The photosynthetic apparatus is composed of proteins encoded by genes from both the nucleus and the chloroplast. To ensure that the photosynthetic complexes are assembled stoichiometrically and to enable their rapid reorganization in response to a changing environment, the plastids emit signals that regulate nuclear gene expression to match the status of the plastids. One of the plastid signals, the chlorophyll intermediate Mg-ProtoporphyrinIX (Mg-ProtoIX) accumulates under stress conditions and acts as a negative regulator of photosynthetic gene expression. By taking advantage of the photoreactive property of tetrapyrroles, Mg-ProtoIX could be visualized in the cells using confocal laser scanning spectroscopy. Our results demonstrate that Mg-ProtoIX accumulated both in the chloroplast and in the cytosol during stress conditions. Thus, the signaling metabolite is exported from the chloroplast, transmitting the plastid signal to the cytosol. Our results from the Mg-ProtoIX over-and underaccumulating mutants copper response defect and genome uncoupled5, respectively, demonstrate that the expression of both nuclear-and plastid-encoded photosynthesis genes is regulated by the accumulation of Mg-ProtoIX. Thus, stress-induced accumulation of the signaling metabolite Mg-ProtoIX coordinates nuclear and plastidic photosynthetic gene expression.

The biogenesis of photosynthetic complexes is assisted by a growing number of trans-acting factors in both chloroplasts and cyanobacteria. We have previously shown that the periplasmic PratA factor from Synechocystis sp. PCC 6803 (Synechocystis 6803) is required for adequate C-terminal processing of the PsbA (D1) subunit of photosystem II (PSII) supporting the idea that the early steps of PSII assembly occur at the plasma membrane. Here we report on the molecular analysis of the interaction between PratA and the D1 protein. Both yeast two-hybrid and glutathione S-transferase pulldown assays revealed that PratA binds to the soluble forms of both mature and precursor D1 C-terminal regions. In agreement with that finding, the binding region was mapped to amino acid positions 314 -328 of D1 by applying a peptide-scanning approach. Approximately 10 -20% of the soluble PratA factor was found to be associated with membranes in a D1-dependent manner. Sucrose density gradient centrifugations allowed the identification of a specific membrane subfraction that contains both PratA and D1 and which might represent a transfer and/or connecting region between plasma and thylakoid membrane. Imaging data obtained with enhanced cyan fluorescent protein-labeled D1 protein in wild-type and pratA mutant backgrounds further supported this notion. Photosystem II (PSII)2 mediates water oxidation and initiates electron flow during light reactions of photosynthesis in cyanobacteria and in chloroplasts of eukaryotes. Cyanobacterial PSII has been characterized in great detail at the structural level (1-4). However, much less is known about the stepwise biogenesis of this molecular machine, which contains at least 19 protein subunits as well as a complex set of cofactors (5-7). In the prokaryotic model organism Synechocystis PCC sp. 6803 (hereafter Synechocystis 6803), several proteins have recently been identified that associate transiently with PSII, and inactivation of these leads to severe defects in PSII activity. Such proteins represent good candidates for putative assembly factors.The protein Slr1471p (OXA1/Alb3/YidC homologous), for instance, has been shown to interact directly with the D1 protein during integration of the latter into the thylakoid membrane (8). The lumenal factor YCF48 (HCF136) is required for both efficient assembly and repair of PSII (9, 10), and the 11-kDa lipoprotein Psb27 has been implicated in facilitating assembly of the manganese cluster of PSII (11, 12). Finally, the 22-kDa Psb29 protein is apparently necessary for accurate biogenesis of the inner antennae of PSII (13).We have previously shown that the tetratricopeptide repeat protein PratA is involved in the biogenesis of PSII: loss of PratA function affects the maturation of the reactioncenter protein D1 (14). In almost all photoautotrophic organisms, D1 is synthesized as a precursor with a C-terminal extension that must be removed to allow assembly of the manganese cluster at the lumenal side of PSII. In Synechocystis 6803, this extension consists of 16 ...

All members of the YidC/Oxa1/Alb3 protein family are evolutionarily conserved and appear to function in membrane protein integration and protein complex stabilization. Here, we report on a second thylakoidal isoform of Alb3, named Alb4. Analysis of Arabidopsis knockout mutant lines shows that Alb4 is required in assembly and/or stability of the CF1CF0-ATP synthase (ATPase). alb4 mutant lines not only have reduced steady-state levels of ATPase subunits, but also their assembly into high-molecular-mass complexes is altered, leading to a reduction of ATP synthesis in the mutants. Moreover, we show that Alb4 but not Alb3 physically interacts with the subunits CF1beta and CF0II. Summarizing, the data indicate that Alb4 functions to stabilize or promote assembly of CF1 during its attachment to the membrane-embedded CF0 part.

Current molecular analyses suggest that initial steps of the biogenesis of cyanobacterial photosystems progress in a membrane subfraction representing a biosynthetic center with contact to both plasma and thylakoid membranes. This special membrane fraction is defined by the presence of the photosystem II assembly factor PratA. The proposed model suggests that both biogenesis of protein complexes and insertion of chlorophyll molecules into the photosystems occur in this intermediate membrane system.