Nam1p, a protein involved in RNA processing and translation, is coupled totranscription through an interaction with yeast mitochondrial RNA polymerase

Alignment of three fungal mtRNA polymerases revealed conserved amino acid s equences in an aminoterminal region of the Saccharomyces cerevisiae enzyme implicated previously as harboring an important functional domain. Phenotyp ic analysis of deletion and point mutations, in conjunction with a yeast tw o-hybrid assay, revealed that Nam1p, a protein involved in RNA processing a nd translation in mitochondria, binds specifically to this domain, The sign ificance of this interaction in vivo was demonstrated by the fact that the temperature-sensitive phenotype of a deletion mutation (rpo41 Delta2), whic h impinges on this amino-terminal domain, is suppressed by overproducing Na m1p. In addition, mutations in the amino-terminal domain result specificall y in decreased steady-state levels of mature mitochondrial CYTB and COXI tr anscripts, which is a primary defect observed in NAM1 null mutant yeast str ains. Finally, one point mutation (R129D) did not abolish Nam1p binding, ye t displayed an obvious COX1/CYTB transcript defect. This mutation exhibited the most severe mitochondrial phenotype, suggesting that mutations in the amino-terminal domain can perturb other critical interactions, in addition to Nam1p binding, that contribute to the observed phenotypes. These results implicate the amino-terminal domain of mtRNA polymerases in coupling addit ional factors and activities involved in mitochondrial gene expression dire ctly to the transcription machinery.