The transposon Tc1 is active in somatic cells of all C. elegans isolates. However, Tc1 activity in the germ line is restricted to certain strains: for example Tc1 is active in the germ line of Bergerac BO, but silenced in Bristol N2. Mori et al. showed that multiple genetic loci are involved in regulation of germ-line transposition (Mori et al. , 1988). In an EMS screen for activation of Tc1 in the germ line of Bristol N2, 43 mutants were isolated that fall into two classes: mutants that are resistant to
pos-1 RNAi, such as
mut-7(
pk204) (Ketting et al. 1999) and
mut-14(
pk738) (see abstract Tijsterman et al. ), and mutants that are wild type for
pos-1 RNAi. The latter class consists of 20 mutants. These mutants are sensitive to RNAi directed against germline-expressed genes (
pos-1,
rba-2 ), as well as somatically-expressed genes (
unc-22 ). Furthermore, one of the mutants,
mut-11 (
pk724) , was tested for
gld-1 cosuppression, and found to be sensitive. Similarly, two previously isolated Tc1 mutators,
mut-4(
st700) and
mut-6(
st702) are completely sensitive to both RNAi (Tabara et al. , 1999; our data) and cosuppression (Ketting and Plasterk, 2000). RNAi-sensitive mutators do share some additional phenotypes with
mut-7(
pk204) . Both RNAi-resistant and RNAi-sensitive mutators are ts-sterile. DAPI-staining of the gonads several mutants shows a reduction of gonad size and the presence of univalents in diakinesis. Like
mut-7 , most RNAi-sensitive mutators are Him. It is possible that the different mutations are directly causing the Him-phenotype and the ts-sterility; on the other hand, these phenotypes might result from the activation of transposons in the germ line, reminiscent of the hybrid dysgenesis syndrome in Drosophila . Initial mapping of the ts-sterility phenotype of fifteen of the RNAi-sensitive mutants indicates this class of mutators consists of at least seven genes. Further progress will be reported. In vitro experiments using Drosophila have shown that in the initiation step of RNAi, dsRNA is cleaved into guide RNAs. Next, these guide RNAs can target a nuclease complex to the mRNA, which is then degraded. We now show that also in C. elegans extracts, exogenous dsRNA is cleaved into 23 bp guide RNAs. So far, one gene has been shown to be involved in the formation of guide RNAs: Drosophila Dicer , encoding a RNase III enzyme. The Dicer gene has one clear homolog in C. elegans : K12H4.8. In order to analyze the role of Dicer in RNAi in C. elegans , a deletion mutant of K12H4.8 was isolated from a chemical deletion library. A 1.7 kb deletion removes the helicase domain and part of the PAZ domain, and is likely to be a complete null allele. The mutant, after outcrossing, is sterile and shows a weak
unc-22 phenotype after feeding of
unc-22 dsRNA. Rescue experiments and phenotypes will be discussed.