Xenopus laevisribosomal protein S1a cDNA sequence

In Xenopus laevis the coordinated synthesis of the over 80 ribosomal proteins (r-proteins) is attained by both posttranscriptional and translational level regulation (1, 2). Also several r-protein genes analyzed present structural similarities probably involved in the common regulatory mechanisms. Our understanding, however, of the structure and expression of these genes is based on the analysis of very few of them and it is thus important to extend these studies to other r-protein genes. We have analyzed the nucleotide sequence of a full length cDNA for Xenopus r-protein SI by using the Maxam and Gilbert technique. This sequence has been named Sla; in fact most genes in Xenopus laevis are present in two gene copies per haploid genome. The cDNA was isolated from a tadpole cDNA library (kindly provided by George Spohr) by screening with the originally cloned S1 cDNA small fragment (3) corresponding to the other copy Sib. The deduced amino acid sequence indicates that Sla is a basic protein of 246 amino acids and has a molecular weight of 26,999. Analysis of new partial cDNA clones obtained for Sib indicates that a sequencing error in the previously published partial sequence (3) had put the code out of frame. Comparison of the full length Sla sequence with the newly determined partial Sib sequence shows that in these 144 aa (carboxy terminal sequence) only two conservative aminoacid substitutions have occurred whereas 24 differences at the nucleotide level are found. This indicates a strong evolutive pressure for protein sequence conservation. Computer search on sequence data banks has revealed that the Xenopus S1 is homologous to the rat r-protein S3 (4), with six aminoacid substitutions, mostly conservative, and a small insertion of three aminoacids close to the carboxyterminal end of the Xenopus protein. Of particular interest are the structures of the 5' and 3' untranslated regions (UTRs) shown in Figure 1. In fact the cDNA sequence shows that the 5' and 3' of r-protein SI mRNA are both very short, 28 and 75 nt respectively, resembling the other Xenopus r-protein mRNAs so far characterized. Moreover the 5' UTR starts with the run of pyrimidines, typical of all Xenopus r-protein mRNAs, and also of other vertebrates. It is possible that these typical structural features of the 5' UTR and 3' UTR are involved in the regulations common to die class of r-protein mRNAs.