6.15:
Export of Mitochondrial and Chloroplast Genes
Organelle genomes, such as those in both mitochondria and chloroplasts, are smaller than those of their prokaryotic ancestors. This is because, during evolution, the majority of their genes were exported to the nucleus while many others were lost before developing into a mitochondrial or chloroplast genome.
These exported genes are known as nuclear integrants of organellar DNA. Specifically, the genes from the mitochondria are nuclear integrants of mitochondrial DNA, and those from the chloroplast are nuclear integrants of plastid DNA.
One theory of why cells may transfer the genes from mitochondria and chloroplasts to the nucleus is that the electron transfer reactions in mitochondria and chloroplasts generate mutation-causing free radicals. The export of these genes reduces exposure to free radicals and the likelihood of harmful mutations.
Additionally, the nucleus has a more effective DNA repair system than either mitochondria or chloroplasts.
As mitochondrial and chloroplast DNA are inherited from a single parent only, they cannot undergo sexual recombination. However, once the genes are incorporated into nuclear DNA, genes from both parents are inherited.
Sexual recombination allows rearrangement of genes from both parents which can prevent the accumulation of undesired mutations and can improve adaptation to the surrounding environment.
Nuclear DNA’s transcription and translation machinery are different from those of mitochondria and chloroplasts; therefore, the exported genes must undergo several modifications to function properly.
These changes include the insertion of new DNA sequences for a promoter and a terminator required for proper mRNA and protein production. A targeting sequence is also added to direct the protein product to the mitochondria or chloroplast.
Most exported genes retain their original function in the mitochondria and chloroplast. However, in some cases, it has led to the development of genes with new functions.
6.15:
Export of Mitochondrial and Chloroplast Genes
A eukaryotic cell can have up to three different types of genetic systems: nuclear, mitochondrial, and chloroplast. During evolution, organelles have exported many genes to the nucleus; this transfer is still ongoing in some plant species. Approximately 18% of the Arabidopsis thaliana nuclear genome is thought to be derived from the chloroplast’s cyanobacterial ancestor, and around 75% of the yeast genome derived from the mitochondria’s bacterial ancestor. This export has occurred irrespective of the location or the size of the gene in the organellar genome; large genes and, in some cases the entire organellar genome, have been found in the nucleus.
Gene transfer to the nucleus is coupled with the loss of the genetic autonomy of the organelle. However, many of the proteins coded by the exported genes are still produced by the nucleus and transported back to the organelle. This is possible as the genes are modified to be compatible with nuclear transcriptional and translational machinery and undergo changes such as the addition of a promoter and a terminator. A targeting sequence is also added, so the resulting proteins get delivered to the specific organelle. This also enables the nucleus to control the supply of these proteins and regulate the biogenesis of the organelles. Sometimes, such exported genes evolve and perform new functions for the organelles other than their parent one. For example, almost 50% of plastid-derived genes in Arabidopsis thaliana carry out non-plastid functions.
There are several theories as to why organisms transfer genes from the organelles to the nucleus. Both mitochondria and chloroplasts generate free radicals which can cause harmful mutations in their DNA. Transfering vulnerable organellar genes to the nucleus may be one of the strategies to protect them from mutations. According to the genetic principle Muller’s ratchet, asexual reproduction leads to the accumulations of deleterious mutations which eventually can cause the extinction of the species. However once transferred to the sexual genome of the nucleus, the exported gene can undergo sexual recombination which helps it to prevent the accumulation of harmful mutations.
Suggested Reading
- Martin, William, and Reinhold G. Herrmann. "Gene transfer from organelles to the nucleus: how much, what happens, and why?." Plant Physiology 118, no. 1 (1998): 9-17.
- Cullis, Christoper Ashley, Barend Juan Vorster, Christell Van Der Vyver, and Karl J. Kunert. "Transfer of genetic material between the chloroplast and nucleus: how is it related to stress in plants?" Annals of Botany 103, no. 4 (2008): 625-633.
- Ku, Chuan, Shijulal Nelson-Sathi, Mayo Roettger, Sriram Garg, Einat Hazkani-Covo, and William F. Martin. "Endosymbiotic gene transfer from prokaryotic pangenomes: Inherited chimerism in eukaryotes." Proceedings of the National Academy of Sciences 112, no. 33 (2015): 10139-10146.