The mitochondrial nucleoid protein, Mgm101p, of Saccharomyces cerevisiae is involved in the maintenance of ρ⁺ and ori/rep-devoid petite genomes but is not required for hypersuppressive ρ^- mtDNA

The Saccharomyces cerevisiae MGM101 gene encodes a DNA-binding protein targeted to mitochondrial nucleoids. MGM101 is essential for maintenance of a functional ρ⁺ genome because meiotic segregants, with a disrupted mgm101 allele, cannot undergo more than 10 divisions on glycerol medium. Quantitative analysis of mtDNA copy number in a ρ⁺ strain carrying a temperature-sensitive allele, mgm101-1, revealed that the amount of mtDNA is halved each cell division upon a shift to the restrictive temperature. These data suggest that mtDNA replication is rapidly blocked in cells lacking MGM101. However, a small proportion of meiotic segregants, disrupted in MGM101, have ρ^- genomes that are stably maintained. Interestingly, all surviving ρ^- mtDNAs contain an ori/rep sequence. Disruption of MGM101 in hypersuppressive (HS) strains does not have a significant effect on the propagation of HS ρ^- mtDNA. However, in petites lacking an ori/rep, disruption of MGM101 leads to either a complete loss or a dramatically decrease stability of mtDNA. This discriminatory effect of MGM101 suggests that replication of ρ⁺ and ori/rep-devoid ρ^- mtDNAs is carried out by the same process. By contrast, the persistence of ori/rep-containing mtDNA in HS petites lacking MGM101 identifies a distinct replication pathway. The alternative mtDNA replication mechanism provided by ori/rep is independent of mitochondrial RNA polymerase encoded by RP041 as a HS ρ^- genome is stably maintained in a mgm101, rpo41 double mutant.