The entire DNA sequence of chromosome III of the yeast Saccharomyces cerevisiae has been determined. This is the first complete sequence analysis of an entire chromosome from any organism. The 315-kilobase sequence reveals 182 open reading frames for proteins longer than 100 amino acids, of which 37 correspond to known genes and 29 more show some similarity to sequences in databases. Of 55 new open reading frames analysed by gene disruption, three are essential genes; of 42 non-essential genes that were tested, 14 show some discernible effect on phenotype and the remaining 28 have no overt function.

In Saccharomyces cerevisiae the nicotinic acid moiety of NAD + can be synthesized from tryptophan using the kynurenine pathway or incorporated directly using nicotinate phosphoribosyl transferase (NPT1). We have identified the genes that encode the enzymes of the kynurenine pathway and for BNA5 (YLR231c) and BNA6 (YFR047c) confirmed that they encode kynureninase and quinolinate phosphoribosyl transferase respectively. We show that deletion of genes encoding kynurenine pathway enzymes are co-lethal with the v vnpt1, demonstrating that no other pathway for the synthesis of nicotinic acid exists in S. cerevisiae. Also, we show that under anaerobic conditions S. cerevisiae is a nicotinic acid auxotroph. ß

The HAP1 gene encodes a complex transcriptional regulator of many genes involved in electron-transfer reactions and is essential in anaerobic or heme-depleted conditions. We show here that strains derived from S288c carry a defective Ty1 element inserted in the 3' region of the HAP1 ORF. This mutant allele acts as a HAP1 null allele in terms of cytochrome c expression and CYC1 UAS1-dependent transcription, but is able to sustain limited growth in heme-depleted conditions.

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We have cloned and sequenced the NAM2 gene of Saccharomyces douglasii, which is a homologue of the NAM2 gene of Saccharomyces cerevisiae. The wild‐type S.douglasii gene possesses the suppressor functions of the mutant S. cerevisiae NAM2‐1 allele, being able to cure a mitochondrial b14 maturase deficiency. By sequence comparisons and direct measurements we have demonstrated that the NAM2 genes encode mitochondrial leucyl tRNA synthetases (EC 6.1.1.4.). Using a derivative of the NAM2 gene, where the expression of the gene is under the control of the UAS GAL10, we have shown that the processing of the pre‐mRNA from the two mosaic genes oxi3 and cob‐box is impaired when transcription of the gene is repressed. These results lead us to conclude that the mitochondrial leucyl tRNA synthetase is involved in protein synthesis and mRNA splicing. Sequence comparisons show that the mitochondrial and Escherichia coli leucyl tRNA synthetases are highly homologous; however, significant features which may be important for the splicing functions of the mitochondrial enzymes are absent from the bacterial enzyme.

Pentatricopeptide repeat (PPR) proteins are particularly numerous in plant mitochondria and chloroplasts, where they are involved in different steps of RNA metabolism, probably due to the repeated 35 amino acid PPR motifs that are thought to mediate interactions with RNA. In non-photosynthetic eukaryotes only a handful of PPR proteins exist, for example the human LRPPRC, which is involved in a mitochondrial disease. We have conducted a systematic study of the PPR proteins in the fission yeast Schizosaccharomyces pombe and identified, in addition to the mitochondrial RNA polymerase, eight proteins all of which localized to the mitochondria, and showed some association with the membrane. The absence of all but one of these PPR proteins leads to a respiratory deficiency and modified patterns of steady state mt-mRNAs or newly synthesized mitochondrial proteins. Some cause a general defect, whereas others affect specific mitochondrial RNAs, either coding or non-coding: cox1, cox2, cox3, 15S rRNA, atp9 or atp6, sometimes leading to secondary defects. Interestingly, the two possible homologs of LRPPRC, ppr4 and ppr5, play opposite roles in the expression of the cox1 mt-mRNA, ppr4 being the first mRNA-specific translational activator identified in S. pombe, whereas ppr5 appears to be a general negative regulator of mitochondrial translation.

We have analysed a new gene, CEM1, from Saccharomyces cerevisiae. Inactivation of this gene leads to a respiratory-deficient phenotype. The deduced protein sequence shows strong similarities with beta-keto-acyl synthases or condensing enzymes. Typically, enzymes of this class are involved in the synthesis of fatty acids or similar molecules. An analysis of the mitochondrial lipids and fatty acids shows no major difference between the wild type and deleted strains, implying that the CEM1 gene product is not involved in the synthesis of the bulk fatty acids. Thus it is possible that the CEM1 protein is involved in the synthesis of a specialized molecule, probably related to a fatty acid, which is essential for mitochondrial respiration.