DNA topoisomerase III can interact with RecQ family DNA helicases functionally and physically in eukaryotic cells, contributing to genome stability. Among four RecQ homologues predicted from the Caenorhabditis elegans genomic DNA sequence, T04A11.6 is the most similar to Bloom syndrome!-s protein in humans. To investigate a possible interaction of the protein with topoisomerase III a (Top3 a ), as observed between Top3 and RecQ homologues in yeast and human, the
top3 a gene expression was suppressed by RNA interference in a C. elegans mutant,
him-6 (
e1104 ) with an amino acid substitution in the RecQ homologue, T04A11.6(communicated by Drs. F. Mueller and C. Wicky). Germ cells in the gonads of the progeny showed severe chromosomal abnormalities and were arrested during mitosis with a subsequent failure of entering meiosis. These phenotypes were also observed in the progeny produced by double RNA interference of the
top3 a and recQ gene expression, though at a reduced level. The severe phenotypes resulting from the double deficiency of
top3 a and recQ contrasted with the results in yeast, where a recQ mutation suppressed some phenotypes of
top3 null mutation. Therefore, in C. elegans, Top3 a and the RecQ homologue (T04A11.6) act either together in a complex or separately in two redundant pathways, which contribute to genome stability during germline mitosis.