Aspects of regulation of ribosomal protein synthesis inXenopus laevis

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Abstract
The work carried out in the authors' laboratories on the structure and expression of ribosomal protein genes inXenopus is reviewed, with some comparisons with other systems. These genes form a class that shares several structural features, especially in the region surrounding the 5′ ends. These similar structures appear to be involved in coregulated expression that is attained at various regulatory levels: transcriptional, transcript processing and stability, and translational. Particular attention is paid here to the one operating at the translational level, which has been studied duringXenopus oogenesis and embryogenesis, and also during nutritional changes ofXenopus cultured cells. This regulation, which responds to the cellular need for new ribosomes, operates by changing the fraction of rp-mRNA engaged on polysomes, leaving each translated rp-mRNA molecule always fully loaded with ribosomes. Responsible for this translational behaviour is the typical 5′UTR, which characterizes all rp-mRNAs analyzed up to now, and that can bindin vitro some proteins, putative trans-acting factors for this translational regulation.