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Published online 3 November 2003. doi:10.1083/jcb.200304040
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© The Rockefeller University Press, 0021-9525/2003/11/503 $8.00
The Journal of Cell Biology, Volume 163, Number 3, 503-510

Article

 Evidence for a two membrane–spanning autonomous mitochondrial DNA replisome

Shelly Meeusen and Jodi Nunnari

Section of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616

Address correspondence to Jodi Nunnari, Section of Molecular and Cellular Biology, University of California, Davis, 1 Shields Ave., Davis, CA 95616. Tel.: (530) 754-9774. Fax: (530) 752-7522. email: jmnunnari{at}ucdavis.edu

The unit of inheritance for mitochondrial DNA (mtDNA) is a complex nucleoprotein structure termed the nucleoid. The organization of the nucleoid as well as its role in mtDNA replication remain largely unknown. Here, we show in Saccharomyces cerevisiae that at least two populations of nucleoids exist within the same mitochondrion and can be distinguished by their association with a discrete proteinaceous structure that spans the outer and inner mitochondrial membranes. Surprisingly, this two membrane–spanning structure (TMS) persists and self-replicates in the absence of mtDNA. We tested whether TMS functions to direct the replication of mtDNA. By monitoring BrdU incorporation, we observed that actively replicating nucleoids are associated exclusively with TMS. Consistent with TMS's role in mtDNA replication, we found that Mip1, the mtDNA polymerase, is also a stable component of TMS. Taken together, our observations reveal the existence of an autonomous two membrane–spanning mitochondrial replisome as well as provide a mechanism for how mtDNA replication and inheritance may be physically linked.

Key Words: mitochondria; mtDNA; nucleoid; replisome; membrane-spanning

The online version of this article includes supplemental material.

Abbreviations used in this paper: mtDNA, mitochondrial DNA; TAC, tripartite attachment complex; TMS, two membrane–spanning structure.


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