Tight Control of Respiration by NADH Dehydrogenase ND5 Subunit Gene Expression in Mouse Mitochondria

Abstract

A mouse cell variant carrying in heteroplasmic form a nonsense mutation in the mitochondrial DNA-encoded ND5 subunit of the respiratory NADH dehydrogenase has been isolated and characterized. The derivation from this mutant of a large number of cell lines containing between 4 and 100% of the normal number of wild-type ND5 genes has allowed an analysis of the genetic and functional thresholds operating in mouse mitochondria. In wild-type cells, ∼40% of the ND5 mRNA level was in excess of that required for ND5 subunit synthesis. However, in heteroplasmic cells, the functional mRNA level decreased in proportion to the number of wild-type ND5 genes over a 25-fold range, pointing to the lack of any compensatory increase in rate of transcription and/or stability of mRNA. Most strikingly, the highest ND5 synthesis rate was just sufficient to support the maximum NADH dehydrogenase-dependent respiration rate, with no upregulation of translation occurring with decreasing wild-type mRNA levels. These results indicate that, despite the large excess of genetic potential of the mammalian mitochondrial genome, respiration is tightly regulated by ND5 gene expression.

ACKNOWLEDGMENTS

This work was supported by Public Health Service grant GM11726 to G.A. and by Eisai Co., Ltd., Tokyo, Japan.

We are very grateful to Anne Chomyn for providing the anti-ND4L antibodies and for critical reading of the manuscript and to Gaetano Villani for helpful discussions. We also thank R. Zedan, A. Drew, and B. Keeley for expert technical assistance.