Efficiency of translation of messenger-RNA before and after fertilization in sea urchins

Abstract

The efficiency of translation, defined as the number of protein molecules produced per mRNA molecule per unit time, was compared before and after the acceleration of protein synthesis which occurs upon fertilization of sea urchin eggs. Examination of protein turnover, extent of protein synthesis on ribosomes, and the size distribution of newly synthesized protein before and after fertilization showed that this acceleration of protein synthesis involved the synthesis of 10 to 15 times more protein molecules on 10 to 15 times as many active ribosomes. To determine whether these increased numbers of active ribosomes were interacting more efficiently with an unchanged number of mRNA molecules or at the same efficiency with an increased number of mRNA molecules, the size of the polysomes and the time a ribosome stayed on the polysomes was compared before and after fertilization. The latter time interval was determined by measuring the average time a newly incorporated amino acid remained in the nascent chains and the size distribution of the nascent chains. New methods giving essentially 100% yield of active polysomes from sea urchin cells made these measurements possible. The results of the experiments show that efficiency of translation is similar in egg and embryos. Protein synthesis, thus, is accelerated at fertilization by the translation of additional mRNA molecules. This translation level control of protein synthesis therefore cannot be a general change in activity of some component in the cellular synthetic machinery such as ribosomes, but must specifically control the activity of a defined population of mRNA molecules.