RIBOSOMAL-PROTEIN L32 OF SACCHAROMYCES-CEREVISIAE REGULATES BOTH SPLICING AND TRANSLATION OF ITS OWN TRANSCRIPT

Ribosomal protein L32 of Saccharomyces cerevisiae regulates the splici ng of its own transcript (1, 2) apparently by interacting with a struc ture composed largely of the 5' exon. However, even in strains overpro ducing L32 mRNA, e.g. from a cDNA copy of the gene, little accumulatio n of L32 is observed after a brief pulse label. When the 5' leader of the RPL32 mRNA is replaced by an exogenous leader, the amount of pulse -labeled L32 increases severalfold, suggesting that L32 regulates the translation of its own mRNA, acting through sequences in the 5' region . This conclusion was confirmed by the observation that in cells carry ing a chimeric gene in which the L32 leader is fused to LacZ coding se quences, the presence of a second gene that overexpresses L32 itself r educes the level of beta-galactosidase by 50%, in spite of a doubling of L32-lacZ fusion mRNA, presumably due to stabilization of the messag e. Mutations within the 5' leader that abolish the regulation of splic ing also abolish the regulation of translation, suggesting that the re gulation of translation by L32 involves a structure similar to that pr oposed for the regulation of splicing. In cells overproducing L32-mRNA about half the excess mRNA was found in ribonucleoproteins of <25 S, unassociated with ribosomal particles. Much of the rest was found in r ibonucleoproteins of 80-120 S.