While all known natural isolates of C. elegans contain multiple copies of the Tc1 transposon, which are active in the soma, Tc1 transposition is fully silenced in the germline of many strains. We mutagenized one such silenced strain and isolated mutants in which Tc1 had been activated in the germline ("mutators"). Interestingly, many other transposons of unrelated sequence had also become active. Most of these mutants are resistant to RNA interference (RNAi). We found one of the mutated genes, mut-7, to encode a protein with homology to RNaseD. This provides support for the notion that RNAi works by dsRNA-directed, enzymatic RNA degradation. We propose a model in which MUT-7, guided by transposon-derived dsRNA, represses transposition by degrading transposon-specific messengers, thus preventing transposase production and transposition.

SUMMARY Some Ts in nuclear DNA of trypanosomes and Leishmania are hydroxylated and glucosylated to yield base J (β-D-glucosyl-hydroxymethyluracil). In Leishmania, about 99% of J is located in telomeric repeats. We show here that most of the remaining J is located at chromosome-internal RNA polymerase II termination sites. This internal J and telomeric J can be reduced by a knockout of J-binding protein 2 (JBP2), an enzyme involved in the first step of J biosynthesis. J levels are further reduced by growing Leishmania JBP2 knockout cells in BrdU-containing medium, resulting in cell death. The loss of internal J in JBP2 knockout cells is accompanied by massive readthrough at RNA polymerase II termination sites. The readthrough varies between transcription units but may extend over 100 kb. We conclude that J is required for proper transcription termination and infer that the absence of internal J kills Leishmania by massive readthrough of transcriptional stops.

Forward genetic screens using novel assays of nematode chemotaxis to soluble compounds identified three independent transposon-insertion mutations in the gene encoding the Caenorhabditis elegans dynein heavy chain (DHC) 1b isoform. These disruptions were mapped and cloned using a newly developed PCR-based transposon display. The mutations were demonstrated to be allelic to the che-3 genetic locus. This isoform of dynein shows temporally and spatially restricted expression in ciliated sensory neurons, and mutants show progressive developmental defects of the chemosensory cilia. These results are consistent with a role for this motor protein in the process of intraflagellar transport; DHC 1b acts in concert with a number of other proteins to establish and maintain the structural integrity of the ciliated sensory endings in C. elegans.

Caenorhabditis elegans (isolate N2 from Bristol, UK) is the first animal of which the complete genome sequence was available. We sampled genomic DNA of natural isolates of C. elegans from four different locations (Australia, Germany, California, and Wisconsin) and found single nucleotide polymorphisms (SNPs) by comparing with the Bristol strain. SNPs are under-represented in coding regions, and many were found to be third base silent codon mutations. We tested 19 additional natural isolates for the presence and distribution of SNPs originally found in one of the four strains. Most SNPs are present in isolates from around the globe and thus are older than the latest contact between these strains. An exception is formed by an isolate from an island (Hawaii) that contains many unique SNPs, absent in the tested isolates from the rest of the world. It has been noticed previously that conserved genes (as defined by homology to genes in Saccharomyces cerevisiae) cluster in the chromosome centers. We found that the SNP frequency outside these regions is 4.5 times higher, supporting the notion of a higher rate of evolution of genes on the chromosome arms.Caenorhabditis elegans is the first animal of which the genome was sequenced (The C. elegans Sequencing Consortium 1998). Recently, the genome sequence of Drosophila has also become available (Adams et al. 2000). C. elegans is a sexually-reproducing animal, but the egg-laying animals are actually hermaphrodites: They produce some sperm that they can use to selffertilize. Self-fertilization quickly results in inbred lines. Although the generation time of C. elegans is ∼3-4 days, it is likely that in the wild the average time of clonal expansion without male-female mating is much longer. The strain Bristol N2, of which the genome sequence was determined, was isolated from mushroom compost in Bristol, UK, before 1956(Nicholas et al. 1959Fatt and Dougherty 1963) and frozen by John Sulston in 1969 (Brenner 1974). This animal occurs worldwide; isolates have been found on all continents except Antarctica (Hodgkin and Doniach 1997). Based on restriction fragment length polymorphisms (RFLPs) associated with Tc1 transposons, at least 20 races were defined. Previous research has indicated that spontaneous mutation rates in C. elegans are low (Anderson 1995), except for transposon insertions in strains that show germ-line transposition. Most strains have been stored frozen since their isolation from nature (Hodgkin and Doniach 1997). For this reason we consider it likely that the single nucleotide polymorphism (SNP) pattern we observe in the strains is identical to that of the original isolate. In this paper we sampled the genome of different natural isolates of C. elegans for SNPs. We investigated the nature of the polymorphisms and determined how they are distributed over the chromosomes and whether we could see differences between coding and noncoding regions. We also investigated how SNPS are distributed over natural isolates from over the globe, and we used this to infer relati...

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