Mitochondrial ribosomal subunits from Saccharomyces cerevisiae rho- mutants

The DExH box protein Suv3p is a component of a yeast mitochondrial 3'-to-5' exoribonuclease that suppresses group I intron toxicity

The yeast mitochondrial protein Suv3p is a putative NTP-dependent RNA helicase. Here we report that in cells lacking Suv3p, there is an approximately 50-fold increase in the excised form of the group I intron omega of the mitochondrial 31S rRNA gene. Surprisingly, little mature 21S rRNA accumulates in those cells; instead, unligated 21S rRNA exons appear. Intron overaccumulation could lead to spliced exon reopening via a reaction known to be catalyzed by group I introns in vitro. We also show that Suv3p is a functional component of a novel mitochondrial NTP-dependent 3'-to-5' exoribonuclease activity that can degrade group I intron RNAs. These findings account for group I intron overaccumulation in cells lacking Suv3p and define a novel function for putative RNA helicases in direct RNA degradation.

Incorporation of the yeast mitochondrial ribosomal protein Mrp2 into ribosomal subunits requires the mitochondrially encoded Var1 protein

Mrp2 is a protein component of the small subunit of mitochondrial ribosomes in the yeast Saccharomyces cerevisiae. We have examined the expression of Mrp2 in yeast mutants lacking mitochondrial DNA and found that the steady-state level of Mrp2 is dramatically decreased relative to wild type. These data suggest that the accumulation of Mrp2 depends on the expression of one or more mitochondrial gene products. The mitochondrial genome of S. cerevisiae encodes two components of the small ribosomal subunit, 15S rRNA and the Var1 protein, both of which are necessary for the formation of mature 37S subunits. Several studies have shown that in the absence of Var1 incomplete subunits accumulate, which lack a limited number of ribosomal proteins. Here, we show that Mrp2 is one of the proteins absent from subunits lacking Var1, indicating that Var1 plays an important role in the incorporation of Mrp2 into mitochondrial ribosomal subunits.

Functional requirement of a site-specific ribose methylation in ribosomal RNA

The product of the PET56 nuclear gene of Saccharomyces cerevisiae was shown to be required for ribose methylation at a universally conserved nucleotide in the peptidyl transferase center of the mitochondrial large ribosomal RNA (21S rRNA). Cells reduced in this activity were deficient in formation of functional large subunits of the mitochondrial ribosome. The purified Pet56 protein catalyzed the site-specific formation of 2'-O-methylguanosine on in vitro transcripts of both mitochondrial 21S rRNA and Escherichia coli 23S rRNA. These results provide evidence for an essential modified nucleotide in rRNA.

Mitochondrial rRNA-containing petite strains of yeast (Saccharomyces cerevisiae) show a normal nuclear-mitochondrial stringent response

The nuclear-mitochondrial stringent response was examined in isonuclear rho+, 21S rRNA-containing rho-, and rho o strains of S. cerevisiae. By 30 min after nutritional downshift, nuclear rDNA transcription falls to 15% of control levels congruently in all strains, as assayed via whole-cell RNA or by hybrid selection of specific double-labeled transcripts. Both in vivo and in vitro, the mitochondrial stringent response is identical between the rho- strain and its parental rho+ strain, and in both, the kinetics and magnitude of the organellar response mirror those of the nuclear response. The data show that mitochondrial transcription and protein synthesis are not required for stringent regulation of either nuclear or mitochondrial rDNA transcription.

The unusual varl gene of yeast mitochondrial DNA

The var1 gene specifies the only mitochondrial ribosomal protein known to be encoded by yeast mitochondrial DNA. The gene is unusual in that its base composition is nearly 90 percent adenine plus thymine. It and its expression product show a strain-dependent variation in size of up to 7 percent; this variation does not detectably interfere with function. Furthermore, var1 is an expandable gene that participates in a novel recombinational event resembling gene conversion whereby shorter alleles are preferentially converted to longer ones. The remarkable features of var1 indicate that it may have evolved by a mechanism analogous to exon shuffling, although no introns are actually present.

The formation of a defective small subunit of the mitochondrial ribosomes in petite mutants of Saccharomyces cerevisiae

The involvement of mitochondrial protein synthesis in the assembly of the mitochondrial ribosomes was investigated by studying the extent to which the assembly process can proceed in petite mutants of Saccharomyces cerevisiae which lack mitochondrial protein synthetic activity due to the deletion of some tRNA genes and/or one of the rRNA genes on the mtDNA. Petite strains which retain the 15-S rRNA gene can synthesize this rRNA species, but do not contain any detectable amounts of the small mitochondrial ribosomal subunit. Instead, a ribonucleoparticle with a sedimentation coefficient of 30 S (instead of 37 S) was observed. This ribonucleoparticle contained all the small ribosomal subunit proteins with the exception of the var1 and three to five other proteins, which indicates that the 30-S ribonucleoparticle is related to the small mitochondrial ribosomal subunit (37 S). Reconstitution experiments using the 30-S particle and the large mitochondrial ribosomal subunit from a wild-type yeast strain indicate that the 30-S particle is not active in translating the artificial message poly(U). The large mitochondrial ribosomal subunit was present in petite strains retaining the 21-S rRNA gene. The petite 54-S subunit is biologically active in the translation of poly(U) when reconstituted with the small subunit (37 S) from a wild-type strain. The above results indicate that mitochondrial protein synthetic activity is essential for the assembly of the mature small ribosomal subunit, but not for the large subunit. Since the var1 protein is the only mitochondrial translation product known to date to be associated with the mitochondrial ribosomes, the results suggest that this protein is essential for the assembly of the mature small subunit.

Mitochondrial and nuclear mutations that affect the biogenesis of the mitochondrial ribosomes of yeast. II. Biochemistry

1. Several nuclear mutants have been isolated which showed thermo- or cryo-sensitive growth on non-fermentable media. Although the original strain carried mitochondrial drug resistance mutations (CR, ER, OR and PR), the resistance to one or several drugs was suppressed in these mutants. Two of them showed a much reduced amount of the mitochondrial small ribosomal subunit (37S) and of the corresponding 16S ribosomal RNA. Two dimensional electrophoretic analysis did not reveal any change in the position of any of the mitochondrial ribosomal proteins. However one of the mitochondrial ribosomal proteins. However one of the mutants showed a striking decrease in the amounts of three ribosomal proteins S3, S4 and S15. 2. Four temperature-sensitive mitochondrial mutations have been localized in the region of the gene coding for the large mitochondrial ribosomal RNA (23S). These mutants all showed a marked anomaly in the mitochondrial large ribosomal subunit (50S) and/or the corresponding 23S ribosomal RNA.

The genetic map of transfer RNA genes of yeast mitochondria: correction and extension

Ninety five rho- mitochondrial DNA's of Saccharomyces cerevisiae were compared for their deletion structure by means of 15 genetic markers and 22 tRNA genes. The patterns of co-deletion and co-retention of different tRNA genes allowed us to determine their positions with respect to each other. The deduced order of tRNA genes was consistent with the order of the genetic markers established by independent genetic approaches. Our previously proposed mitochondrial tRNA gene map has been revised and extended. Transfer RNA genes, corresponding to all 20 aminoacids, and two isoacceptor tRNA genes were localized. The possible position of each tRNA gene has been indicated on the physical map of mitochondrial DNA. Seventeen tRNA genes are carried by a narrow region representing less than 20% of the wild type genome.