Molecular cloning and analysis of the nuclear gene MRP-L6 coding for a putative mitochondrial ribosomal protein from Saccharomyces cerevisiae

Mutations in nuclear genes encoding mitochondrial ribosome proteins restore pollen fertility in S male-sterile maize

The interaction of plant mitochondrial and nuclear genetic systems is exemplified by mitochondria-encoded cytoplasmic male sterility (CMS) under the control of nuclear restorer-of-fertility genes. The S type of CMS in maize is characterized by a pollen collapse phenotype and a unique paradigm for fertility restoration in which numerous nuclear restorer-of-fertility lethal mutations rescue pollen function but condition homozygous-lethal seed phenotypes. Two nonallelic restorer mutations recovered from Mutator transposon-active lines were investigated to determine the mechanisms of pollen fertility restoration and seed lethality. Mu Illumina sequencing of transposon-flanking regions identified insertion alleles of nuclear genes encoding mitochondrial ribosomal proteins RPL6 and RPL14 as candidate restorer-of-fertility lethal mutations. Both candidates were associated with lowered abundance of mitochondria-encoded proteins in developing maize pollen, and the rpl14 mutant candidate was confirmed by independent insertion alleles. While the restored pollen functioned despite reduced accumulation of mitochondrial respiratory proteins, normal-cytoplasm plants heterozygous for the mutant alleles showed a significant pollen transmission bias in favor of the nonmutant Rpl6 and Rpl14 alleles. CMS-S fertility restoration affords a unique forward genetic approach to investigate the mitochondrial requirements for, and contributions to, pollen and seed development.

HFA1 encoding an organelle-specific acetyl-CoA carboxylase controls mitochondrial fatty acid synthesis in Saccharomyces cerevisiae

The Saccharomyces cerevisiae gene, HFA1, encodes a >250-kDa protein, which is required for mitochondrial function. Hfa1p exhibits 72% overall sequence similarity (54% identity) to ACC1-encoded yeast cytoplasmic acetyl-CoA carboxylase. Nevertheless, HFA1 and ACC1 functions are not overlapping because mutants of the two genes have different phenotypes and do not complement each other. Whereas ACC1 is involved in cytoplasmic fatty acid synthesis, the phenotype of hfa1Delta disruptants resembles that of mitochondrial fatty-acid synthase mutants. They fail to grow on lactate or glycerol, and the mitochondrial cofactor, lipoic acid, is reduced to <10% of its normal cellular concentration. Other than Acc1p, the N-terminal sequence of Hfa1p comprises a canonical mitochondrial targeting signal together with a matrix protease cleavage site. Accordingly, the HFA1-encoded protein was specifically assigned by Western blotting of appropriate cell fractions to the mitochondrial compartment. Removal of the mitochondrial targeting sequence abolished the competence of HFA1 DNA to complement hfal null mutants. Conversely and in contrast to the intact HFA1 sequence, the signal sequence-free HFA1 gene complemented the mutational loss of cytoplasmic acetyl-CoA carboxylase. Expression of HFA1 under the control of the ACC1 promoter restored cellular ACC activity in ACC1-defective yeast mutants to wild type levels. From this finding, it is concluded that HFA1 encodes a specific mitochondrial acetyl-CoA carboxylase providing malonyl-CoA for intraorganellar fatty acid and, in particular, lipoic acid synthesis.

Isolation and characterization of a species-specific DNA fragment for identification of Candida (Torulopsis) glabrata by PCR

A PCR specific for Candida glabrata that amplifies a mitochondrial rRNA gene fragment was developed by analysis of C. glabrata-specific agarose gel bands, which were generated by arbitrarily primed PCR. The expected PCR product was successfully amplified with genomic DNA from 95 C. glabrata isolates but not from a number of other fungal isolates.

Sequence and expression analysis of a novel Xenopus laevis cDNA that encodes a protein similar to bacterial and chloroplast ribosomal protein L24

We report here the cloning and the characterization of a Xenopus laevis cDNA that encodes a basic protein of 276 amino acids with a central core region, which shows a substantial degree of homology to bacterial and chloroplast ribosomal protein L24, and additional diverged N- and C-terminal polypeptide extensions. The N-terminal extension displays similarities to the mitochondrial targetting sequence, thereby suggesting that the cDNA probably codes for a mitochondrial ribosomal protein. Although the gene was expressed ubiquitously, at fairly constant levels, during embryogenesis, the abundance of the transcripts in the different tissues varies with the mRNA levels in the kidney, adipose tissue, muscle and liver being greater than that present in the brain, heart, ovary and lung.

Mitochondrial ribosomal proteins (MRPs) of yeast

Mitochondrial ribosomal proteins (MRPs) are the counterparts in that organelle of the cytoplasmic ribosomal proteins in the host. Although the MRPs fulfil similar functions in protein biosynthesis, they are distinct in number, features and primary structures from the latter. Most progress in the eludication of the properties of individual MRPs, and in the characterization of the corresponding genes, has been made in baker's yeast (Saccharomyces cerevisiae). To date, 50 different MRPs have been determined, although biochemical data and mutational analysis propose a total number which is substantially higher. Surprisingly, only a minority of the MRPs that have been characterized show significant sequence similarities to known ribosomal proteins from other sources, thus limiting the deduction of their functions by simple comparison of amino acid sequences. Further, individual MRPs have been characterized functionally by mutational studies, and the regulation of expression of MRP genes has been described. The interaction of the mitochondrial ribosomes with transcription factors specific for individual mitochondrial mRNAs, and the communication between mitochondria and the nucleus for the co-ordinated expression of ribosomal constituents, are other aspects of current MRP research. Although the mitochondrial translational system is still far from being described completely, the yeast MRP system serves as a model for other organisms, including that of humans.

Identification and characterization of the genes for mitochondrial ribosomal proteins of Saccharomyces cerevisiae

We have purified 13 large subunit proteins of the mitochondrial ribosome of the yeast Saccharomyces cerevisiae and determined their partial amino acid sequences. To elucidate the structure and function of these proteins, we searched for their genes by comparing our sequence data with those deduced from the genomic nucleotide sequence data of S. cerevisiae and analyzed them. In addition, we searched for the genes encoding proteins whose N-terminal amino acid sequences we have reported previously [Grohmann, L., Graack, H.-R., Kruft, V., Choli, T., Goldschmidt-Reisin, S. & Kitakawa, M. (1991) FEBS Lett. 284, 51-56]. Thus, we were able to identify and characterize 12 new genes for large subunit proteins of the yeast mitochondrial ribosome. Furthermore, we determined the N-terminal amino acid sequences of seven small subunit proteins and subsequently identified the genes for five of them, three of which were found to be new.

Prediction and identification of new natural substrates of the yeast mitochondrial intermediate peptidase

Most mitochondrial precursor proteins are processed to the mature form in one step by mitochondrial processing peptidase (MPP), while a subset of precursors destined for the matrix or the inner membrane are cleaved sequentially by MPP and mitochondrial intermediate peptidase (MIP). We showed previously that yeast MIP (YMIP) is required for mitochondrial function in Saccharomyces cerevisiae. To further define the role played by two-step processing in mitochondrial biogenesis, we have now characterized the natural substrates of YMIP. A total of 133 known yeast mitochondrial precursors were collected from the literature and analyzed for the presence of the motif RX(decreases)(F/L/I)XX(T/S/G)XXXX(decreases), typical of precursors cleaved by MPP and MIP. We found characteristic MIP cleavage sites in two distinct sets of proteins: respiratory components, including subunits of the electron transport chain and tricarboxylic acid cycle enzymes, and components of the mitochondrial genetic machinery, including ribosomal proteins, translation factors, and proteins required for mitochondrial DNA metabolism. Representative precursors from both sets were cleaved to predominantly mature form by mitochondrial matrix or intact mitochondria from wild-type yeast. In contrast, intermediate-size forms were accumulated upon incubation of the precursors with matrix from mip1 delta yeast or intact mitochondria from mip1ts yeast, indicating that YMIP is necessary for maturation of these proteins. Consistent with the fact that some of these substrates are essential for the maintenance of mitochondrial protein synthesis and mitochondrial DNA replication, mip1 delta yeast undergoes loss of functional mitochondrial genomes.

The NAM9-1 suppressor mutation in a nuclear gene encoding ribosomal mitochondrial protein of Saccharomyces cerevisiae

The nuclear gene NAM9 from Saccharomyces cerevisiae (Sc) codes for a protein which, on the basis of sequence homology, was previously postulated to be a mitochondrial (mt) equivalent of the Escherichia coli (Ec) S4 ribosomal protein (r-protein) [Boguta et al., Mol. Cell. Biol. 12 (1992) 402-412]. The mt-r character of the NAM9 product is now confirmed by cross-reaction with the antisera for the Sc mt r-proteins. The NAM9-1 mutation, characterized previously as the nuclear suppressor of some ochre mt mit- mutants, is found to be a single nucleotide substitution changing Ser82 to Leu within the part of NAM9 corresponding to the S4 region involved in interaction with the 16S rRNA. This indicates that the mechanism of NAM9-1 suppression could be analogous to the suppression due to ram (ribosomal ambiguity) mutations in the Ec structural gene encoding r-protein S4. The NAM9-1 mutation leads also to defect in respiratory growth in the background of the wild-type mit+ genome.

Cloning and analysis of the nuclear gene MRP-S9 encoding mitochondrial ribosomal protein S9 of Saccharomyces cerevisiae

The Saccharomyces cerevisiae nuclear gene MRP-S9 was identified as part of the European effort in sequencing chromosome II. MRP-S9 encodes for a hydrophilic and basic protein of 278 amino acids with a molecular mass of 32 kDa. The C-terminal part (aa 153-278) of the MRP-S9 protein exhibits significant sequence similarity to members of the eubacterial and chloroplast S9 ribosomal-protein family. Cells disrupted in the chromosomal copy of MRP-S9 were unable to respire and displayed a characteristic phenotype of mutants with defects in mitochondrial protein synthesis as indicated by a loss of cytochrome c oxidase activity. Additionally, no activities of the gluconeogenetic enzymes, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase, could be observed under conditions of glucose de-repression. The respiration-deficient phenotype could not be restored by transformation of the disruption strain with a wild-type copy of MRP-S9, indicating that MRP-S9 disruption led to rho- or rho0 cells. Sequence similarities of MRP-S9 to other members of the ribosomal S9-protein family and the phenotype of disrupted cells are consistent with an essential role of MRP-S9 is assembly and/or function of the 30s subunit of yeast mitochondrial ribosomes.

Implications of a functional large ribosomal RNA with only three modified nucleotides

The sequence and structure of the peptidyl transferase region of large subunit ribosomal RNA is highly conserved and specific modified nucleotides could be important structural or functional elements in the catalytic center responsible for peptide bond formation. In fact, it has not been possible to reconstitute active E coli 50S subunits from in vitro transcripts of 23S rRNA and total 50S proteins. It is significant therefore, that the PET56 gene of yeast encodes an essential ribose methyltransferase that specifically modifies a universally conserved nucleotide, G2270, in the peptidyl transferase center of the mitochondrial large ribosomal RNA (21S). Since the loss of this modification in yeast mitochondrial 21S rRNA severely affects the assembly of 54S subunits, it is likely that the analogous 2'-O-methylguanosine at position 2251 (Gm2251) in E coli 23S rRNA is also required for the assembly of 50S subunits. Gm could be a critical structural determinant for the correct folding of the rRNA, the binding of one or more ribosomal proteins, or the interaction of the rRNA with tRNA. Previous work has shown that the mitochondrial large rRNAs are minimally modified relative to the E coli and eukaryotic cytoplasmic rRNAs. By direct chemical analysis using combined high performance liquid chromatography-mass spectrometry, the modification status of the yeast mitochondrial rRNAs was reexamined, revealing the presence of Gm, Um and pseudouridine (psi) in 21S rRNA. The Um was mapped to nucleotide 2791, which corresponds to the ribose methylated and universally conserved U2552 in E coli 23S rRNA, and the psi has been recently mapped to position 2819, which corresponds to psi 2580 in E coli 23S rRNA. The retention of Um and psi nucleotides in the peptidyl transferase center of the otherwise minimally modified mitochondrial rRNAs suggests that these modifications, like Gm2270, might be essential for ribosome assembly or function or both.

The yeast nuclear gene MRP-L13 codes for a protein of the large subunit of the mitochondrial ribosome

The nuclear gene MRP-L13 of Saccharomyces cerevisiae, which codes for the mitochondrial ribosomal protein YmL13, has been cloned and characterized. It is a single-copy gene residing on chromosome XI. Its nucleotide sequence was found to be identical to that of the previously reported ORF YK105. A comparison of the predicted protein sequence of the MRP-L13 gene product and the actual N-terminal amino-acid sequence of the isolated YmL13 protein indicated that the mature protein is preceded by a mitochondrial signal peptide of 86 amino-acid residues, which is the longest among all known mitochondrial ribosomal proteins of S. cerevisiae. No sequence similarity was found to any other ribosomal protein in the current databases. The transcription of MRP-L13 was found to be repressed in the presence of glucose. Its protein product is not strictly essential for mitochondrial functions, but disruption of the gene by insertion of LEU2 noticeably affected cellular growth on non-fermentable carbon sources.