During evolution, plant mitochondrial genomes underwent multiple rearrangements leading to the loss of genetic information. One of the consequences is that the number of mitochondrial encoded tRNA genes is not sufficient to ensure plant mitochondrial translation. In order to compensate this deficiency, cytosolic tRNAs have to be imported into mitochondria. Although this phenomenon is an essential process for mitochondrial biogenesis in most eukaryotic cells, it remains poorly understood. The unicellular green alga Chlamydomonas represents an attractive model organism for the study of this mechanism. There is a high and selective import of cytosolic tRNA into mitochondria and the manipulation of the nuclear and mitochondrial genomes is possible. One of the goals of the project pertains the comprehension of the regulation of this process inside mitochondria. For this, we will manipulate the mitochondrial genome of Chlamydomonas reinhardtii in order to measure the effect on the balance of imported and non-imported tRNAs. A second major aim is to determine the role of the components of the tRNA import machinery in C. reinhardtii and thus we plan to selectively inactivate the expression of the corresponding genes by RNA silencing.