Single-molecule imaging reveals cytoplasmic translation of P granule-enriched mRNAs in C. elegans

Germ granules are condensates in germ plasm, a specialized cytoplasm that segregates to the embryonic germline. In Drosophila, translation of nanos mRNA occurs at the surface of germ granules, suggesting that the granules promote translation. In C. elegans, however, germ (P) granules are not essential for Nanos expression. Using single-molecule imaging in C. elegans embryos, we map the distribution of translating and non-translating molecules of the Nanos homolog nos-2 and two other maternal mRNAs enriched in P granules. In early germline blastomeres, these mRNAs are not translated and distribute between the cytoplasm and P granules. At translation onset, mRNA molecules in the cytoplasm are translated, while most mRNA molecules in the P granules remain non-translating. nos-2 translation requires a rise in the concentration of the RNA-binding protein POS-1, which occurs independently of P granules. Consistent with low translation inside the granules, P granules are depleted of ribosomes and 43S pre-initiation complexes. Our observations suggest that germ granules promote Nanos protein expression by concentrating Nanos mRNA in germline precursors, but do not directly promote translation. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=100 SRC="FIGDIR/small/735846v1_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@1875b06org.highwire.dtl.DTLVardef@16919f6org.highwire.dtl.DTLVardef@1278c27org.highwire.dtl.DTLVardef@1628645_HPS_FORMAT_FIGEXP M_FIG C_FIG SynopsisGerm granules are condensates proposed to regulate the translation of mRNAs like Nanos that code for germ cell fate determinants. Using single-molecule imaging in C. elegans embryos, this study shows that P granule scaffolds concentrate mRNAs in germline precursors, but do not control the activity of translational regulators. - P granules concentrate mRNAs but are depleted of ribosomes and 43S pre-initiation complexes - Translation occurs mainly in the cytoplasm where ribosomes are most abundant - nanos translation onset is timed by a rise in POS-1, which counteracts the repressor SPN-4; both enrich in P granules but act independently.