Sea urchin egg and embryo ribosomes: Differences in translational activity in a cell-free system

To determine whether ribosomes have a role in the postfertilization activation of protein synthesis in sea urchin eggs, we measured the translational activity of ribosomes isolated from unfertilized eggs and embryos of Strongylocentrotus purpuratus. Numerous previous studies have indicated few if any differences in the activity of such ribosomes. However, by using improved physiological isolation and in vitro conditions, we have found important differences in the activities of egg and embryo ribosomes. Ribosomes obtained from blastula polyribosomes were active in translating reticulocyte mRNA in a ribosome-dependent cell-free translation system, whereas ribosomes obtained from unfertilized eggs became fully active only after a characteristic, reproducible delay of up to 15 min at 26°C. The extent of this delay varied with incubation pH, but not with concentrations of K⁺, Mg²⁺, initiation factors, or mRNA. However, at incubation pH between 6.90 and 7.65, the egg ribosomes were always less active than blastula ribosomes.