Mitochondrial translation products during release from glucose repression in Saccharomyces cerevisiae

Control of Translation at the Initiation Phase During Glucose Starvation in Yeast

Glucose is one of the most important sources of carbon across all life. Glucose starvation is a key stress relevant to all eukaryotic cells. Glucose starvation responses have important implications in diseases, such as diabetes and cancer. In yeast, glucose starvation causes rapid and dramatic effects on the synthesis of proteins (mRNA translation). Response to glucose deficiency targets the initiation phase of translation by different mechanisms and with diverse dynamics. Concomitantly, translationally repressed mRNAs and components of the protein synthesis machinery may enter a variety of cytoplasmic foci, which also form with variable kinetics and may store or degrade mRNA. Much progress has been made in understanding these processes in the last decade, including with the use of high-throughput/omics methods of RNA and RNA:protein detection. This review dissects the current knowledge of yeast reactions to glucose starvation systematized by the stage of translation initiation, with the focus on rapid responses. We provide parallels to mechanisms found in higher eukaryotes, such as metazoans, for the most critical responses, and point out major remaining gaps in knowledge and possible future directions of research on translational responses to glucose starvation.

Divergent overlapping transcripts at the PET122 locus in Saccharomyces cerevisiae

PET122 is one of three nuclear genes specifically required for translation of the mitochondrial mRNA for cytochrome c oxidase subunit III in Saccharomyces cerevisiae. The nucleotide sequence of 2,862 base pairs (bp) of yeast genomic DNA encompassing the PET122 locus shows very close spacing between the PET122 gene (254 codons) and two unidentified open reading frames, termed ORF2 and ORF3. ORF2 is encoded by the same strand of DNA as PET122 and is located 53 bp downstream of PET122, while ORF3 is encoded on the opposite strand and is located 215 bp upstream of PET122. Five transcripts, with sizes of 2.9, 2.3, 2.1, 1.5, and 1.4 kilobases (kb), are produced from this locus. The 2.1- and 1.4-kb transcripts encode ORF3, the 1.5-kb transcript encodes ORF2, and the 2.9- and 2.3-kb transcripts encode PET122. A particularly interesting feature of the ORF3-PET122-ORF2 transcription unit is a 535-base overlap between the 2.3-kb PET122 transcript produced from one strand and a 2.1-kb ORF3 transcript produced from the opposite strand. Similarly, the 2.9-kb PET122 transcript overlaps the 2.1-kb ORF3 transcript by more than 900 bases and the 1.5-kb ORF3 transcript by at least 200 bases. Hence, these pairs of transcripts are antisense to one another and have the potential to regulate, in an interdependent fashion, the posttranscriptional expression of ORF3 and PET122.

Divergent overlapping transcripts at the PET122 locus in Saccharomyces cerevisiae

PET122 is one of three nuclear genes specifically required for translation of the mitochondrial mRNA for cytochrome c oxidase subunit III in Saccharomyces cerevisiae. The nucleotide sequence of 2,862 base pairs (bp) of yeast genomic DNA encompassing the PET122 locus shows very close spacing between the PET122 gene (254 codons) and two unidentified open reading frames, termed ORF2 and ORF3. ORF2 is encoded by the same strand of DNA as PET122 and is located 53 bp downstream of PET122, while ORF3 is encoded on the opposite strand and is located 215 bp upstream of PET122. Five transcripts, with sizes of 2.9, 2.3, 2.1, 1.5, and 1.4 kilobases (kb), are produced from this locus. The 2.1- and 1.4-kb transcripts encode ORF3, the 1.5-kb transcript encodes ORF2, and the 2.9- and 2.3-kb transcripts encode PET122. A particularly interesting feature of the ORF3-PET122-ORF2 transcription unit is a 535-base overlap between the 2.3-kb PET122 transcript produced from one strand and a 2.1-kb ORF3 transcript produced from the opposite strand. Similarly, the 2.9-kb PET122 transcript overlaps the 2.1-kb ORF3 transcript by more than 900 bases and the 1.5-kb ORF3 transcript by at least 200 bases. Hence, these pairs of transcripts are antisense to one another and have the potential to regulate, in an interdependent fashion, the posttranscriptional expression of ORF3 and PET122.

Chromosomal localization and expression of CBS1, a translational activator of cytochrome b in yeast

Translation of mitochondrial cytochrome b RNA in yeast requires the product of the nuclear gene CBS1, a 27.5 kDa soluble mitochondrial protein. In this paper we show that the CBS1 gene is located on chromosome IV immediately adjacent to COX9, the gene coding for cytochrome c oxidase subunit VIIa. CBS1 is transcribed as a very low abundant 900 b RNA. Transcription starts at a single position 101 bp upstream of the CBS1 initiation codon. At positions -39 to -27 of its leader sequence it contains a small open reading frame of 4 codons. By monitoring the beta-galactosidase activity of a CBS1/lacZ fusion construct we show that expression of CBS1 is subjected to regulation by oxygen and by glucose: the beta-galactosidase activity is elevated threefold in glycerol or galactose grown cells compared to that in glucose grown cells. A further threefold reduction of the activity is observed in anaerobically grown cells. In accordance with this result is the observation that the steady-state level of CBS1 mRNA of anaerobically grown cells is ninefold lower than that of aerobically cultured cells.

Control of the Saccharomyces cerevisiae regulatory gene PET494: transcriptional repression by glucose and translational induction by oxygen

The product of the Saccharomyces cerevisiae nuclear gene PET494 is required to promote the translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit III (coxIII). The level of cytochrome c oxidase activity is affected by several different environmental conditions, which also influence coxIII expression. We have studied the regulation of PET494 to test whether the level of its expression might modulate coxIII translation in response to these conditions. A pet494::lacZ fusion was constructed and used to monitor PET494 expression. PET494 was regulated by oxygen availability: expression in a respiration-competent diploid strain grown anaerobically was one-fifth the level of expression in aerobically grown cells. However, since PET494 mRNA levels did not vary in response to oxygen deprivation, regulation by oxygen appears to occur at the translational level. This oxygen regulation was not mediated by heme, and PET494 expression was independent of the heme activator protein HAP2. The regulation of PET494 expression by carbon source was also examined. In cells grown on glucose-containing medium, PET494 was expressed at levels four- to sixfold lower than in cells grown on ethanol and glycerol. However, addition of ethanol to glucose-containing medium induced PET494 expression approximately twofold. PET494 transcript levels varied over a fourfold range in response to different carbon sources. The effects on PET494 expression of mutations in the SNF1, SNF2, SSN6, and HXK2 genes were also determined and found to be minimal.

Control of the Saccharomyces cerevisiae regulatory gene PET494: transcriptional repression by glucose and translational induction by oxygen

The product of the Saccharomyces cerevisiae nuclear gene PET494 is required to promote the translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit III (coxIII). The level of cytochrome c oxidase activity is affected by several different environmental conditions, which also influence coxIII expression. We have studied the regulation of PET494 to test whether the level of its expression might modulate coxIII translation in response to these conditions. A pet494::lacZ fusion was constructed and used to monitor PET494 expression. PET494 was regulated by oxygen availability: expression in a respiration-competent diploid strain grown anaerobically was one-fifth the level of expression in aerobically grown cells. However, since PET494 mRNA levels did not vary in response to oxygen deprivation, regulation by oxygen appears to occur at the translational level. This oxygen regulation was not mediated by heme, and PET494 expression was independent of the heme activator protein HAP2. The regulation of PET494 expression by carbon source was also examined. In cells grown on glucose-containing medium, PET494 was expressed at levels four- to sixfold lower than in cells grown on ethanol and glycerol. However, addition of ethanol to glucose-containing medium induced PET494 expression approximately twofold. PET494 transcript levels varied over a fourfold range in response to different carbon sources. The effects on PET494 expression of mutations in the SNF1, SNF2, SSN6, and HXK2 genes were also determined and found to be minimal.

Nuclear functions required for cytochrome c oxidase biogenesis in Saccharomyces cerevisiae: multiple trans-acting nuclear genes exert specific effects on expression of each of the cytochrome c oxidase subunits encoded on mitochondrial DNA

Fourteen nuclear complementation groups of mutants that specifically affect the three mitochondrially-encoded subunits of yeast cytochrome c oxidase have been characterized. Genes represented by these complementation groups are not required for mitochondrial transcription, transcript processing, or translation per se but are required for the expression of one of the three genes--COX1, COX2, or COX3--which encode the cytochrome c oxicase subunits I, II, or III, respectively. Five of these genes affect the biogenesis of cytochrome c oxidase subunit I, 3 affect the biogenesis of subunit II, 3 affect the biogenesis of subunit III and 3 affect the biogenesis of both cytochrome c oxidase subunit I and cytochrome b, the product of COB. Among the 5 complementation groups of mutants that affect the expression of COX1, 2 lack COX1 transcripts, 1 produces incompletely processed COX1 transcripts, and 2 contain normal levels of normal-sized COX1 transcripts. In contrast, all 3 complementation groups which affect the expression of COX2 and all 3 complementation groups which affect the expression of COX3 exhibit no, or little, detectable difference with respect to the wild type pattern of transcripts. The 3 complementation groups which affect the expression of both COX1 and COB all have aberrant COX1 and COB transcript patterns. These findings indicate that multiple trans-acting nuclear genes are required for specific expression of each COX gene encoded on mitochondrial DNA and suggest that their products act at different steps in the expression of these mitochondrial genes.

PET111, a Saccharomyces cerevisiae nuclear gene required for translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit II

Mutations in the nuclear gene PET111 are recessive and specifically block accumulation of cytochrome c oxidase subunit II (coxII), the product of a mitochondrial gene. However, the coxII mRNA is present in pet111 mutants at a level approximately one-third that of wild type. The simplest explanation for this phenotype is that PET111 is required for translation of the coxII mRNA. The reduced steady-state level of this mRNA is probably a secondary effect, caused by increased degradation of the untranslated transcript. Mitochondrial suppressors of pet111, carried on rho-mtDNAs, bypass the requirement for PET111 in coxII translation. Three suppressors are fusions between the coxII structural gene and other mitochondrial genes, that encode chimeric proteins consisting of the N-terminal portions of other mitochondrially coded proteins fused to the coxII precursor protein. When present together with rho+ mtDNA in a heteroplasmic state, these suppressors allow coxII synthesis in pet111 mutants. Thus in wild type, the PET111 product, or something under its control, probably acts at a site coded in the proximal portion of the gene for coxII to promote translation of the mRNA. PET111 was isolated by molecular cloning and genetically mapped to a position approximately midway between rna1 and SUP8 on chromosome XIII.

Product of Saccharomyces cerevisiae nuclear gene PET494 activates translation of a specific mitochondrial mRNA

The product of Saccharomyces cerevisiae nuclear gene PET494 is known to be required for a posttranscriptional step in the accumulation of one mitochondrial gene product, subunit III of cytochrome c oxidase (coxIII). Here we show that the PET494 protein probably acts in mitochondria by demonstrating that both a PET494-beta-galactosidase fusion protein and unmodified PET494 are specifically associated with mitochondria. To define the PET494 site of action, we isolated mutations that suppress a pet494 deletion. These mutations were rearrangements of the mitochondrial gene oxi2 that encodes coxIII. The suppressor oxi2 genes had acquired the 5'-flanking sequences of other mitochondrial genes and gave rise to oxi2 transcripts carrying the 5'-untranslated leaders of their mRNAs. These results demonstrate that in wild-type cells PET494 specifically promotes coxIII translation, probably by interacting with the 5'-untranslated leader of the oxi2 mRNA.

The yeast nuclear gene CBS1 is required for translation of mitochondrial mRNAs bearing the cob 5' untranslated leader

Mitochondrial translation of the cob mRNA to yield apocytochrome b is specifically dependent on the nuclear gene CBS1, while mitochondrial translation of the oxi2 mRNA to yield cytochrome oxidase subunit III (cox III) is specifically dependent on the nuclear gene PET494. Chimeric oxi2 mRNAs bearing the 5' leaders of other mitochondrial mRNAs, transcribed from rho- mitochondrial DNAs termed MSU494, are translated in pet494 mutants. In this study, we examined translation of coxIII from MSU494-encoded chimeric mRNAs in zygotes of defined nuclear and mitochondrial genotype. CoxIII was translated from a chimeric mRNA bearing the cob leader only when the zygotes contained a wild-type CBS1 gene. CoxIII translation from an mRNA bearing the 5' leader of the mitochondrial gene aap1 was not dependent on CBS1 activity. We conclude that the product of the nuclear gene CBS1, or something under its control, acts in the mitochondrion on the cob mRNA 5' leader to activate translation of down-stream coding sequences.

Product of Saccharomyces cerevisiae nuclear gene PET494 activates translation of a specific mitochondrial mRNA

The product of Saccharomyces cerevisiae nuclear gene PET494 is known to be required for a posttranscriptional step in the accumulation of one mitochondrial gene product, subunit III of cytochrome c oxidase (coxIII). Here we show that the PET494 protein probably acts in mitochondria by demonstrating that both a PET494-beta-galactosidase fusion protein and unmodified PET494 are specifically associated with mitochondria. To define the PET494 site of action, we isolated mutations that suppress a pet494 deletion. These mutations were rearrangements of the mitochondrial gene oxi2 that encodes coxIII. The suppressor oxi2 genes had acquired the 5'-flanking sequences of other mitochondrial genes and gave rise to oxi2 transcripts carrying the 5'-untranslated leaders of their mRNAs. These results demonstrate that in wild-type cells PET494 specifically promotes coxIII translation, probably by interacting with the 5'-untranslated leader of the oxi2 mRNA.

Two yeast nuclear genes, CBS1 and CBS2, are required for translation of mitochondrial transcripts bearing the 5'-untranslated COB leader

Mutations in one of the yeast nuclear genes CBS1 or CBS2 both prevent the excision of the maturase-coding introns bI2, bI3 and bI4 from the mitochondrial COB precursor transcript. Mutant strain MK2 (cbs1-1) has recently been reported to be primarily defective in the translation of COB transcripts, as it can be suppressed by a fusion of the COB structural gene with the 5' untranslated leader of the mitochondrial OLI1 gene (G. Rödel, A. Körte and F. Kaudewitz, Curr Genet 9: 641-648). Here I report that the effect of mutation cbs2-1, too, is suppressed by this gene rearrangement. CBS2 is the second nuclear gene identified which is involved in the translation of mitochondrial transcripts bearing the 5' untranslated COB leader. Gene specific translation control appears to be a major mode of regulation of mitochondrial gene expression in Saccharomyces cerevisiae.

Mitochondrial transcription and processing of transcripts during release from glucose repression in 'resting cells' of Saccharomyces cerevisiae

Mitochondrial transcription and processing of transcripts have been investigated at different stages of release from glucose repression in resting cells of Saccharomyces cerevisiae. Transcripts were identified by hybridization with nick-translated or terminally labelled gene-specific probes. This allowed the determination of the steady-state levels of individual transcripts in the mitochondrial RNA population. Results showed different gene-specific patterns of response to respiratory induction: no increase in the level of transcripts (oxi2); a rapid increase in the steady-state levels of all transcripts (cob); a very strong increase in the processing of the high-molecular-mass precursors (oxi3 and oli2); an increase in the level of stable circular transcripts (oxi3). As a whole the results indicate specific and differentiated effects of release from glucose repression on the expression of the different mitochondrial genes and demonstrate the importance of processing events in mitochondrial regulation.