loci on chromosome IV. The functional significance of the upstream motifs was not clear; it seemed probable that they could act as promoters, and that 21U-RNAs were individually transcribed, but it was also feasible that these sequences were sites for targeted cleavage of longer transcripts, or that they were recognition sequences for RNA dependent RNA polymerases. Interestingly, the upstream motif was conserved in the related nematode C. briggsae, but none of the 21U-RNAs themselves were. Very few 21U-RNAs matched other sequences in the genome, but in total they encoded a huge diversity of sequence. It appeared that evolutionary pressure was maximising this sequence complexity rather than maintaining sequence identity. The most important questions on 21U-RNAs were therefore clear from the start; what are the targets? What’s the function of the system?
In 2008, parallel studies from the labs of Craig Mello and Eric Miska further characterised 21U-RNAs (Batista et al. Das et al). As well as bringing their total number to 15,722, they discovered that they were solely expressed in the germline and associate with a Piwi-family Argonaute protein, PRG-1. prg-1 null mutants display dramatic reductions in germ cell numbers as well as an independent temperature sensitive fertility defect. 21U-RNAs fail to accumulate in prg-1 mutants, and co-immunoprecipitate with PRG-1. Like many other Piwi proteins, PRG-1 is localised to the perinuclear nuage in the germline (P-granules).
21U-RNAs are therefore the piwi-interacting RNAs (piRNAs) of C. elegans. Like piRNAs in vertebrates and Drosophila, they are 5’ monophosphorylated, have 5’ uridine residues, and are 3’ modified. Most importantly they interact with a Piwi-family AGO, and are implicated in germline functions. However, they do display some major differences not encountered in piRNAs from other clades; they are only 21nt long (whilst those in vertebrates and Drosophila are 24-30nt in length) and most strangely they appear to be individually transcribed. As with 21U-RNAs, piRNAs in other animals are generated from large genomic clusters. However, in Drosophila and mammals individual piRNAs derive from larger cluster transcripts. Another important difference with other piRNA systems was the lack of a ping-pong piRNA amplification system.
As the main known role for PIWI/piRNA systems is the silencing of transposable elements, Das et al. and Batista et al. looked for evidence of a similar function for the PRG-1/21U-RNA system. Das et al. screened worms mutant for both prg-1 and the closely related gene prg-2 for signs of transposon desilencing. The only transposon found to be affected was Tc3. Expression of the Tc3 transposase mRNA was higher, and the reversion rate of mutations caused by Tc3 insertions increased 1000 fold in the double mutants (interestingly, the reversion rate was up only 100 fold in prg-1 single mutants; the only evidence of prg-2 having a functional role in the 21U-RNA system). Das et al. identified a single 21U-RNA that mapped to the sense strand of the Tc3 transposase gene. They then found that a large number of endogeneous siRNAs (ie WAGO associated 22G-RNAs) targeted against both the transposase gene and the terminal inverted repeats (TIR) were strongly depleted in the prg1+2 mutants. Batista et al reported slightly different findings: They identified a different 21U-RNA mapping to the TIR of Tc3 in the same orientaion as the transposase gene. When they searched for endo-siRNAs against Tc3 that were depleted in the prg-1 mutants, they only found that those targeted against the TIR were affected. The discrepancies between the two studies could indicate slightly different roles for the two PRG proteins.
Importantly, both studies found that the PRG-1/21U-RNA system functions upstream of the WAGO/22G-RNA system in transposon control. The specifics were rather hazy though. The 21U-RNAs matching Tc3 sequence were both orientated sense to the transposase gene, and so would not be able to base-pair with the transposase mRNA. The 21U-RNA identified by Batista et al. wasn’t even directed against a part of the transposon expected to be transcribed. However, the 22G-RNAs that were sensitive to prg-1 function were generally antisense to the direction of transposase transcription. These findings suggested a model in which transcription from downstream genomic regions may generate antisense Tc3 transcripts which would be recognised by a PRG-1/21U-RNA complex. This would then trigger the RdRP-dependent synthesis of 22G-RNAs against the transposon transcripts, that would then silence the transposons most probably through alterations to chromatin structure.
Linking the PRG-1/21U-RNA system to the WAGO/22G-RNA was a major step, and could be a general mechanism for 21U-RNA action. The 22G-RNA mediated stage of the process can be viewed as equivalent to the ping-pong amplified secondary piRNA stage in Drosophila transposon silencing. However, these findings raised important questions; If this is a system for the control of mobile elements, why was only one transposon found to be desilenced in prg-1 mutants, and so few 21U-RNAs found to match transposon sequences? And what about the vast number of 21U-RNAs without identity to other genomic sequences? A similar phenomenon is seen in mammals, in which a huge pool of ‘pachytene’ piRNAs without known targets or functions, are found. To explain this enigma, Batista et al. suggested that 21U-RNAs could base-pair imperfectly with their targets. microRNAs are able to recognise their targets by base-pairing within a ‘seed’ region driving less stringent pairing between the rest of the molecules. If 21U-RNAs work by a similar mode, the whole worm transcriptome could be under piRNA-directed regulation. Only modest changes in general gene expression were found in the prg-1 mutants, so it’s unclear what this global genome regulation or surveillance system really means.
I get the impression that a dichotomy of interpretations has arisen between the two principal labs studying 21U-RNAs. This can be broadly expressed as Eric Miska considering 21U-RNAs as a mechanism primarily directed against genomic parasites, whilst Craig Mello favours a broader ‘global surveillance system’ interpretation. The next post, on the recent papers from these labs, will try to dissect these different interpretations and clarify the current state of knowledge of C. elegans piRNAs.
Stop Press: A new paper has just cleared up the contentious issues of the function of the upstream motif, and whether 21U-RNAs are individually transcribed. Cecere et al. show that the upstream motifs are indeed promoters. Forkhead family transcription factors bind the motif, and drive the separate expression of the thousands of 21U-RNAs.
Ruby JG, Jan C, Player C, Axtell MJ, Lee W, Nusbaum C, Ge H, & Bartel DP (2006). Large-scale sequencing reveals 21U-RNAs and additional microRNAs and endogenous siRNAs in C. elegans. Cell, 127 (6), 1193-207 PMID: 17174894
Das PP, Bagijn MP, Goldstein LD, Woolford JR, Lehrbach NJ, Sapetschnig A, Buhecha HR, Gilchrist MJ, Howe KL, Stark R, Matthews N, Berezikov E, Ketting RF, Tavaré S, & Miska EA (2008). Piwi and piRNAs act upstream of an endogenous siRNA pathway to suppress Tc3 transposon mobility in the Caenorhabditis elegans germline. Molecular cell, 31 (1), 79-90 PMID: 18571451
Batista PJ, Ruby JG, Claycomb JM, Chiang R, Fahlgren N, Kasschau KD, Chaves DA, Gu W, Vasale JJ, Duan S, Conte D Jr, Luo S, Schroth GP, Carrington JC, Bartel DP, & Mello CC (2008). PRG-1 and 21U-RNAs interact to form the piRNA complex required for fertility in C. elegans. Molecular cell, 31 (1), 67-78 PMID: 18571452
Cecere G, Zheng GX, Mansisidor AR, Klymko KE, & Grishok A (2012). Promoters Recognized by Forkhead Proteins Exist for Individual 21U-RNAs. Molecular cell PMID: 22819322