The C-terminal region of mitochondrial single-subunit RNA polymerases contains species-specific determinants for maintenance of intact mitochondrial genomes.

Functional conservation of mitochondrial RNA polymerases was investigated in vivo by heterologous complementation studies in yeast. It turned out that neither the full-length mitochondrial RNA polymerase of Arabidopsis thaliana, nor a set of chimeric fusion constructs from plant and yeast RNA polymerases can substitute for the yeast mitochondrial core enzyme Rpo41p when expressed in Deltarpo41 yeast mutants. Mitochondria from mutant cells, expressing the heterologous mitochondrial RNA polymerases, were devoid of any mitochondrial genomes. One important exception was observed when the carboxyl-terminal domain of Rpo41p was exchanged with its plant counterpart. Although this fusion protein could not restore respiratory function, stable maintenance of mitochondrial petite genomes (rho(-))(-) was supported. A carboxyl-terminally truncated Rpo41p exhibited a comparable activity, in spite of the fact that it was found to be transcriptionally inactive. Finally, we tested the carboxyl-terminal domain for complementation in trans. For this purpose the last 377 amino acid residues of yeast mitochondrial Rpo41p were fused to its mitochondrial import sequence. Coexpression of this fusion protein with C-terminally truncated Rpo41p complemented the Deltarpo41 defect. These data reveal the importance of the carboxyl-terminal extension of Rpo41p for stable maintenance of intact mitochondrial genomes and for distinct species-specific intramolecular protein-protein interactions.