RNA processing in yeast mitochondria: characterization of mit(-) mutants disturbed in the synthesis of subunit I of cytochrome c oxidase

Copper metabolism in Saccharomyces cerevisiae: an update

Copper is an essential element in all forms of life. It acts as a cofactor of some enzymes and is involved in forming proper protein conformations. However, excess copper ions in cells are detrimental as they can generate free radicals or disrupt protein structures. Therefore, all life forms have evolved conserved and exquisite copper metabolic systems to maintain copper homeostasis. The yeast Saccharomyces cerevisiae has been widely used to investigate copper metabolism as it is convenient for this purpose. In this review, we summarize the mechanism of copper metabolism in Saccharomyces cerevisiae according to the latest literature. In brief, bioavailable copper ions are incorporated into yeast cells mainly via the high-affinity transporters Ctr1 and Ctr3. Then, intracellular Cu⁺ ions are delivered to different organelles or cuproproteins by different chaperones, including Ccs1, Atx1, and Cox17. Excess copper ions bind to glutathione (GSH), metallothioneins, and copper complexes are sequestered into vacuoles to avoid toxicity. Copper-sensing transcription factors Ace1 and Mac1 regulate the expression of genes involved in copper detoxification and uptake/mobilization in response to changes in intracellular copper levels. Though numerous recent breakthroughs in understanding yeast's copper metabolism have been achieved, some issues remain unresolved. Completely elucidating the mechanism of copper metabolism in yeast helps decode the corresponding system in humans and understand how copper-related diseases develop.

A GC cluster repeat is a hotspot for mit- macro-deletions in yeast mitochondrial DNA

In a random collection of mit- mutations of the yeast strain 777-3A we find that deletions are exceptionally frequent in the OXI3 gene, a large mosaic gene coding for subunit I of cytochrome oxidase. About 10% of all oxi3-mutants carry the same macro-deletion, del-A, extending from the 5' non-translated leader of OXI3 to intron 5b of this gene. Determination of the respective wild-type sequences and of the del-A junction sequence revealed that the end-points of the deletion are in two GC clusters with 31 bp sequence identity which are located at a distance of 11.3 kb. We speculate that not only the sequence identity of the two GC clusters but also the palindromic structure of these putatively mobile elements of yeast mitochondrial DNA (mtDNA) plays a role in deletion formation.

RNA splicing in yeast mitochondria: DNA sequence analysis of mit- mutants deficient in the excision of introns aI1 and aI2 of the gene for subunit I of cytochrome c oxidase

We have characterized two yeast mutants deficient in the splicing of transcripts of the mitochondrial gene for cytochrome c oxidase subunit I (coxI). Both map to the first intron (aI1). RNA blot analysis shows that in addition to a reduced (mutant M15-190) or blocked (mutant M12-193) excision of the mutated intron aI1, the mutants are unable to excise the adjacent aI2 intron, the reading frame of which displays an amino acid sequence similarity to aI1. Splicing of the downstream introns is not affected, however. Sequence analysis of the first mutant DNA (M12-193) reveals a premature termination of the intron-encoded open reading frame, followed by two alterations at a short distance downstream. The other (M15-190) contains 11 separate changes. Although these occur in the intron reading frame, their main effect on RNA splicing may be exerted through the disturbance of intron secondary structure proposed for the 5' end of several group II introns. The implications of these findings in relation to maturase function and structure of intron aI1 are 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).

Excised group II introns in yeast mitochondria are lariats and can be formed by self-splicing in vitro

Excised group II introns in yeast mitochondria appear as covalently closed circles under the electron microscope. We show that these circular molecules are branched and resemble the lariats arising through splicing of nuclear pre-mRNAs in yeast and higher eukaryotes. One member of this intron class (aI5c in the gene for cytochrome c oxidase subunit I) is capable of self-splicing in vitro, giving correct exon-exon ligation and resulting in the appearance of both linear and lariat forms of the excised intron. Nuclease digestion of the latter molecules reveals the presence of a complex oligonucleotide with the probable structure AGU, which thus resembles the branch point formed in the spliceosome-dependent reactions undergone by nuclear pre-mRNAs. Unlike group I introns, this group II intron is not demonstrably dependent on GTP for self-splicing and circularization of the isolated, linear intron is not observed. A model accounting for these observations is presented.