Suppressors of cis-acting splicing-deficient mutations that affect the ribozyme core of a group II intron.

Many of the cis-dominant mutations that lead to respiratory deficiency by preventing maturation of specific yeast mitochondrial transcripts are found to affect the ribozyme core of group I and group II introns. We have searched for suppressors of mutations in the ribozyme-encoding sections of a group II intron, the first intron in the COX1 gene of Saccharomyces cerevisiae, which was independently subjected to in vitro site-directed mutagenesis. Three of the original mutants bore multiple mutations, which act synergistically, since for most individual mutations, suppressors could be obtained that ensured at least partial recovery of respiratory competence and splicing. Out of a total of ten suppressor mutations that were identified, three were second-site substitutions that restored postulated base-pairings in the ribozyme core. Remarkably, and as is observed for group I introns, at least half of the cis-dominant mutations in the first two group II introns of the COX1 gene affect sites that have been shown to participate in RNA tertiary interactions. We propose that this bias reflects cooperativity in the formation of ribozyme tertiary but not secondary structure, on the one hand, and the need for synergistic effects in order to generate a respiratory-deficient phenotype in the laboratory on the other. Finally, a novel in vivo splicing product of mutant cells is attributed to bimolecular splicing at high concentrations of defective transcripts.

Biochemical evidence that Saccharomyces cerevisiae YGR262c gene, required for normal growth, encodes a novel Ser/Thr-specific protein kinase.

Saccharomyces cerevisiae YGR262c gene, whose disruption causes severely defective growth, encodes a putative protein kinase shorter than any other protein kinase biochemically characterized to date and lacking some of the conserved features of these enzymes. Here we show that the product of the YGR262c gene, piD261, expressed in E. coli with a C-terminal (His)6 tag, is a bona fide Ser/Thr protein kinase as judged from its capability to autophosphorylate and to phosphorylate casein and osteopontin in the presence of [gamma-32P]ATP. In contrast, no phosphorylation of histones, myelin basic protein, phosvitin, bovine serum albumin and poly(Glu/Tyr)4:1 could be detected. Mn2+ or, less effectively, Co2+ are required for piD261 catalytic activity, which is conversely undetectable in the presence of Mg2+, a behaviour unique among Ser/Thr protein kinases.

Phosphorylation and activation of the atypical kinase p53-related protein kinase (PRPK) by Akt/PKB.

p53-related protein kinase (PRPK), the human homologue of yeast Bud32, belonging to a small subfamily of atypical protein kinases, is inactive unless it is previously incubated with cell lysates. Here we show that such an activation of PRPK is mediated by another kinase, Akt/PKB, which phosphorylates PRPK at Ser250. We show that recombinant PRPK is phosphorylated in vitro by Akt and its phospho-form is recognized by a Ser250-phospho-specific antibody; that cell co-transfection with Akt along with wild-type PRPK, but not with its Ser250Ala mutant, results in increased PRPK phosphorylation; and that the phosphorylation of p53 at Ser15, the only known substrate of PRPK, is markedly increased by co-transfection of Akt with wild-type PRPK, but not PRPK dead mutant, and is abrogated by cell treatment with the Akt pathway inhibitor LY294002. Our data disclose an unanticipated mechanism by which PRPK can be activated and provide a functional link between this enigmatic kinase and the Akt signaling pathway.

A controversy concerning the structure of the mitochondrial genome of a yeast petite mutant: resolution by sequence analysis.

Sequence analysis was used to define the repeat unit that constitutes the mitochondrial genome of a petite (rho-) mutant of the yeast Saccharomyces cerevisiae. This mutant has retained and amplified in tandem a 2,547 bp segment encompassing the second exon of the oxi3 gene excised from wild-type mtDNA between two direct repeats of 11 nucleotides. The identity of the mtDNA segment retained in this petite has recently been questioned (van der Veen et al., 1988). The results presented here confirm the identity of this mtDNA segment to be that determined previously by restriction mapping (Carignani et al., 1983).

Antibodies against a fused gene product identify the protein encoded by a group II yeast mitochondrial intron.

In the mitochondrial genome of Saccharomyces cerevisiae, introns aI1 and aI2 of the gene encoding the COX1 subunit are the only group II introns with open reading frames (ORFs); these can be translated into two homologous proteins, the maturase aI1 and aI2. These proteins are structurally related to viral reverse transcriptases and have been shown genetically to be involved in pre-mRNA splicing and in the deletion of introns from mitochondrial DNA. To identify these mitochondrial proteins and study their properties more directly, we raised antibodies against a part of the intron aI2 ORF translation product. For this purpose, we constructed series of fusion genes, by joining parts of the genes for protein A or lacZ to different portions of the intron aI2. These were expressed in Escherichia coli as hybrid polypeptides, which were used for the production and identification of specific antibodies against the yeast mitochondrial protein. The antibodies recognized the 57 kDa protein (maturase aI2) that accumulates in two yeast mutants deficient in the splicing of aI2. This protein corresponds to the translation product of the 3' part of intron aI2 and accumulates unaltered in the two cis-acting mutants.

Petite deletion map of the mitochondrial oxi3 region in Saccharomyces cerevisiae.

Fifty eight mitochondrial mutants (p + mit- mutants), all deficient in cytochrome oxidase activity and previously assigned to the genetic region oxi3 on the mitochondrial DNA, were mapped by the method of "petite deletion mapping". This procedure resulted in the identification of at least twenty one different classes of oxi3 mutants, which could be arranged in a linear order. Moreover, it provided a set of twenty three p- petite mutants, each containing a differentially deleted mit DNA segment included in the oxi3 region. The two sets of mutants, p+ oxi3- and p- oxi3+, will be of interest for a further genetic and physical analysis of this mitochondrial DNA segment which spans over about ten thousand base pairs and controls the subunit I of cytochrome oxidase.

Analysis of an 11.6 kb region from the right arm of chromosome VII of Saccharomyces cerevisiae between the RAD2 and the MES1 genes reveals the presence of three new genes.

Sequence analysis of an 11,628 bp DNA segment from the right arm of Saccharomyces cerevisiae chromosome VII revealed the presence of the 5' end of the RAD2 gene, the MES1 gene and six open reading frames (ORFs) each longer than 300 bp. Four of these ORFs are expressed genes, as indicated by transcript analysis. One of them, YGR261c, which specifies a putative beta-adaptine, corresponds to gene YKS5, which has recently been identified as a suppressor of loss of casein kinase 1 function. The remaining three ORFs are new genes; of these, YGR260w encodes a protein showing similarity to the S. cerevisiae allantoate permease and YGR262c specifies a putative protein kinase.

The sequence of a 6.3 kb segment of yeast chromosome III reveals an open reading frame coding for a putative mismatch binding protein.

We report the sequence of a 6.3 kb segment of DNA mapping near the end of the right arm of chromosome III of Saccharomyces cerevisiae. The sequence reveals a major open reading frame coding for a putative protein of 1047 amino acids with a striking similarity to the bacterial proteins involved in recognition of mismatched DNA base pairs. This is particularly interesting as the existence of a yeast mismatch repair system similar to that of bacteria has been postulated for some years, but a yeast protein homologous to the bacterial mismatch binding protein had not been identified. The results of a comparison of the putative yeast mismatch binding protein with the bacterial mismatch binding proteins and with two cognate mammalian sequences, support the idea that a similar mismatch repair system may be present also in mammalian cells. The possibility that all of these proteins may have evolved from a common ancestral gene is also discussed.

Expression of the mitochondrial split gene coding for cytochrome oxidase subunit I in S. cerevisiae: RNA splicing pathway.

We have studied the splicing pathway leading to the synthesis of cytochrome oxidase subunit I (COX I) mRNA, by analysing the transcription pattern of several oxi3- splicing deficient mutants located in the first four introns of the gene. The four introns contain long open reading frames (ORFs) in phase with the upstream exons. All the mutations block the excision of the mutated intervening sequence (IVS) from the pre-mRNA, and accumulate characteristic novel polypeptides of sizes which could correspond to the translation products of the intron's ORF. Most of the mutations do not affect the splicing of the following intervening sequences; only in the case of mutations in the aI1 intron is a polar effect observed on the splicing of the second intron, aI2. Our results indicate that the splicing of these two intervening sequences which both belong to the class II of introns described by Michel et al. (1982), is controlled by the activity of the maturases encoded by their respective ORFs and that the translation of the aI2 maturase depends on the previous excision of aI1 IVS. (Moreover, the aI1 maturase, which accumulates in some mutants, can efficiently splice aI2 IVS when the translation of the latter's proper maturase cannot occur).

Critical sequences within mitochondrial introns: cis-dominant mutations of the "cytochrome-b-like" intron of the oxidase gene.

We have established the DNA sequence of two cis-dominant mutations located in the fourth intron, a14, of the yeast mitochondrial gene oxi3. These mutations prevent the synthesis of subunit I of cytochrome oxidase. Both mutations affect a very short DNA sequence located several hundred base pairs from the intron-exon junctions. An identical sequence is found in the cob-box gene; and this sequence is critical for the excision of the cytochrome b intron. Our interpretation is that this short sequence represents a common signal that must be recognized by the box7-encoded mRNA maturase, in conjunction with the mitochondrial ribosome, to splice out the introns in the two nonhomologous genes, cob-box and oxi3.

Extra-chromosomal inheritance of rhodamine 6G resistance in Saccharomyces cerevisiae.

Rhodamine 6G was found to be a specific inhibitor of aerobic growth of yeast, having no effect on fermentative growth. A single step spontaneous mutant of S. cerevisiae resistant to rhodamine 6G was isolated, which showed cross-resistance to the ATPase inhibitors venturicidin and triethyltin, to the uncoupler 1799, to bongkrekic acid and to cycloheximide, but not to oligomycin or to the inhibitors of mito chondrial protein synthesis, chloramphenicol and erythromycin. The genetic analysis of this mutant showed that both nuclear and cytoplasmic (but apparently not mitochondrial) factors may be involved in the determination of the mutation. The behaviour is discussed as a possible function for 2 micron circular (omicron) DNA.

A putative serine/threonine protein kinase gene on chromosome III of Saccharomyces cerevisiae.

We have sequenced a gene on chromosome III of Saccharomyces cerevisiae which codes for a putative serine/threonine protein kinase of 726 amino acids (calculated molecular weight 82 kDa). We have called this gene KIN82. The amino acid sequence of KIN82 is most similar to the cyclic nucleotide-dependent protein kinase subfamily and the protein kinase C subfamily. Gene disruption of KIN82 did not produce any phenotype when tested under a variety of conditions. Reduced stringency hybridizations revealed the presence of another genomic sequence with high homology to the carboxy-terminal catalytic domain of KIN82.

Comparative analysis of the region of the mitochondrial genome containing the ATPase subunit 9 gene in the two related yeast species Saccharomyces douglasii and Saccharomyces cerevisiae.

We have determined the nucleotide sequence of a region of the mitochondrial genome of the yeast Saccharomyces douglasii which contains the ATPase subunit 9 gene and part of the intergenic sequences that surround it. The gene is 228 nucleotides long and encodes a polypeptide of 76 aa. A comparison of the coding sequence with that of S. cerevisiae reveals the presence of three silent transitions. A high level of similarity is also found between regions involved in the initiation of transcription and mRNA processing. More interestingly, a region of similarity situated outside the known regulatory regions has been identified. As the intergenic regions are generally highly divergent, the remarkable conservation of these non-coding sequences suggests that their structure may be relevant to the expression of this region of the mitochondrial DNA.

An mRNA maturase is encoded by the first intron of the mitochondrial gene for the subunit I of cytochrome oxidase in S. cerevisiae.

We have localized ten oxi3- mutations in the first, al1, intron of the coxl gene. All are splicing deficient, being unable to excise the intron. Complementation experiments disclose several domains in the intron al1: the 5'-proximal and 3'-proximal domains harbor cis-dominant mutations, while trans-recessive ones are located in the intron's open reading frame. Comprehensive analyses of allele-specific polypeptides accumulating in mutants show that they result from the translation of the intron's ORF. We conclude that a specific mRNA maturase involved in splicing of oxidase mRNA is encoded by the intron al1 in a manner similar to the cytochrome b mRNA maturase.