Structure and transcription of the mitochondrial genome in heteroplasmic strains of Saccharomyces cerevisiae

A missense mutation in the nuclear gene coding for the mitochondrial aspartyl-tRNA synthetase suppresses a mitochondrial tRNA(Asp) mutation

The nuclear suppressor allele NSM3 in strain FF1210-6C/170-E22 (E22), which suppresses a mutation of the yeast mitochondrial tRNA(Asp)gene in Saccharomyces cerevisiae, was cloned and identified. To isolate the NSM3 allele, a genomic DNA library using the vector YEp13 was constructed from strain E22. Nine YEp13 recombinant plasmids were isolated and shown to suppress the mutation in the mitochondrial tRNA(Asp)gene. These nine plasmids carry a common 4. 5-kb chromosomal DNA fragment which contains an open reading frame coding for yeast mitochondrial aspartyl-tRNA synthetase (AspRS) on the basis of its sequence identity to the MSD1 gene. The comparison of NSM3 DNA sequences between the suppressor and the wild-type version, cloned from the parental strain FF1210-6C/170, revealed a G to A transition that causes the replacement of amino acid serine (AGU) by an asparagine (AAU) at position 388. In experiments switching restriction fragments between the wild type and suppressor versions of the NSM3 gene, the rescue of respiratory deficiency was demonstrated only when the substitution was present in the construct. We conclude that the base substitution causes the respiratory rescue and discuss the possible mechanism as one which enhances interaction between the mutated tRNA(Asp)and the suppressor version of AspRS.

Ordered processing of the polygenic transcripts from a mitochondrial tRNA gene cluster in K. lactis

In Saccharomyces cerevisiae, transcription of the mitochondrial genome starts at multiple initiation sites and is followed by the processing of multigenic transcripts at the 5' and 3' termini of tRNA sequences and in some intergenic regions. We have used a comparative approach to investigate the structure and function of the latter processing sites. We present here an analysis of the transcripts of a cluster of tRNA genes from the mitochondrial genome of Kluyveromyces lactis. The gene order of this cluster is the same as that of the cluster in S. cerevisiae but the sequence of the intergenic regions is different. A detailed analysis of transcripts has been performed using S1 mapping and primer extension techniques. The results can be summarized as follows: (1) transcription of the cluster very probably starts at initiation sites having the nonanucleotide sequence TTATAAGTA (which acts as a promoter in S. cerevisiae) and yields polygenic transcripts; (2) processing of these transcripts seems to occur through an ordered pathway of endonucleolytic events in which some tRNA sequences are preferentially excised and some endonucleolytic cuts occur more readily than others; (3) in two intergenic regions, strong signals indicate the existence of processing events. The sequences around these sites are similar in sequence and localization to S. cerevisiae intergenic processing sites, indicating a possible functional importance in maintaining a conserved order of tRNA genes in different species of yeasts.

Novel hybrid maturases in unstable pseudorevertants of maturaseless mutants of yeast mitochondrial DNA

Unstable pseudorevertants of mitochondrial mutants of Saccharomyces cerevisiae lacking the maturase function encoded by the fourth intron of the cytochrome b gene (bI4) were isolated. They were found to be heteroplasmic cells owing their regained ability to respire (and grow on glycerol medium) to the presence of a rearranged (rho-) mtDNA that contains an in-frame fusion of the reading frames of the group I introns bI4 and intron 4 alpha of the coxl gene encoding subunit I of cytochrome c oxidase (aI4 alpha). The products of those gene fusions suppress the bI4 maturase deficiency still present in those heteroplasmic cells. Similar heteroplasmic pseudorevertants of a group II maturaseless mutant of the first intron of the coxI gene were characterized; they result from partial deletion of the coxI gene that fuses the reading frames of introns 1 and 2. These heteroplasms provide independent support for the existence of RNA maturases encoded by group I and group II introns. Also, since the petite/mit- heteroplasms arise spontaneously at very high frequencies they provide a system that can be used to obtain mutants unable to form or maintain heteroplasmic cells.