Conversion at large intergenic regions of mitochondrial DNA in Saccharomyces cerevisiae

DNA recombination-initiation plays a role in the extremely biased inheritance of yeast [rho-] mitochondrial DNA that contains the replication origin ori5

Hypersuppressiveness, as observed in Saccharomyces cerevisiae, is an extremely biased inheritance of a small mitochondrial DNA (mtDNA) fragment that contains a replication origin (HS [rho(-)] mtDNA). Our previous studies showed that concatemers (linear head-to-tail multimers) are obligatory intermediates for mtDNA partitioning and are primarily formed by rolling-circle replication mediated by Mhr1, a protein required for homologous mtDNA recombination. In this study, we found that Mhr1 is required for the hypersuppressiveness of HS [ori5] [rho(-)] mtDNA harboring ori5, one of the replication origins of normal ([rho(+)]) mtDNA. In addition, we detected an Ntg1-stimulated double-strand break at the ori5 locus. Purified Ntg1, a base excision repair enzyme, introduced a double-stranded break by itself into HS [ori5] [rho(-)] mtDNA at ori5 isolated from yeast cells. Both hypersuppressiveness and concatemer formation of HS [ori5] [rho(-)] mtDNA are simultaneously suppressed by the ntg1 null mutation. These results support a model in which, like homologous recombination, rolling-circle HS [ori5] [rho(-)] mtDNA replication is initiated by double-stranded breakage in ori5, followed by Mhr1-mediated homologous pairing of the processed nascent DNA ends with circular mtDNA. The hypersuppressiveness of HS [ori5] [rho(-)] mtDNA depends on a replication advantage furnished by the higher density of ori5 sequences and on a segregation advantage furnished by the higher genome copy number on transmitted concatemers.

MGM101, a nuclear gene involved in maintenance of the mitochondrial genome in Saccharomyces cerevisiae

A nuclear mutation, mgm101, results in temperature sensitive loss of mitochondrial DNA (mtDNA) in the yeast Saccharomyces cerevisiae. The corresponding gene, MGM101, was isolated from a genomic DNA library by complementation. Sequence analysis shows that MGM101 encodes a positively charged protein of 269 amino acids with a calculated molecular weight of 30 kDa. This analysis also reveals that MGM101 is adjacent to the ribosomal protein gene RPS7A on chromosome X and hybridization indicates it occurs in single copy. Creation of a null mutant by targeted disruption showed that the gene has no essential cellular function, aside from its participation in mitochondrial genome maintenance. As no counterpart has been identified in databases it is a novel protein whose role has yet to be determined. Expression of MGM101 is low on glucose medium but on galactose there is a two-fold increase in the level of the transcript.

Transformation of Saccharomyces cerevisiae with nonhomologous DNA: illegitimate integration of transforming DNA into yeast chromosomes and in vivo ligation of transforming DNA to mitochondrial DNA sequences

When the yeast Saccharomyces cerevisiae was transformed with DNA that shares no homology to the genome, three classes of transformants were obtained. In the most common class, the DNA was inserted as the result of a reaction that appears to require base pairing between the target sequence and the terminal few base pairs of the transforming DNA fragment. In the second class, no such homology was detected, and the transforming DNA was integrated next to a CTT or GTT in the target; it is likely that these integration events were mediated by topoisomerase I. The final class involved the in vivo ligation of transforming DNA with nucleus-localized linear fragments of mitochondrial DNA.

Transformation of Saccharomyces cerevisiae with nonhomologous DNA: illegitimate integration of transforming DNA into yeast chromosomes and in vivo ligation of transforming DNA to mitochondrial DNA sequences

When the yeast Saccharomyces cerevisiae was transformed with DNA that shares no homology to the genome, three classes of transformants were obtained. In the most common class, the DNA was inserted as the result of a reaction that appears to require base pairing between the target sequence and the terminal few base pairs of the transforming DNA fragment. In the second class, no such homology was detected, and the transforming DNA was integrated next to a CTT or GTT in the target; it is likely that these integration events were mediated by topoisomerase I. The final class involved the in vivo ligation of transforming DNA with nucleus-localized linear fragments of mitochondrial DNA.

A mobile group II intron of a naturally occurring rearranged mitochondrial genome in Kluyveromyces lactis

Mitochondrial intron content is variable in the yeast Kluyveromyces lactis. Strains can be divided into three classes depending on the structure of the cytochrome oxidase subunit 1 (COX1) gene: (1) those containing intron K1 cox1.1, (2) those containing K1 cox1.2, 3 and 4 and, (3) those that contain all four introns. In addition, strains belonging to the first class (designated Type B strains), have an altered mitochondrial gene order relative to strains from classes (2) and (3) (Type A, Hardy et al. 1989). Crossing experiments reveal that K1 cox1.1 (a group II intron) transfers at high frequency (89%) to mitochondrial genomes lacking this intron. By contrast, the mobility of the remaining introns (all group I) is of the order of 7%.