The unicellular green alga Chlamydomonas reinhardtii as an experimental system to study chloroplast RNA metabolism

Chloroplasts are typical organelles of photoautotrophic eukaryotic cells wh ich drive a variety of functions, including photosynthesis. For many years the unicellular green alga Chlamydomonas reinhardtii has served as an exper imental organism for studying photosynthetic processes. The recent developm ent of molecular tools for this organism together with efficient methods of genetic analysis and the availability of many photosynthesis mutants has n ow made this alga a powerful model system for the analysis of chloroplast b iogenesis. For example, techniques have been developed to transfer recombin ant DNA into both the nuclear and the chloroplast genome. This allows both complementation tests and analyses of gene functions in vivo. Moreover, sit e-specific DNA recombinations in the chloroplast allow targeted gene disrup tion experiments which enable a "reverse genetics" to be performed. The pot ential of the algal system for the study of chloroplast biogenesis is illus trated in this review by the description of regulatory systems of gene expr ession involved in organelle biogenesis. One example concerns the regulatio n of trans-splicing of chloroplast mRNAs, a process which is controlled by both multiple nuclear- and chloroplast-encoded factors. The second example involves the stabilization of chloroplast mRNAs. The available data lead us predict distinct RNA elements, which interact with trans-acting factors to protect the RNA against nucleolytic attacks.