The photosynthetic complexes have a dual genetic origin as their subunits are encoded both by the chloroplast and nuclear genomes. Their assembly thus depends on a concerted interplay between these two genetic systems. Genetic analysis of the biogenesis of the photosynthetic apparatus in the green alga C. reinhardtii and the land plants Arabidopsis and maize revealed a large number of nucleus-encoded factors which act as chloroplast trans-acting activators of chloroplast gene expression [7]. They are mostly involved in different posttranscriptional steps of chloroplast gene expression including RNA processing, RNA stability, splicing and translation. A remarkable feature is that several of these factors act in a gene-specific manner indicating that altogether hundreds of factors of this sort must be acting in the plastids as activators of gene expression. Another important point is that many of these factors interact specifically with the 5′untranslated region (5′UTR) of their plastid target gene. It is therefore possible to make the expression of a reporter gene dependent on one of these factors by fusing the reporter gene to the 5′UTR of the corresponding target gene (Fig. 1). We have exploited these unique features of plastid gene expression for developing a robust inducible and repressible chloroplast gene expression system in C. reinhardtii [8,9]. This system opens new interesting possibilities for the study of both basic and applied aspects of chloroplast biology and photosynthesis.