FEEDBACK INHIBITION OF THE YEAST RIBOSOMAL-PROTEIN GENE CRY2 IS MEDIATED BY THE NUCLEOTIDE-SEQUENCE AND SECONDARY STRUCTURE OF CRY2 PRE-MESSENGER-RNA

The Saccharomyces cerevisiae CRY1 and CRY2 genes, which encode ribosom al protein rp59, are expressed at a 10:1 ratio in wild-type cells. Del etion or inactivation of CRY1 leads to 5- to l0 fold-increased levels of CRY2 mRNA. Ribosomal protein 59, expressed from either CRY1 or CRY2 , represses expression of CRY2 but not CRY1. cis-Acting elements invol ved in repression of CRY2 were identified by assaying the expression o f CRYL2-lacZ gene fusions and promoter fusions in CRY1 CRY2 and cryl-D elta CRY2 strains. Sequences necessary and sufficient for regulation l ie within the transcribed region of CRY2, including tire 5' exon and t he first 62 nucleotides of the intron. Analysis of CRY2 point mutation s corroborates these results and indicates that both the secondary str ucture and sequence of the regulatory region of CRY2 pre-mRNA are nece ssary for repression. The regulatory sequence of CRY2 is phylogenetica lly conserved; a very similar sequence is present in the 5' end of the RP59 gene of the yeast Kluyveromyces lactis. Wild-type cells contain very low levels of both CRY2 pre-mRNA and CRY;, mRNA. Increased levels of CRY2 pre-mRNA are present in mtr mutants, defective in mRNA transp ort, and in upfl mutants, defective in degradation of cytoplasmic RNA, suggesting that in wild-type repressed cells, unspliced CRY2 pre-mRNA is degraded in the cytoplasm. Taken together, these results suggest t hat feedback regulation of CRY2 occurs posttranscriptionally. A model for coupling ribosome assembly and regulation of ribosomal protein gen e expression is preposed.