Selective recruitment of masked maternal mRNA from messenger ribonucleoprotein particles containing FRGY2 (mRNP4)

In Xenopus, the germ cell-specific Y-box (CTGATTGGCCAA) factor, FRGY2, facilitates in vitro transcription in egg extracts from oocyte-selective promoters containing a Y-box. This same factor is a major component of the messenger ribonucleoprotein (mRNP) storage particles of the oocyte. These particles store maternal mRNAs and inhibit their translation. FRGY2 is identical to mRNP4 and homologous to mRNP3, two previously described oocyte-specific mRNP proteins. We demonstrate that FRGY2 associates with a broad spectrum of mRNAs exhibiting no apparent sequence specificity. These interactions suggest that FRGY2 has a general role in packaging mRNA analogous to that of histone with DNA. All mRNAs examined that accumulate in oocytes appear within these mRNP storage particles, including messages coding for proteins such as FRGY2 and TFIIIA, which are translated in the oocyte. Moreover, we show that mRNAs that are translationally repressed in oocytes, such as messages coding for the proteins histone H1 and FRGY1, accumulate only within the particles. These mRNAs are subsequently recruited from the particles to the ribosomes and utilized for translation during embryogenesis prior to transcriptional activation of the zygotic genome. We propose that the assembly of mRNP storage particles represents a default state and that translational regulation is achieved via specific recruitment of the messages from the mRNP fraction to the ribosomes.