Biochemical research on oogenesis: protein synthesis in whole cells and in cell-free extracts of Xenopus laevis immature ovaries.

Nearly all tRNA molecules in previtellogenic oocytes of Xenopus laevis are included in nucleoprotein particles sedimenting at 42S. The tRNA-binding sites of these particles have several properties in common with those of the ribosomes. This suggests that the 42S particles might behave like unprogrammed ribosomes and be the site of a template-independent polymerization of amino acids. We expected this reaction to be insensitive to protein synthesis inhibitors, such as cycloheximide and puromycin. We found that these antibiotics almost completely inhibit the incorporation of labeled amino acids into protein, when added to the incubation medium of whole ovaries or free oocytes. In cell-free extracts of ovaries, the incorporation of amino acids is partially insensitive to cycloheximide and puromycin. When such extracts are fractionated by sucrose density centrifugation and incubated with ATP, a major peak of amino acid incorporation can be detected, which nearly coincides with the 42S particle peak.