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The gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), an ATP binding cassette (ABC) transporter that functions as a phosphorylationand nucleotide-regulated chloride channel, is mutated in cystic fibrosis (CF) patients. Deletion of a phenylalanine at amino acid position 508 (⌬F508) in the first nucleotide binding domain (NBD1) is the most prevalent CF-causing mutation and results in defective protein processing and reduced CFTR function, leading to chloride impermeability in CF epithelia and heterologous systems. Using a STE6/CFTR⌬F508 chimera system in yeast, we isolated two novel ⌬F508 revertant mutations, I539T and G550E, proximal to and within the conserved ABC signature motif of NBD1, respectively. Western blot and functional analysis in mammalian cells indicate that mutations I539T and G550E each partially rescue the CFTR⌬F508 defect. Furthermore, a combination of both revertant mutations resulted in a 38-fold increase in CFTR⌬F508-mediated chloride current, representing 29% of wild type channel activity. The G550E mutation increased the sensitivity of CFTR⌬F508 and wild type CFTR to activation by cAMP agonists and blocked the enhancement of CFTR⌬F508 channel activity by 2 mM 3-isobutyl-1-methylxanthine. The data show that the ⌬F508 defect can be significantly rescued by second-site mutations in the nucleotide binding domain 1 region, that includes the LSGGQ consensus motif. Cystic fibrosis (CF)1 is the most frequent lethal genetic disease associated with a single gene in Caucasians (1). CF results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes an ATP binding cassette (ABC) transporter that functions as a phosphorylation and nucleotide-regulated chloride channel located in the apical membrane of epithelial cells (2, 3). The ABC transporters constitute a large family of ubiquitously expressed proteins, mostly involved in ATP-driven translocation of diverse substrates across biological membranes (4, 5). It has been proposed that a functional ABC transporter has a minimal structural requirement of two membrane-spanning domains and two nucleotide binding domains (NBDs) (5). The NBDs, or ABC cassettes, share 30 -50% sequence identity (6) and are characterized by the presence of three conserved motifs; Walker A and Walker B motifs are present in several nucleotide binding and hydrolyzing proteins (7), and the ABC-signature motif, located just upstream of the Walker B, is diagnostic of ABC cassettes (5, 6).The deletion of the Phe-508 (⌬F508) in the first nucleotide binding domain (NBD1) of CFTR is the most frequent CFcausing mutation, present in 90% of CF chromosomes. ⌬F508 impairs normal protein maturation and trafficking to the plasma membrane (8, 9), presumably through a localized effect on the folding of the NBD1 domain (10 -12). This misfolding results in retention of CFTR⌬F508 by the endoplasmic reticulum-associated quality control and in subsequent degradation with the participation of the cytoplasmic proteasome (13). The CFTR...

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The DNA sequences of eight yeast ribosomal protein genes have been compared for the purpose of identifying homologous regions which may be involved in the coordinate regulation of ribosomal protein synthesis. A 12 bp homology was identified in the 5' DNA sequence preceding the structural gene for 6 out of 8 yeast ribosomal protein genes. In each case the homologous sequence was found at a position approximately 300 bp preceding the transcription start of the ribosomal protein gene. This homology was not identified in any non-ribosomal protein gene examined. Additional homologies between ribosomal protein genes were identified in the transcribed regions, including the untranslated 5' and 3' DNA regions flanking the coding regions.

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The temperature-sensitive mutation rna2 causes the accumulation of higher molecular weight transcripts from the ribosomal protein 51 (rp5l) gene of yeast and many other yeast ribosomal protein genes. We have determined the DNA sequence of the rp5l gene, confirming that it contains an intron and that the higher molecular weight transcript is an intron-containing precursor RNA. These data and other experiments suggest that the rna2 mutation affects mRNA processing (splicing) and that the presence of an intron is sufficient to render expression of a gene sensitive to the rna2 mutation. To test these hypotheses, we have inserted the rpSl intron into the coding region of a hybrid Escherichia coli 13-galactosidase gene, thereby interrupting the open reading frame subsequent to the initiating methionine codon. De-spite the presence of the intron, the f3-galactosidase gene is expressed in yeast. Thus, the rp5l intron is properly excised from the normally intronless gene. The presence of the rpSl intron causes the 1B-galactosidase activity to be sensitive to the rna2 mutation, consistent with the notion that this mutation affects gene expression at the level of splicing. The experiments suggest that an intron-containing /8-galactosidase gene can be used in a general way to study mRNA splicing.The temperature-sensitive rna2 mutation of Saccharomyces cerevisiae causes a dramatic decline in the synthesis of most ribosomal proteins when cells carrying the rna2 mutation are shifted to the restrictive temperature. This effect appears to be quite specific for ribosomal protein synthesis-i.e., the rate of synthesis of total protein and of most nonribosomal proteins is relatively unaffected (1). After a shift to the restrictive temperature, mRNA isolated from cells that carry the rna2 mutation is specifically depleted of most ribosomal protein mRNAs, suggesting that the decline in ribosomal protein synthesis is due to a decline in ribosomal protein mRNA levels (2). For many ribosomal proteins, the decrease in mature mRNA levels in cells carrying the rna2 mutation is associated with increased levels of higher molecular weight transcripts (3)(4)(5). In the case of ribosomal protein 51 (rp5l), the mature mRNA and the higher molecular weight transcript were shown by nuclease S1 mapping to be spliced and unspliced transcripts, respectively (3).These data suggest that many yeast ribosomal protein genes contain introns and that the rna2 mutation interferes with ribosomal protein mRNA processing. Here we present the rp51 DNA sequence and identify the rpSl intron. We also demonstrate that the rp51 intron, when inserted into a yeast CYCJlacZ fusion gene (6), is correctly spliced from the fusion transcript, thereby allowing 03-galactosidase expression. Splicing of the rp5l intron from the CYCl-lacZ fusion transcript is further shown to be defective in cells carrying the rna2 mutation when shifted to the nonpermissive temperature. MATERIALS AND METHODSStrains. The haploid S. cerevisiae strain RY26 (a, rna2, ura3-1, ura3-2, adel, ade...