Multiple maternal proteins coordinate to restrict the translation ofC. elegans nanos-2to primordial germ cells

Abstract

Although germ cell formation has been relatively well understood in worms and insects, how germ cell-specific developmental programs are initiated is not clear. In Caenorhabditis elegans, translational activation of maternal nos-2 mRNA is the earliest known molecular event specific to the germline founder cell P4. Cis-elements in nos-2 3′UTR have been shown to mediate translational control, however the trans-acting proteins are not known. Here, we provide evidence that four maternal RNA-binding proteins, namely OMA-1, OMA-2, MEX-3 and SPN-4, bind nos-2 3′UTR to suppress its translation, and POS-1, another maternal RNA-binding protein, relieves this suppression in P4. The POS-1: SPN-4 ratio in P4 increases significantly over its precursor, P3; and POS-1 competes with SPN-4 for binding to nos-2 RNA in vitro. We propose temporal changes in the relative concentrations of POS-1 and SPN-4, through their effect on the translational status of maternal mRNAs such as nos-2, initiate germ cell-specific developmental programs in C. elegans.

RNA recognition by the embryonic cell fate determinant and germline totipotency factor MEX-3

“…In the early embryo, NOS-2 is not observed until the 16-cell stage in the germline precursor P 4 . However, in mex-3 mutant embryos, NOS-2 is expressed ectopically throughout the entire embryo (11).…”

“…mex-3 mutant embryos show ectopic expression of PAL-1, and the anterior blastomeres take on a C-like fate resulting in excess muscle in the anterior (3). The gene encoding the Nanos homolog NOS-2 is also dependent upon MEX-3 for its protein expression pattern (11). This protein is required for the proper development of primordial germ cells and their incorporation into the somatic gonad.…”

“…Translational reporter experiments suggest that MEX-3 is regulating its targets in a 3ЈUTR dependent manner (10,11). An RNA reporter containing the 3ЈUTR of pal-1 fused to lacZ is translated in early embryos in a pattern that matches endogenous PAL-1 (10).…”

“…In the absence of MEX-3, ectopic expression of the reporter is observed in oocytes and anterior blastomeres. Furthermore, recent work demonstrates that MEX-3 represses nos-2 mRNA translation through its 3ЈUTR (11). Using a GFP::H2B translational reporter transgene, several cis-regulatory elements within the 3ЈUTR required for the spatial and temporal control of nos-2 mRNA translation (subA-E) were identified (12).…”

“…Using a GFP::H2B translational reporter transgene, several cis-regulatory elements within the 3ЈUTR required for the spatial and temporal control of nos-2 mRNA translation (subA-E) were identified (12). Both in vitro and in vivo experiments suggest that MEX-3 represses nos-2 translation by interacting with a repeat sequence present within the regulatory elements subB and subC (11), yet the precise specificity determinants remain unknown.…”