Cohesin maintains cohesion of the sister chromatids after replication, and is essential for proper segregation of chromosomes. Recent studies have shown that progression of anaphase depends on the cleavage of cohesin subunit Rad21/Scc1 by separin protease in yeast. Yeast cohesins are bound between sisters along the entire chromosomes and are cleaved simultaneously at the onset of anaphase, whereas in vertebrates cohesins on the arm region dissociate without cleavage and are replaced by condensins in prophase, and only cohesins at the centromeric regions hold sister chromotids together until they are cleaved. The cohesin complex contains either Rad21/Scc1or Rec8, Scc3, Smc1, and Smc3. This subunit composition is highly conserved from yeast to humans. Almost all the organisms have at least two distinct cohesin complexes, in which different types of subunits are contained. For instance, yeast has two complexes, one including Rad21/Scc1 for mitosis and the other including Rec8 for meiosis. In the nematode C. elegans we identified two genes homologous to
rad21 by genome-wide database search. Only C. elegans has two genes homologous to
rad21 so far. To see whether the two types of cohesin complexes function differently and how they are developmentally regulated, we initiated molecular and functional characterization of the two RAD-21 homologs in C. elegans . One of the two genes (F10G7.4; hereafter
rad-21.1 ) turned out to be essential for embryogenesis, because we observed 100% embryonic lethality by RNAi-by-soaking of this gene. When closely observed, each cell division in
rad-21.1 -depleted embryos took longer than in wild type, although the patterns of cell lineage seemed unaffected. The appearance of nuclei was abnormal, but the amount of DNA in each nucleus seemed equal and not affected judging from DAPI staining. Whether there are any minor defects of chromosome segregation in
rad-21.1 (RNAi) embryos is under investigation by FISH analysis.
rad-21.1 was also essential for fertility and larval development, because complete sterility (by RNAi-by-L1-soaking) and larval arrest as escaper (by dsRNA injection) were observed. We produced polyclonal antibodies against RAD-21.1 protein, and found that in early-satge embryos RAD-21.1 protein localize to the chromosomes in a cell cycle dependent manner, as has been seen in vertebrates. RAD-21.1 could also be detected in the germ cell nuclei in larval-stage worms and in condensed germ cell nuclei in adult gonads. The other gene (K08A8.3;
rad-21.2 ) appeared to function in larval development, because we observed larval arrest by RNAi, although the penetrance was not high (aproximately 50%). The level of transcription of
rad-21.2 was lower than
rad-21.1 , judged by nothern blotting and whole mount in situ hybridization. We found that RAD-21.2 protein was localized to almost all nuclei in late embryos and larvae, also suggesting the function of RAD-21.2 in the larval development. Unlike RAD-21.1, RAD-21.2 was not detected in nuclei in early embryos, nor in germ cell nuclei in larvae. From these results, we speculate that the two
rad-21 genes perform different functions and are regulated developmentally in C.elegans .