Coordinate expression of ribosomal protein genes during Xenopus development.

The expression of ribosomal protein and rRNA genes during Xenopus oogenesis results in the synthesis of sufficient ribosomes to support development of the swimming tadpole. cDNA clones for ribosomal proteins L13, L15, L23, and S22 have been isolated and used as probes to examine ribosomal protein gene transcripts during oogenesis and embryogenesis. Our results show that ribosomal protein mRNAs attain maximal steady-state levels in stage II oocytes concomitant with the onset of vitellogenesis. Approximately 50% of ribosomal protein mRNAs are associated with polysomes throughout oogenesis, resulting in a constant rate of ribosomal protein synthesis in stage III through stage VI oocytes. In contrast, the polysomal to nonpolysomal distribution of bulk poly(A)+ RNA increases during oogenesis, resulting in a five- to eightfold stimulation in the rate of overall protein synthesis. Following fertilization, maternal ribosomal protein mRNAs are degraded. Accumulation of de novo ribosomal protein transcripts is first detectable during gastrulation, but ribosomal protein mRNAs do not enter polysomes until stage 30 tailbud embryos. We find no discernible structural or functional differences between ribosomal protein transcripts in the polysomal and the nonpolysomal fractions for the observed stages of oocytes and embryos. These results are consistent with a model in which control of ribosomal protein synthesis is regulated at the translational level during Xenopus development.