STRUCTURAL AND FUNCTIONAL CHARACTERIZATIONS OF MUNG BEAN MITOCHONDRIAL NUCLEOIDS
Although many studies indicated that the nucleoid is the fundamental segregating unit of mitochondrial DNA (mtDNA), the relation between mitochondrial nucleoids division and mitochondrion propagation remains unclear. In our investigation, we aimed to ascertain whether mitochondrial nucleoids play a role in mitochondrial biogenesis. Higher-plant mitochondrial nucleoids isolated by conventional procedures are composed of a chromatin-like structure associated with a membrane component. We also identified a similar chromatin-like structure in situ, which underwent discrete changes during seedling development. These purified nucleoids possess a membrane system with cardiolipin and membrane proteins of inner- and outer-mitochondrial-membrane origin. Immunoblotting analysis revealed that actin protein was copurified with mitochondrial nucleoids, and formaldehyde crosslinking further revealed that certain proteins, including actin, are associated with mtDNA. Mitochondrial nucleoids are self-sufficient in terms of template, primers and enzymes to synthesize DNA in vitro in a pattern that mimics DNA synthesis in isolated mitochondria. Pulse-field gel electrophoresis showed most newly synthesized DNA to be well bound. Less than 50% of DNA migrated from the well, and 50- to 200-kb smear bands did not have prominent size classes. Equally important, this self-sufficiency showed RNA transcription but not translation activity in vitro in isolated nucleoids. Higher-plant mitochondrial nucleoids appear to be not merely static entities for mtDNA packaging and maintenance but function in mitochondrial biogenesis.
Lo, Y. S., Hsiao, L. J., Jane, W. N., Charng, Y. C., Dai, H., and Chiang, K. S. 2004. GFP targeted mitochondria show heterogeneity of size, morphology and dynamics in transgenic Nicotiana tabacum L. plants in vivo. Int. J. Plant Sci. 165: 949-955.
Dai, H., Lo, Y. S., Litvinchuk, A., Wang, Y. T., Jane, W. N., Hsiao, L. J., and Chiang, K. S. 2005. Structural and functional characterizations of mung bean mitochondrial nucleoids. Nucleic Acids Res. 33: 4725-4739.
