Identification of the Sequences Responsible for the Splicing Phenotype of the Regulatory Intron of the Li Ribosomal

Splicing of the regulated third intron of the LI ribosomal protein gene ofXenopus laevis has been studied in vivo by oocyte microinjection of wild-type and mutant SP6 precursor RNAs and in vitro in the heterologous HeLa nuclear extract. We show that two different phenomena combine to produce the peculiar splicing phenotype of this intron. One, which can be defined constitutive, shows the same features in the two systems and leads to the accumulation of spliced mRNA, but in very small amounts. The low efficiency of splicing is due to the presence of a noncanonical 5 ' splice site which acts in conjunction with sequences present in the 3 ' portion of the intron. The second leads to the massive conversion of the pre-mRNA into site specific truncated molecules. This has the effect of decreasing the concentration of the pre-mRNA available for splicing. We show that this aberrant cleavage activity occurs only in the in vivo oocyte system and depends on the presence of an intact Ul RNA. Several eukaryotic ribosomal protein (r-protein) genes have been shown to be controlled at posttranscriptional levels such as splicing (6, 9, 12), RNA stability (34, 35), and translation (3, 5, 25, 27, 32). An interesting example is provided by the gene coding for the r-protein Li of Xenopu