OVEREXPRESSION OF THE RNR1 GENE RESCUES SACCHAROMYCES-CEREVISIAE MUTANTS IN THE MITOCHONDRIAL-DNA POLYMERASE-ENCODING MIP1 GENE

A multicopy suppressor gene which rescues the temperature-sensitive growth defect of Saccharomyces cerevisiae mutants in the mitochondrial DNA (mtDNA) polymerase-encoding MIP1 gene has been isolated and identified as the RNR1 gene. This gene, whose transcript is cell cycle-regulated and mainly expressed at the G1 to S phase transition, encodes the large subunit of ribonucleotide reductase. This enzyme catalyses a limiting step in the production of deoxynucleotides needed for DNA synthesis. The presence of a high copy number of the RNR1 gene also decreases the accumulation of rho(-) mutants observed in diploids that harbour a single copy of the MIP1 gene. In cell cycle-synchronised cells, the presence of a high copy number of RNR1 does not modify its cell cycle transcription regulation and increases its transcript level by a factor of 10 throughout the cell cycle. Our results show that an increased supply of dNTPs in mitochondria can stimulate the mtDNA polymerase activity and indicate that the dNTP concentration may be rate limiting for the replication of mtDNA.