Translational repression restricts expression of the C. elegans Nanos homolog NOS-2 to the embryonic germline

Abstract

Members of the nanos gene family are evolutionarily conserved regulators of germ cell development. In several organisms, Nanos protein expression is restricted to the primordial germ cells (PGCs) during early embryogenesis. Here, we investigate the regulation of the Caenorhabditis elegans nanos homolog nos-2. We find that the nos-2 RNA is translationally repressed. In the adult germline, translation of the nos-2 RNA is inhibited in growing oocytes, and this inhibition depends on a short stem loop in the nos-2 3'UTR. In embryos, nos-2 translation is repressed in early blastomeres, and this inhibition depends on a second region in the nos-2 3'UTR. nos-2 RNA is also degraded in somatic blastomeres by a process that is independent of translational repression and requires the CCCH finger proteins MEX-5 and MEX-6. Finally, the germ plasm component POS-1 activates nos-2 translation in the PGCs. A combination of translational repression, RNA degradation, and activation by germ plasm has also been implicated in the regulation of nanos homologs in Drosophila and zebrafish, suggesting the existence of conserved mechanisms to restrict Nanos expression to the germline.

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

“…The expression pattern of the reporter harboring the wild-type nos-2 3ЈUTR matches the previously reported endogenous NOS-2 expression pattern (12,22) (Fig. 5B).…”

“…To address this question, EMSA was performed to determine the binding affinity of MEX-3 to the five known regulatory elements of nos-2 (subA-E) (Fig. S4B) (12) and to a 21-nucleotide fragment of the pal-1 UTR that harbors one MRE. We anticipated that MEX-3 would bind to nos-2 subB, nos-2 subC, and the fragment from the pal-1 UTR, but not to nos-2 subA, subD, or subE sequences that lack the MRE.…”

“…4). The UUUA half site is located within a conserved repeat element that has previously been shown to be required for MEX-3 dependent regulation (11,12).…”

“…Each MRE is highlighted in gray. The conserved octamer is surrounded by a box, which was reported previously to be required for translation (12). Half of each MRE site was mutated to CCCC (shown in bold) to make the nos-2 MREmut reporter construct.…”

“…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). 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.…”