Worm Breeder's Gazette 16(2): 28

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

A genetic link between RNAi, transposon silencing and cosuppression.

Rene F. Ketting, Ronald H.A. Plasterk

The Netherlands Cancer Institute, Division of Molecular Biology, Center for Biomedical Genetics, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands

Originally discovered in plants, the phenomenon of cosuppression by transgenic DNA has since been observed in diverse organisms, ranging from fungi to animals: introduction of transgenic copies of a gene results in reduced expression of the transgene as well as the endogenous gene. The effect depends on sequence identity between transgene and endogenous gene. Some cases of cosuppression resemble RNA interference, as RNA appears to be both an important initiator as well as a target in these processes. In C. elegans several cases of cosuppression have been described 1. We used three genes for which this is the case: fem-1 2, gld-1 3 and mrt-2 4.
For fem-1 it was already described that the promoter region is required for the cosuppression effect 2, suggesting that RNA production from the transgene is essential. We found that also for gld-1 and mrt-2 induced cosuppression the promoter regions are required. In addition we could show that transgenes containing only the gld-1 promoter do not induce cosupprssion. Together these data further support the idea that transcriptional activity of transgenes is essential to induce cosuppression, and that the effect is not based on a promoter competition effect.
Both RNAi and cosuppression have been implicated in the silencing of transposons. We here report that mutants that have a defect in transposon silencing and RNAi (mut-2(r459)I, mut-7(pk204)III, mut-8(pk716)I and mut-9(pk734)IV) are in addition resistant to cosuppression. Of this set, the mut-7 gene has been cloned; it encodes a protein that has a catalytic domain homologous to RNaseD 5. Taken together this suggests that RNA interference and cosuppression in C. elegans are mediated at least in part by the same molecular machinery, possibly via a MUT-7 catalysed RNA-guided degradation of mRNA's.
In addition to the RNAi resistant mutators we also tested the involvement of the non-RNAi resistant mutator mut-6(st702)IV. We found that this mutator locus does not confer resistance to cosuppression.
We also tested the involvement of the rde-1 gene in cosuppression. rde-1 encodes a protein that shares homology with eIF2C, and is essential for RNAi, but does not result in germline transposon activation 6. Interestingly we found that this gene is not required for efficient cosuppression.
Alltogether, these data show that RNAi and cosuppression do have a mechanistic link, and are involved in the silencing of transposable elements, but also that RNAi and cosuppression are clearly not identical processes.

1 Zalevsky et al., 1998 West Coast Worm Meeting abstract 66
2 Gaudet et al., Mol.Biol.Cell. 7, 1107-1121 (1996)
3 Jones and Schedl, Genes Dev. 9, 1491-1504 (1995)
4 Ahmed and Hodgkin, 1999 International Worm Meeting abstract 60
5 Ketting et al., Cell 99, 133-141 (1999)
6 Tabara et al., Cell 99, 123-132 (1999)