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Cyanobacteria form a very important bacterial phylum that emerged at the dawn of life on Earth. These organisms evolved an efficient photoautotrophic metabolism based on oxidation of water molecules and the reduction of CO2 to organic matter using energy of photons. This unique process involves an endogenous thylakoid membrane system and requires two large chlorophyll-protein assemblies called photosystem I and II (PSI and PSII). In the last decades an amazing amount of progress has been achieved in understanding the structure of photosystems, as well as the mechanisms by which they are assembled from individual components. In contrast, we have very limited knowledge regarding the membrane compartment(s) in which the core photosystem subunits are synthesized and assembled into mature photosynthetic complexes. It appears that synthesis and early stages of photosystem biogenesis are inherently related to biogenesis of the whole thylakoid membrane system, and there is an urgent need to localize these processes in the cell.
The unicellular cyanobacterium Synechocystis PCC 6308 provides a unique combination of molecular genetics and physiological characteristics, and is frequently used as a tool for photosynthetic studies. Recently, a distinct region called PratA-defined membrane (PDM) has been identified in Synechocystis in the vicinity of the plasma membrane, and appears to be a likely site for both commencement of PSII biogenesis, and the terminal steps of chlorophyll biosynthesis.
At this time, available data regarding PDM are inconclusive, however in an attempt to clarify this, we have commenced localizing the proteins critical for synthesis of chlorophyll-binding PSII subunits such as YidC insertase, factors like Sll0933 and Ycf48 known to assist in PSII biogenesis, and enzymes related to chlorophyll biosynthesis. Additionally, we are trying to trace the steps of thylakoid membrane biogenesis, by analyzing the ‘greening’ of the nitrogen depleted/repleted cells.
Employing various Synechocystis mutants as controls, we prepared Tokuyasu cryo-sections and localized selected proteins by specific antibodies. To date, our data has not highlighted any specific membrane region where biogenesis could occur, as the analyzed proteins are quite regularly scattered thru the thylakoid membranes. We have however observed an intriguing membrane structure, including putative vesicles protruding from the plasma membrane. Our data contradict the current model based on PDM, and indicate both that the biogenesis of thylakoid membranes could involve vesicular transport, and that the synthesis of photosystems might be located in hypothetical biosynthetic rafts in thylakoids.
