The interdependency of cellular and subcellular DNA

Evolution of eukaryotic cells was certainly a remarkable step in the emergence of complex organisms. This revolutionary step had occurred through the endosymbiotic relationship of different types of bacteria – survival of bacteria within another bacterium. According to the endosymbiotic theory, it is known that mitochondria and chloroplast in plant cell were free-living bacteria once. This theory is greatly supported by the existence of primitive bacterial DNA within these sub-cellular organelles. Mitochondria and chloroplasts both have their own DNA. The most surprising thing is that they had their own standalone DNA, previously. But today we observed that these bacterial DNAs in our cells or plant cells cannot function without the help of nuclear DNA. What happened to these DNAs and how they became dependent?

Now, we have to look at the cellular functioning these days. To survive and to enable a perfect cellular machinery, all three genomes need to work together. Both chloroplast and mitochondrial DNA are dependent on nuclear DNA. Most of the interactions are nucDNA ⇔ mtDNA and nucDNA ⇔ cpDNA. For example, major protein complexes of photosynthetic membranes contain some proteins encoded by cpDNA and some by nucDNA. In case of nucDNA, gene expression machinery includes both nucleus and cytoplasmic ribosomes, but expression of cpDNA is entirely occurred within the plastid. Proteins produced in the cytoplasm are forwarded to the chloroplast by virtue of transit peptides, located at the N-terminus of specific amino acid sequences. This transit peptide determines the ultimate locations within the chloroplast such as thylakoid or stroma and even orientation within the membrane, that is, inward or outward facing.

figure

Figure: The flow of genetic information between chloroplast and nucleus

Rubisco, a major carboxylating enzyme, has two subunits. The small one is encoded by a nuclear gene rbcS while the larger one is encoded by chloroplast gene rbcL. Rubisco is a key carboxylating enzyme in most species, huge amount of this enzyme is synthesized within the cell. Because it is in fact, a relatively inefficient catalyst. There is specific signal between these subcellular compartments to ensure that sufficient quantities of each protein. However, it is still a mystery that how signal from chloroplasts transduced into nucleus.

Similar interactions between nuclear DNA and mitochondrial DNA is observed in case of many respiratory complexes of mitochondrial membranes. Furthermore, protein synthesis in mitochondria is greatly facilitated by some nuclear-encoded tRNAs. Cytoplasmic male sterility is also resulted from the faulty interaction between mitochondrial genome and nuclear genome. The mitochondrial gene cmsT results male sterility due to prevention or inhibition of normal stamen development. This gene encodes an integral membrane protein of inner mitochondrial membrane which is about 13kDa. However, the mechanism of male sterility is still unclear but this could be due to high energy requirement for stamen development.

Therefore, nuclear genome plays a dominant role over the mtDNA and cpDNA to coordinate overall cellular activity. But is there any interaction between mitochondria and chloroplasts? No direct evidence is not found yet to define this inter-relation, although some DNA sequences are found to be similar in both organelles.

Reference:

Plant in action: http://plantsinaction.science.uq.edu.au/edition1/?q=content/10-3-2-genome-interactions

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