cdl-1 (cell death lethal) mutants show several embryonic defects: 1) delay in appearance of cell corpses and accumulation of cell corpses in late embryogenesis, 2) defects in elongation, 3) failure in attachment of the pharynx to the buccal cavity. To understand the
cdl-1 function, we cloned the
cdl-1 gene. By transformation assay, we found that a cosmid T19E10 could rescue the
cdl-1 phenotypes. RNAi for ORFs on T19E10 revealed that some of R06F6.1(RNAi) F1s showed
cdl-1-like phenotype, although most of them arrested at the early embryonic stage. We sequenced the corresponding coding region from
cdl-1 mutants and identified mutations in two alleles, thus concluding that R06F6.1 is the
cdl-1 gene.
cdl-1 encodes a member of the stem-loop binding protein(SLBP) family, which binds to the 3'-stem-loop of core histone mRNAs. It has been described that metazoan core histone mRNAs have a stem-loop structure instead of a poly-A sequence, and SLBPs have been implicated in post-transcriptional regulation of core histone mRNAs. In C. elegans, most core histone genes contain a conserved stem-loop sequence in their 3'-UTR sequence. We confirmed the interaction between R06F6.1 protein and the stem-loop structure by yeast three-hybrid system. This result suggests that R06F6.1 may also function in the post-transcriptional regulation of core histones. To examine the early embryonic phenotypes in
cdl-1(RNAi) embryos, we observed them by DAPI stiaining and Nomarski optics. In these embryos, chromosomes were less condensed during mitosis, cytokinesis occured before completion of the nuclear division, and nuclear fragments existed in some blastmeres. These observations suggest that the chromatin structure, especially its condensation, might be defective in
cdl-1(RNAi) embryos. We then performed RNAi with core histone genes, the probable targets of CDL-1. Most RNAi embryos showed early arrest phenotype similar to
cdl-1(RNAi) embryos, which supports the hypothesis that CDL-1 regulates the expression of core histones. We are currently trying to examine whether the original
cdl-1 phenotypes are also caused by the defect of core histone expression.