Changes upon fertilization in the distribution of RNA-containing particles in sea urchin eggs.

Abstract The distribution of labeled RNA's synthesized and conserved during oogenesis has been examined in homogenates of unfertilized and of fertilized eggs of the sea urchin Lytechinus pictus . The results showed that appreciable numbers of ribosomal aggregates were present in homogenates of unfertilized eggs. About two-thirds of the aggregates were associated with low-speed sedimentable material. Approximately two to three times more ribosomal aggregates were present in homogenates of eggs 2 hours after fertilization. Half of these were in the low-speed pellets of the homogenate. Thus, the rise in the number of ribosomal aggregates after fertilization was confined to the supernatant fraction of homogenates. Ribosomal preparations of unfertilized eggs were compared with those of fertilized eggs for their ability to incorporate endogenously valine- 14 C into protein in a cell-free system. Equivalent preparations showed that those from fertilized eggs incorporated about fifteen times more amino acids into protein in half an hour than those from unfertilized eggs. RNA's were extracted with phenol from the ribosomal and the postribosomal region of sucrose density gradients of homogenates of labeled unfertilized eggs. The former showed predominantly 28 S and 18 S radioactivity; the latter yielded primarily (about 75%) polydisperse radioactivity, and the remainder 28 S and 18 S. Most (about 75%) of the heterogeneously sedimenting postribosomal label occurred in the low-speed supernatant fraction of the homogenate and relatively little in the corresponding pellet. The amount of postribosomal radioactivity in homogenates of eggs preloaded with labeled RNA did not decrease by 2 hours after fertilization. The results are discussed in relation to the presence of masked mRNA in unfertilized eggs.