Quantitative changes in protein synthesis during oogenesis in Xenopus laevis.

Protein synthesis rates in Xenopus laevis oocytes from stage 1 through stage 6 were measured. In addition, the translational efficiencies, total RNA contents, and percentages of ribosomes in polysomes in growing oocytes at several stages were determined. Stage 1 oocytes synthesize protein at a mean rate of 0.18 ng hr-1 while stage 6 oocytes make protein at a rate of 22.8 ng hr-1. Polysomes from growing and full-grown oocytes sedimented in a sucrose gradient with a peak value of 300 S, corresponding to a weight-average packing density of 10 ribosomes per mRNA. Ribosome transit times of endogenous mRNAs were essentially unchanged at all stages examined. While the oocyte's total ribosomal RNA content was observed to increase about 115-fold during oogenesis, the percentage of ribosomes in polysomes remained constant at approximately 2%. Taken together, the data suggest that the 127-fold increase in protein synthesis which occurs during Xenopus oogenesis involves the progressive recruitment onto polysomes of mRNA from the maternal stockpile.