A test for masked message: The template activity of messenger ribonucleoprotein particles isolated from sea urchin eggs

The hypothesis that the "masked message" of unfertilized eggs consists of nontranslatable mRNP particles was directly tested by in vitro translation of mRNPs in a system derived from wheat germ. Three classes of mRNPs were tested: particles prepared from sea urchin eggs in buffers containing 0.35 M K⁺, particles prepared from sea urchin eggs in 0.35 M Na⁺, and particles released with EDTA in 0.35 M K⁺ from polysomes of sea urchin embryos cultured in the presence of actinomycin D. The mRNA content of particles was monitored by determination of poly(A) content. The wheat germ system used is quantitatively stimulated by addition of mRNA derived from eggs or from any of the classes of mRNPs used. Particles prepared from eggs with Na⁺ or released from polysomes contain less protein than particles isolated from eggs in K⁺, and as expected these particles are fully translatable in vitro. Particles prepared from eggs in buffers containing 0.35 M K⁺ produce little or no stimulation in the in vitro system. That this lack of translation represents in vivo masking is indicated by several considerations: (1) The nontranslatable particles were prepared in 0.35 M K⁺ and 5 mM Mg²⁺, ion concentrations similar to those found in echinoderm eggs; (2) density and sedimentation rate characteristics of the particles are little changed by isolation; (3) RNA extracted from isolated particles is fully translatable; and (4) particles prepared from polysomes or under conditions which destabilize RNPs are translatable. These data support the masking hypothesis for the protein synthesis repression system of eggs.