Worm Breeder's Gazette 15(3): 23 (June 1, 1998)
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Transposons In The C.elegans Germline Are Regulated At The Transcriptional Level.
Rene F. Ketting,
Henri G.A.M. van Luenen,
Ronald H.A. Plasterk
Department of Molecular Biology, The Netherlands Cancer
Institute, Plesmanlaan 121, 1066 CX Amsterdam, the
NetherlandsGermline transposition in C.elegans is severely repressed compared to somatic transposition. For example in Bristol N2, germline transposition is not detectable whereas transposition events in somatic cells can readily be detected. We want to understand the mechanism underlying this phenomenon.
To do this, we isolated a new mutator allele, mut-7(pk204), that was generated using EMS mutagenesis, using a similar approach as described by John Collins (1,2). One important difference between the two approaches is that, in contrast to the screen by Collins et al., this screen starts with a strain that shows no transposition activity in the germline at all. We recently did another screen to get more mutator alleles, but instead of using the unc-54(r323::Tc1) allele as transposition detector we now used the unc-22(st136::Tc1) allele. We screened about 12.000 mutagenised genomes and found 46 putative mutators. They are in the process of being retested, but the majority is turning out to be real mutators. This provides us with an extensive collection of mutants, with which it will be possible to further examine transposition regulation in the germline.
The mut-7(pk204) allele activates multiple transposons (Tc1, Tc3, Tc4 and Tc5), and displays a slightly higher X-chromosome non-disjunction. Both these phenotypes are increased at elevated temperatures. The mutation also causes temperature sensitive sterility. At 25C between 80 and 90% of the oocytes remain unfertilized, or die for another, unknown reason.
In collaboration with Bill Kelly and Andy Fire we also looked at germline gene expression from a transgene. Simple transgenic arrays are usually silenced in the germline, even if the promoter driving the construct is normally active in the germline. We tested a transgene containing let-858 fused to GFP (3). This GFP fusion should be expresses in the nuclei of all cells, including the germline. However, the germlines of these animals stay dark. When mut-7(pk204) is introduced, the germline nuclei turn green, indicating that the transgene is not properly silenced anymore.
>From this we conclude that mut-7(pk204) causes improper gene expression in the germ-line. This could also lead to expression of normally silent transposase genes, resulting in transposase protein in the germ-line. As we know that transposase protein alone is sufficient to catalyze the transposition reaction (at least for Tc1 (4)), transposons will now start jumping in the germline.
This model of a general derepression of germline transcription provides an explanation for the intriguing observation that one mutation could activate diverse transposons such as Tc1, Tc3, Tc4 and Tc5 (1,2).
1) John Collins, Bonnie Saari and Philip Anderson (1987) Nature 328: 726-728.
2) Rene F.Ketting, Miriam Th Smits and Ronald H.A.Plasterk (1996) WBG 14(2):24.
3) William G.Kelly and Andrew Fire (1995) WBG 14(1):63.
4) Jan C.Vos, Ivo de Baere and Ronald H.A.Plasterk (1996) Genes & Dev. 10:755-761.