A DNA Helicase Required for Maintenance of the Functional Mitochondrial Genome in Saccharomyces cerevisiae

Abstract

A novel DNA helicase, a homolog of several prokaryotic helicases, including Escherichia coli Rep and UvrD proteins, is encoded by the Saccharomyces cerevisiae nuclear genome open reading frame YOL095c on the chromosome XV. Our data demonstrate that the helicase is localized in the yeast mitochondria and is loosely associated with the mitochondrial inner membrane during biochemical fractionation. The sequence of the C-terminal end of the 80-kDa helicase protein is similar to a typical N-terminal mitochondrial targeting signal; deletions and point mutations in this region abolish transport of the protein into mitochondria. The C-terminal signal sequence of the helicase targets a heterologous carrier protein into mitochondria in vivo. The purified recombinant protein can unwind duplex DNA molecules in an ATP-dependent manner. The helicase is required for the maintenance of the functional ([rho⁺]) mitochondrial genome on both fermentable and nonfermentable carbon sources. However, the helicase is not essential for the maintenance of several defective ([rho⁻]) mitochondrial genomes. We also demonstrate that the helicase is not required for transcription in mitochondria.

ACKNOWLEDGMENTS

We thank R. Stuart and W. Neupert for the antibodies against mitochondrial marker proteins and helpful discussions. We are grateful to M. Makarowa and H. Holkeri for their help with spore dissection analysis. We thank J. Remme, A. Stenlund, and our colleagues in the department for the comments on the manuscript.

This work has been supported by Estonian Science Foundation grant 2888 to J.S.