Targeted Gene Knockout Reveals a Role in Meiotic Recombination for ZHP-3, a Zip3-Related Protein in Caenorhabditis elegans

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Abstract
The meiotically expressed Zip3 protein is found conserved from Saccharomyces cerevisiae to humans. In baker's yeast, Zip3p has been implicated in synaptonemal complex (SC) formation, while little is known about the protein's function in multicellular organisms. We report here the successful targeted gene disruption of zhp-3 (K02B12.8), the ZIP3 homolog in the nematode Caenorhabditis elegans. Homozygous zhp-3 knockout worms show normal homologue pairing and SC formation. Also, the timing of appearance and the nuclear localization of the recombination protein Rad-51 seem normal in these animals, suggesting proper initiation of meiotic recombination by DNA double-strand breaks. However, the occurrence of univalents during diplotene indicates that C. elegans ZHP-3 protein is essential for reciprocal recombination between homologous chromosomes and thus chiasma formation. In the absence of ZHP-3, reciprocal recombination is abolished and double-strand breaks seem to be repaired via alternative pathways, leading to achiasmatic chromosomes and the occurrence of univalents during meiosis I. Green fluorescent protein-tagged C. elegans ZHP-3 forms lines between synapsed chromosomes and requires the SC for its proper localization.