Polysome structure in sea urchin eggs and embryos: an electron microscopic analysis.

Modified chromatin-spreading techniques have been used to prepare reproducible spreads of polysomes for EM visualization. We have analyzed polysomes of sea urchin (Lytechinus pictus) eggs and embryos to elucidate some of the events which occur during the burst of protein synthetic activity following fertilization. We have confirmed that the rise in protein synthesis after fertilization is concomitant with the recruitment of messenger RNA molecules into polysomes. The presence of ribosome-free tails is a structural feature of polysomes in which the mRNA is being translated for the first time. This structural marker has allowed us to derive an elongation rate of 1.5-1.8 amino acids/sec and a transit time of 5 min for the average-sized polysome in L. pictus embryos at 16 degrees C. We have also observed a difference between the structure and the average size of polysomes in the egg and embryo. Egg polysomes are longer (average size = 11.96 ribosomes, n = 671) than 1-hr embryo polysomes (average size = 7.14 ribosomes, n = 863) and 10% of the egg polysomes contain visible gaps while less than 1% of the 1-hr embryo polysomes contain internal ribosome-free regions. We speculate that such differences reflect the lower translation efficiency of the relatively dormant egg cell.