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Cross-strand binding of TFAM to a single mtDNA molecule forms the mitochondrial nucleoid

Journal article
Authors C. Kukat
K. M. Davies
C. A. Wurm
H. Spahr
N. A. Bonekamp
I. Kuhl
F. Joos
P. L. Polosa
C. B. Park
Viktor Posse
Maria Falkenberg
S. Jakobs
W. Kuhlbrandt
N. G. Larsson
Published in Proceedings of the National Academy of Sciences of the United States of America
Volume 112
Issue 36
Pages 11288-11293
ISSN 0027-8424
Publication year 2015
Published at Institute of Biomedicine, Department of Medical Biochemistry and Cell Biology
Pages 11288-11293
Language en
Links dx.doi.org/10.1073/pnas.1512131112
Keywords nucleoids, mitochondria, cryo-ET, STED, nanoscopy, TRANSCRIPTION FACTOR-A, ELECTRON-MICROSCOPY, SUPERRESOLUTION, FLUORESCENCE, DNA REPLICATION, PROTEIN, ORGANIZATION, VISUALIZATION, MUTATIONS, COMPLEXES, MECHANISM, Multidisciplinary Sciences
Subject categories Basic Medicine

Abstract

Mammalian mitochondrial DNA (mtDNA) is packaged by mitochondrial transcription factor A (TFAM) into mitochondrial nucleoids that are of key importance in controlling the transmission and expression of mtDNA. Nucleoid ultrastructure is poorly defined, and therefore we used a combination of biochemistry, super-resolution microscopy, and electron microscopy to show that mitochondrial nucleoids have an irregular ellipsoidal shape and typically contain a single copy of mtDNA. Rotary shadowing electron microscopy revealed that nucleoid formation in vitro is a multistep process initiated by TFAM aggregation and cross-strand binding. Superresolution microscopy of cultivated cells showed that increased mtDNA copy number increases nucleoid numbers without altering their sizes. Electron cryo-tomography visualized nucleoids at high resolution in isolated mammalian mitochondria and confirmed the sizes observed by superresolution microscopy of cell lines. We conclude that the fundamental organizational unit of the mitochondrial nucleoid is a single copy of mtDNA compacted by TFAM, and we suggest a packaging mechanism.

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