THE S12 RIBOSOMAL-PROTEIN OF PODOSPORA-ANSERINA BELONGS TO THE S19 BACTERIAL FAMILY AND CONTROLS THE MITOCHONDRIAL GENOME INTEGRITY THROUGHCYTOPLASMIC TRANSLATION

In the filamentous fungus Podospora anserina, two nuclear genes are in volved in the premature death syndrome associated with a site-specific deletion of the mitochondrial DNA: a mutant allele of the AS1 gene, e ncoding the cytoplasmic ribosomal protein S12, and an uncharacterized gene closely linked to the mating-type locus. We describe here the clo ning and the sequencing of the wild-type and two mutant alleles of the AS1 gene. The P. anserina S12 protein belongs to the bacterial S19 ri bosomal protein family and shows 72% identity with the S15 human ribos omal protein. Transformation experiments have shown that the AS1-4 mut ation itself is responsible for the premature death phenotype and that it corresponds to a Gly to Asp change in the highly conserved COOH-te rminal part of the protein. Use of antibodies directed against S12 did not permit detection of the mutant ribosomal protein inside the mitoc hondria. However, cross-reactions were observed with at least one mito chondrial ribosomal protein displaying a higher molecular weight than S12. The mitochondrial protein does not seem to be a by-product of the AS1 gene but is more likely the mitochondrial homologue of S12. These results strongly suggest that the mutant S12 protein acts indirectly to promote the mitochondrial deletion, via the cytoplasmic translation .