The role of plastid-encoded RNA polymerase-associated proteins (PAPs) in chloroplast biogenesis of plants

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Chloroplasts are the specific cell organelles of plants which perform the process of photosynthesis. They represent the structural and functional base for the energetic and metabolic support of the terrestrial biosphere including mankind. Without chloroplasts life of higher order as we know it today would be impossible. Despite this eminent role of these organelles for life on Earth understanding of essential molecular regulatory steps of their biogenesis is still elusive. Chloroplasts typically emerge from undifferentiated precursors, the proplastids, or from arrested developmental stages, the etioplasts. These differentiation processes are strictly light-dependent as becoming evident from a dark-grown seedling exposed to light. Within a few hours the yellow, chlorophyll-free cotyledons become green and develop photosynthetically active chloroplasts. This requires active and rapid regulation of thousands of genes that encode structural and functional components of chloroplasts. Most of these genes are encoded in the cell nucleus and their gene products need to be imported into the plastids after cytosolic translation. However, a small set of genes is encoded in the plastid-own genome, the plastome, and need to be expressed by the organelles themselves. Therefore, for a successful biogenesis of chloroplasts a tight coordination of the nuclear and plastid genomes is mandatory. An important key role in this coordination plays the plastid encoded RNA polymerase (PEP). This enzyme transcribes the genes of the plastome together with another nuclear encoded RNA polymerase (NEP). The PEP is comprised of four core subunits that are highly similar to those of prokaryotic RNA polymerases representing a remnant of the endosymbiotic ancestry of plastids. All four subunits are encoded in the plastome and are expressed exclusively by the NEP enzyme. Biochemical and molecular-genetic experiments have subsequently shown, that the PEP core complex is further expanded by the physical addition of 12 nuclear-encoded subunits during the initial steps of chloroplast biogenesis. These additional subunits were named PEP-associated proteins (PAPs) and they are of eminent importance since lack of any of them blocks chloroplast development ending finally in an albino phenotype of the corresponding mutant. PAPs, thus, represent apparently key components of chloroplast biogenesis. In this project we aim to unravel their specific role during chloroplast biogenesis. We emphasize to study their involvement in the phytochrome-mediated light signalling network, their expression regulation as well a their time-dependent assembly into the PEP complex. Special focus will be given to their potential function as retrograde signals from plastids towards the cell nucleus, a fascinating new idea for this still not understood signalling route that potentially allows plastids to control their own biogenesis.