SELECTIVE RECRUITMENT OF MASKED MATERNAL MESSENGER-RNA FROM MESSENGER-RIBONUCLEOPROTEIN PARTICLES CONTAINING FRGY2 (MESSENGER RNP4)

In Xenopus, the germ cell-specific Y-box (CTGATTGGCCAA) factor, FRGY2, facilitates in vitro transcription in egg extracts from oocyte-select ive promoters containing a Y-box. This same factor is a major componen t of the messenger ribonucleoprotein (mRNP) storage particles of the o ocyte. These particles store maternal mRNAs and inhibit their translat ion. FRGY2 is identical to mRNP4 and homologous to mRNP3, two previous ly described oocyte-specific mRNP proteins. We demonstrate that FRGY2 associates with a broad spectrum of mRNAs exhibiting no apparent seque nce specificity. These interactions suggest that FRGY2 has a general r ole 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 TF IIIA, which are translated in the oocyte. Moreover, we show that mRNAs that are translationally repressed in oocytes, such as messages codin g for the proteins histone H1 and FRGY1, accumulate only within the pa rticles. 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 t he assembly of mRNP storage particles represents a default state and t hat translational regulation is achieved via specific recruitment of t he messages from the mRNP fraction to the ribosomes.