STRUCTURE AND EXPRESSION OF RIBOSOMAL-PROTEIN GENES IN XENOPUS-LAEVIS

In Xenopus laevis, as well as in other vertebrates, ribosomal proteins (r-proteins) are coded by a class of genes that share some organizati onal and structural features. One of these, also common to genes codin g for other proteins involved in the translation apparatus synthesis a nd function, is the presence within their introns of sequences coding for small nucleolar RNAs. Another feature is the presence of common st ructures, mainly in the regions surrounding the 5' ends, involved in t heir coregulated expression. This is attained at various regulatory le vels: transcriptional, posttranscriptional, and translational. Particu lar attention is given here to regulation at the translational level, which has been studied during Xenopus oogenesis and embryogenesis and also during nutritional changes of Xenopus cultured cells. This regula tion, which responds to the cellular need for new ribosomes, operates by changing the fraction of rp-mRNA (ribosomal protein mRNA) engaged o n polysomes. A typical 5' untranslated region characterizing all verte brate rp-mRNAs analyzed to date is responsible for this translational behaviour: it is always short and starts with an 8-12 nucleotide polyp yrimidine tract. This region binds in vitro some proteins that can rep resent putative trans-acting factors for this translational regulation .