The Role of Proteases in Regulating Gene Expression and Assembly Processes in the Chloroplast

Chloroplasts house many biochemical processes that are essential for plant viability. Foremost, among these is photosynthesis, which requires the protein-rich thylakoid membrane system. The activation of chloroplast genes encoding thylakoid membrane proteins and the targeting and assembly of these proteins together with their nuclear-encoded partners are essential for the elaboration of the thylakoid membrane. Several nuclear-encoded proteins that regulate chloroplast gene expression and that mediate the targeting of proteins to the thylakoid membrane have been identified in recent years, and many more remain to be discovered. The abundance of such proteins is critical and is likely to be determined to a significant extent by their stability, which in turn, is influenced by chloroplast protease activities. The primary goal of this project was to link specific proteases to specific substrates, and in particular, to specific regulatory and assembly proteins. We proposed a two-pronged approach, involving genetic analysis of the consequences of the mutational loss of chloroplast proteases, and biochemical analysis of the degradation pathways of specific proteins that have been shown to control chloroplast gene expression. Our initial bioinformatic analysis of chloroplast proteases allowed us to identify the set of pro teases that is targeted to the chloroplast. We used that information to recover three Arabidopsis mutants with T - DNA insertions in specific chloroplast protease genes. We carried out the first analysis of the stability of a regulator of chloroplast gene expression (CRS2), and found that the protein is much less stable than are typical components of the photosynthetic apparatus. Genetic reagents and analytical methods were developed that have set the stage for a rapid advancement of our understanding of chloroplast proteolysis. The results obtained may be useful for manipulating the expression of transgenes in the chloroplast and for engineering plants whose photosynthetic activity is optimized under harsh environmental conditions.