Transport of Metals: A Key Process in Oxygenic Photosynthesis

Abstract

Metals play important roles in all phases of oxygenic photosynthesis in cyanobacteria, eukaryotic algae and green plants. For the photosynthetic electron transport reactions in the thylakoid membranes, iron, copper, manganese and magnesium are essential cofactors in various proteins and pigment-protein complexes. Zinc, iron and magnesium also play critical roles during the carbon-fixation reactions. In addition, iron, copper and zinc are constituents of superoxide dismutase and other protective enzymes that are essential to maintain the integrity and function of the photosynthetic apparatus in its highly reactive environment. Inside any living cell, concentrations of various metals are maintained within specific ranges. If the concentration of any metal is below a lower threshold level, organisms suffer from this metal ion deficiency. On the other hand, excess amount of many metals can be toxic. Since metals are both essential and potentially toxic, they are under strict homeostatic control that requires a balance between their uptake and efflux. Usually, metals are transported across cell and organellar membranes via specific transporters. Many families of metal uptake and efflux transporters have recently been described in both prokaryotes and eukaryotes. In 1996, the publication of the complete genome sequence of the cyanobacterium Synechocystis sp. PCC 6803, an oxygenic photosynthetic organism, provided valuable information about a number of potential metal transporters in this organism. During the past few years, genetic and biochemical dissection of some of these transporters has yielded important functional data about both the transport processes and their regulations, for a number of metals. Recent completion of the genome sequence of the Arabidopsis thaliana has opened additional exciting opportunities for functional genomic analysis of metal homeostasis and its influence on photosynthesis. In this chapter, we discuss transport of iron, copper, manganese, zinc and magnesium, primarily using the Synechocystis 6803 paradigm.