An RNA-dependent RNA polymerase is required for paramutation in maize

Paramutation, an allele-dependent transfer of epigenetic information, which results in the heritable silencing of one allele by another, requires the mop1 (mediator of paramutation 1) gene to occur in maize. Current proposed and demonstrated models of paramutation include trans- acting RNAs, and interactions between chromatin protein complexes and encoded tandem repeats. To elucidate the mechanism of this phenomena, Alleman et al. identified by positional cloning that mop1 coded for an RNA- dependent RNA polymerase (RDRP) homolog gene, in a mutated form that has a stop codon upstream to the polymerase domain. Therefore, their results provide support for the trans- acting RNA model. This discovery is in agreement with other investigations that also found an involvement of RNAi pathways in RNA- directed DNA methylation, for example with one previously discussed here; although Alleman and colleagues failed to detect any specific siRNA for this proposed mechanism of paramutation in maize, and they provided possible explanations for this:

We propose that the level of transcription occurring in the three genotypes is not sufficient to trigger paramutation or to maintain transcriptional silencing [...] and that the RDRP encoded by mop1 is required to produce a higher threshold of RNA [...] that mediates these processes. [...] An intriguing question is why the transcribed single-copy sequence does not induce silencing. A hypothesis about the importance of tandem arrays in maintaining silencing in centromeric heterochromatin offers a possible explanation. Another idea is that the unique junction fragments created by the tandem repeats have specific properties. Because the penetrance of paramutation increases with the number of repeats, a mechanism that senses the number of repeat junctions needs to be postulated.

[...] Our results add the classical paramutation phenomenon to a growing list of processes involving the RNA-mediated formation of chromatin domains. Some examples include dosage compensation in flies, X-inactivation in mammals, RNA-directed DNA methylation in plants, meiotic silencing of unpaired chromatin in Neurospora and C. elegans, and transcriptional silencing of centromeres, transgenes and transposons in multiple species. Like paramutation, most of these phenomena share epigenetic regulation, long-range interactions and chromosome dynamics. Unique features of b1 paramutation [in maize] are its extreme penetrance and faithful transmission through meiosis.

Now that these results proposed that mop1 is a RDRP homolog, it would be interesting to characterize the activities of this yet hypothetical protein, and its structural properties as well.

Alleman M. et al. 2006. Nature 442:295-298

epigenetics

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