Mitochondria ribosomal protein genes in many eukaryotic species have been transferred from the mitochondria to nuclear genome

Abstract

Mitochondria of eukaryotic cells are derived from ancient bacterial cells, and currently adopts an endosymbiotic relationship within the host cells. Sequencing and examination of the mitochondrial genome reveals that it is relatively gene sparse, possibly with many genes either transferred to the nuclear genome or lost through redundancy. Work examining the complement of genes in mitochondrial genomes of many common eukaryotic model organisms revealed that mitochondrial ribosomal protein genes are no longer encoded by the mitochondrial genome. Specifically, this work reveals that apart from Arabidopsis thaliana and Oryza sativa, mitochondrial ribosomal protein genes are no longer encoded by the mitochondrial genome. Species without ribosomal protein genes in their mitochondrial genome are: Caenorhabditis elegans, Danio rerio, Drosophila melanogaster, Homo sapiens, and Mus musculus. Such revelations hold important implications for our understanding of essentiality of encoding ribosomal protein genes in the mitochondrial genome. The apparent answer is no, with the mitochondrial genome of A. thaliana, and O. sativa useful for studying the processes involved in progressive loss of mitochondrial ribosomal protein genes to the nuclear genome. At the organelle level, lack of mitochondrial ribosomal protein genes in the mitochondrial genome meant that ribosomal proteins could be efficiently synthesized in the cytoplasm and transported back to the mitochondria. But, more importantly, revelations in this work also points to the alternative scenario that the mitochondrial ribosome no longer plays an important physiological role with the remaining mitochondrial ribosomal proteins serving as vestiges of evolution neglect and redundancy. Overall, many eukaryotic species no longer encode mitochondrial ribosomal protein genes in their mitochondrial genome. This meant that mitochondrial ribosomal proteins could be efficiently transported back to the mitochondria or that the mitochondrial ribosome is no longer needed. In effect, contemporary sequencing studies of the mitochondria of different model eukaryotic species may be witnessing the progressive loss of mitochondrial ribosomal protein genes through the effects of evolutionary selection pressure.