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Translational regulation of ribosomal protein synthesis in Xenopus cultured cells: mRNA relocation between polysomes and RNP during nutritional shifts.

Translational control of ribosomal protein mRNA was analyzed in a Xenopus cell line during growth-rate changes induced by serum deprivation and readdition. After being transferred into serum-free medium, the cells rapidly decrease their DNA, RNA and protein synthesis, while addition of serum to the culture after a few hours of deprivation causes a rapid recovery. During these growth-rate changes, we observed a shift in ribosomal protein mRNA distribution between polysomes and RNP. The proportion of mRNA on polysomes for the four ribosomal proteins analyzed changed from 70-80% during rapid growth to 25-35% during the downshift and back to 70-80% after the upshift. Northern blot analysis showed that ribosomal protein mRNA level was constant during the shifts even in the presence of the transcriptional inhibitor actinomycin D. This indicates that the distribution changes were due to a reversible transfer of ribosomal protein mRNA between polysomes and RNP without altering mRNA stability. We have also compared the kinetics of ribosomal protein mRNA distribution changes with the kinetics of the changes in the partition of ribosomes between free monomers and polysomes. The results obtained show that the change in ribosomal protein mRNA localization is very fast, allowing short-term adjustments of ribosome synthesis rate. Moreover, our observations are consistent with the hypothesis that the amount of free ribosomes present in the cell could affect ribosomal protein mRNA utilization.