During germline development, the somatic gonad induces the germ line to proliferate. Induction occurs via the GLP-1/Notch signaling pathway and prevents germ cells from entering meiosis.
ego-1 mutations were identified in a genetic screen for recessive enhancers of
glp-1 (Qiao et al., 1995). Later studies showed that EGO-1 is a putative cellular RNA-dependent RNA Polymerase (RdRP) and has several germline functions. EGO-1 activity ensures a robust response to RNA interference and promotes germline proliferation, meiotic progression, and gametogenesis (Smardon et al., 2000, Vought et al., in press). EGO-1 is also involved in silencing unpaired chromosomes (see abstract by Maine et al). Our working model is that the
ego-1 developmental defects partially result from defects in RNA metabolism, perhaps from the failure to produce or amplify small, non-coding RNA important for various cellular processes. To better understand the role of EGO-1 in germline development, we screened for interaction partners by yeast two-hybrid approach. We tested candidate interactors by RNAi to identify those that function in the germ line, particularly those whose function might be related to
ego-1. We also sought to identify candidates that function in RNA metabolism. One candidate interactor is the C. elegans ortholog of a conserved eukaryotic translation initiation factor, eIF5B, encoded by Y54F10BM.2. eIF5B is essential for the joining of 40S and 60S ribosome subunits, and absence of eIF5B function diminishes translation initiation in vivo (Olsen et al., 2003, Pestova et al., 2000). We found that knockdown of eIF5B by RNAi causes germline proliferation defects and enhances a partial
glp-1(lf) phenotype. We characterized the gene structure by sequencing our cDNA clone and ESTs obtained from the C. elegans EST project. We isolated a deletion allele of the eIF5B gene,
bc367, and determined that it encodes a severely truncated polypeptide. Therefore,
bc367 is likely to be null.
bc367 mutants arrest development at L2 stage. We are currently testing whether
bc367 enhances the partial
glp-1(lf) phenotype. To further study the
bc367 germline phenotype, we are generating a multi-copy transgenic array to rescue the somatic defects. Some other translation factors are involved in transposon silencing, a process that also involves components of the RNAi machinery (Vastenhouw et al., 2003). Our future experiments include testing whether eIF5B is involved in transposon silencing.