The late larval/adult C. elegans germline contains both proliferating and meiotic germ cells. Proliferating germ cells reside at the distal end of each gonad arm; as germ cells move proximally, they enter meiotic prophase and progress through gametogenesis. Germ cell proliferation is induced by GLP-1/Notch receptor signaling. GLP-1 signaling promotes proliferation by inhibiting the activities of the GLD-1 and GLD-2 pathways, which function redundantly to promote meiosis and/or inhibit proliferation. GLD-1 is likely to, at least in part, induce entry into meiotic prophase by inhibiting the translation of mRNAs that promote proliferation while GLD-2 is likely to, at least in part, promote entry into meiotic prophase by activating the translation of meiotic mRNAs. We previously identified several ego genes, which interact genetically with GLP-1 signaling [ e nhancers of weak g lp- 1 loss-of-function (lf)] and function to promote germline proliferation and/or inhibit meiotic development (Qiao et al . 1995; Smardon et al . 2000). ego genes encode a diverse set of proteins, including (1) LAG-1, a transcriptional regulator that is a core component of GLP-1/Notch signaling, (2) EGO-1, an RdRP that promotes several aspects of germline development (see abstract by Yu et al . ), and (3) ATX-2, a post-transcriptional regulator that promotes both proliferation and the oocyte fate (see abstract by She et al .). Here we identify the
ego-5 gene as encoding the B subunit of the DNA polymerase (pol) alpha-primase complex, which is thought to function in assembly and nuclear transport of the complex but lacks catalytic activity. This finding has led us to investigate whether regulation of DNA replication proteins might be an aspect of the proliferation/meiosis decision. To investigate the specificity of the interaction between
ego-5 and
glp-1 , we asked whether a weak
glp-1 lf allele could be enhanced by knockdown of (1) other components of the DNA polymerase alpha-primase complex, (2) other DNA replication proteins, and (3) other proteins required for the mitotic cell cycle. In the absence of GLP-1 signaling, germ cells both prematurely cease proliferation AND prematurely enter meiosis; this phenotype is distinct from a simple mitotic defect, where germ cells cease proliferating, but do not enter meiosis. We find that weak
glp-1 lf is also enhanced under conditions of partial RNAi-mediated knockdown of other (catalytic) subunits of the DNA pol alpha-primase complex and by depletion of some other DNA replication proteins. In contrast, depletion of various other cell cycle proteins (e.g. ,
cdk2) does not enhance weak
glp-1 lf indicating that a general mitotic cell cycle progression defect does not induce meiotic entry. These results suggest that partial depletion of DNA replication factors may mimic a process that is a normal part of the switch to meiotic development.
gld-1 promotes meiotic entry, and we have recently identified ~100 new putative GLD-1 mRNA targets based on a co-immunoprecipitation/amplification/microarray detection strategy. Among the targets are mRNAs that encode DNA replication proteins, including a component of the DNA pol alpha-primase complex. In other organisms, meiotic S phase is several-fold slower than mitotic S phase (see Forsburg, 2002). While the reasons for this difference are unclear, the reduced rate of DNA replication in meiotic S phase may be necessary for the association of factors with the chromosomes that are required for later meiotic events (e.g., recombination between homologs). Taken together, our data suggest a model where GLD-1 promotes entry into meiosis, in part, via translational repression of mRNAs that encode certain DNA replication proteins; reducing the level of certain replication proteins may decrease the rate of DNA replication, which in turn may be an important aspect of the switch from mitotic to meiotic S phase. Qiao, et al . (1995) Genetics 141, 551-569 Smardon, et al . (2000) Current Biology 10, 169-178. Forsburg (2002) Mol Cell 9, 701-711.