All eukaryotic organisms face the challenge to coordinate the activity of different genomes and to ensure the stochiometric assembly of multiprotein complexes whose subunits have their genetic orginin in the nucleus, the mitochondria or, in the case of plants and algae, in the chloroplasts. This thesis work bears on one aspect of the coordination between genomes, namely how the nucleus influences gene expression in the chloroplast in the model organism Chlamydomonas reinhardtti. Many nucleus-encoded proteins have been identified with crucial functions in chloroplast biogenesis and I have concentrated here on two proteins, Tab2 and Mab1, which turn out to be both required for the synthesis of PsaB and therefore for the generation of the Photosystem I.