THE COLD SENSITIVITY OF A MUTANT OF SACCHAROMYCES-CEREVISIAE LACKING A MITOCHONDRIAL HEAT-SHOCK PROTEIN-70 IS SUPPRESSED BY LOSS OF MITOCHONDRIAL-DNA

SSH1, a newly identified member of the heat shock protein (hsp70) mult igene family of the budding yeast Saccharomyces cerevisiae, encodes a protein localized to the mitochondrial matrix. Deletion of the SSH1 ge ne results in extremely slow growth at 23 degrees C or 30 degrees C, b ut nearly wild-type growth at 37 degrees C. The matrix of the mitochon dria contains another hsp70, Ssc1, which is essential for growth and r equired for translocation of proteins into mitochondria. Unlike SSC1 m utants, an SSH1 mutant showed no detectable defects in import of sever al proteins from the cytosol to the matrix compared to wild type. Incr eased expression of Ssc1 partially suppressed the cold-sensitive growt h defect of the SSH1 mutant, suggesting that when present in increased amounts, Ssc1 can at least partially carry out the normal functions o f Ssh1, Spontaneous suppressors of the cold-sensitive phenotype of an SSH1 null mutant were obtained at a high frequency at 23 degrees C, an d were all found to be respiration deficient. 15 of 16 suppressors tha t were analyzed lacked mitochondrial DNA, while the 16th had reduced a mounts. We suggest that Ssh1 is required for normal mitochondrial DNA replication, and that disruption of this process in ssh1 cells results in a defect in mitochondrial function at low temperatures.