Partial suppression of Oxa1 mutants by mitochondria-targeted signal recognition particle provides insights into the evolution of the cotranslational insertion systems

Biolistic Transformation of Chlamydomonas reinhardtii and Saccharomyces cerevisiae Mitochondria

Chlamydomonas reinhardtii and Saccharomyces cerevisiae are currently the two micro-organisms in which genetic transformation of mitochondria is routinely performed. The generation of a large variety of defined alterations as well as the insertion of ectopic genes in the mitochondrial genome (mtDNA) are possible, especially in yeast. Biolistic transformation of mitochondria is achieved through the bombardment of microprojectiles coated with DNA, which can be incorporated into mtDNA thanks to the highly efficient homologous recombination machinery present in S. cerevisiae and C. reinhardtii organelles. Despite a low frequency of transformation, the isolation of transformants in yeast is relatively quick and easy, since several natural or artificial selectable markers are available, while the selection in C. reinhardtii remains long and awaits new markers. Here, we describe the materials and techniques used to perform biolistic transformation, in order to mutagenize endogenous mitochondrial genes or insert novel markers into mtDNA. Although alternative strategies to edit mtDNA are being set up, so far, insertion of ectopic genes relies on the biolistic transformation techniques.

Vestiges of the Bacterial Signal Recognition Particle-Based Protein Targeting in Mitochondria

The main bacterial pathway for inserting proteins into the plasma membrane relies on the signal recognition particle (SRP), composed of the Ffh protein and an associated RNA component, and the SRP-docking protein FtsY. Eukaryotes use an equivalent system of archaeal origin to deliver proteins into the endoplasmic reticulum, whereas a bacteria-derived SRP and FtsY function in the plastid. Here we report on the presence of homologs of the bacterial Ffh and FtsY proteins in various unrelated plastid-lacking unicellular eukaryotes, namely Heterolobosea, Alveida, Goniomonas, and Hemimastigophora. The monophyly of novel eukaryotic Ffh and FtsY groups, predicted mitochondrial localization experimentally confirmed for Naegleria gruberi, and a strong alphaproteobacterial affinity of the Ffh group, collectively suggest that they constitute parts of an ancestral mitochondrial signal peptide-based protein-targeting system inherited from the last eukaryotic common ancestor, but lost from the majority of extant eukaryotes. The ability of putative signal peptides, predicted in a subset of mitochondrial-encoded N. gruberi proteins, to target a reporter fluorescent protein into the endoplasmic reticulum of Trypanosoma brucei, likely through their interaction with the cytosolic SRP, provided further support for this notion. We also illustrate that known mitochondrial ribosome-interacting proteins implicated in membrane protein targeting in opisthokonts (Mba1, Mdm38, and Mrx15) are broadly conserved in eukaryotes and nonredundant with the mitochondrial SRP system. Finally, we identified a novel mitochondrial protein (MAP67) present in diverse eukaryotes and related to the signal peptide-binding domain of Ffh, which may well be a hitherto unrecognized component of the mitochondrial membrane protein-targeting machinery.