It is widely accepted that the energy-producing organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from once free-living ancestors. In the course of evolution, the vast majority of genes have been relocated to the nucleus. If the function of the proteins encoded by these genes is still important in today’s world, they are imported into the organelles after being translated on cytosolic (eukaryotic) ribosomes. But, why are some genes kept in the miniature genomes of mitochondria and chloroplasts? It has been proposed that genes are retained in organelles because their codon usage or base composition might prevent the efficient expression of organellar genes in the nucleus, because some proteins might be difficult to import across the double organelle membranes, or because their expression must be tightly coupled to the redox state of the respective electron transport chains.
In our Opinion paper in Trends in Plant Science (2024), we propose that mRNA targeting is a previously unappreciated mechanism to explain gene retention in chloroplasts. In brief, we propose that mRNAs encoding proteins central to photosynthesis have to interact with the thylakoid membrane systems and that this is critical for localizing the encoded proteins to the membrane, and coordinating early steps of complex assembly.
We developed this idea, based on recent experimental data, together with Conrad Mullineaux (London) and Annegret Wilde (Freiburg).