Zinc Starvation Response in a Cyanobacterium Revealed

The cyanobacterium Anabaena needs a variety of metals for its cellular biochemistry. The general problem is to supply all the different metals to the right enzymes, thereby avoiding toxic side reactions and interference between these metals. Anabaena has to deal with surplus metals but also with metal starvation, and this problem has to be solved for many environments, each with a different availability of metal cations. In this issue, Napolitano et al. (34) report a study in which they unraveled the zinc starvation response in Anabaena. Zinc is an important essential trace element in all organisms. More than 100 zinc-dependent or -binding proteins are known in Escherichia coli (39), e.g., a fructose-bisphosphate aldolase, DNA primase, carbonic anhydrase, alkaline phosphatase, and RNA polymerase. In mammals, including humans, zinc is required for proper brain function and for insulin and semen release and acts as cellular signal (30). The story told by Napolitano et al. (34) is wonderful to read. They identified the main transcriptional regulator to cope with zinc deficiency in Anabaena, Zur, and dozens of genes under Zur control. Most interesting among those were putative zinc-binding metallochaperones, other regulatory proteins, and, surprisingly, TonB-dependent outer membrane proteins, which may be involved in active transport of zinc or zinc chelates across the outer membrane. Anabaena might even synthesize and excrete a zinc chelator (a “zincophore”? ), reminiscent of siderophores for iron or chalkophores for copper acquisition. To place this work (34) in a proper context, it is important to reiterate what is known about Anabaena, how this bacterium grows, why it needs metal cations, how zinc homeostasis in the context of general metal homeostasis might function, and what is new in the report by Napolitano et al. (34) (Fig. 1). Anabaena sp. PCC 7120 is a cyanobacterium. Without these organisms, life on this planet would be different. They are a major phylum of the superkingdom Bacteria, and their ecological niche is a physiological one, namely, oxygenic photosynthesis. Cyanobacteria are the only organisms able to perform this reaction, either as free-living organisms or as plastid endosymbionts (or slaves? ) of eukaryotic cells. The product of oxygenic photosynthesis is molecular oxygen. With a half-cell potential (Eo=) of 816 mV (61), transfer of electrons from NADH (Eo= 320 mV) releases about 238 kJ/mol under standard conditions (molar concentrations; pH 7), enough to conserve 3 ATP per electron pair transferred. No other respiratory electron acceptor allows such a high energy conservation by electron transfer-dependent phosphorylation. Production of molecular oxygen by cyanobacteria led to two major oxygenation events about 2.4 billion and 700 million years ago, which may have sparked evolution of eukaryotes and multicellular organisms, respectively (22, 25, 45). Thus, cyanobacteria have efficiently changed the biogeochemistry of our planet. Anabaena needs transition metals to perform oxygenic photosynthesis and for nitrogen fixation. Oxygenic photosynthesis occurs in thylakoids, a specialized internal membrane system that contains photosystems I and II. Both contain chlorophyll molecules, which harbor a Mg at their center. A Mn-containing water-splitting complex attached to photosystem II replenishes electrons that are transferred after light absorption from photosystem II to a plastoquinone. The water-splitting complex contains at its core a Mn4CaO5 cluster with two chloride anions in its vicinity (59), oxidizes water to molecular oxygen, and donates the resulting electrons one by one to the center of photosystem II. In the subsequent steps, electrons are transferred from the plastoquinone to an iron-containing cytochrome b6f complex, via the copper-dependent soluble protein plastocyanin to the iron-containing photosystem I, and from here, after being pushed by another photon, to NADP . This process also conserves energy via the proton motive force and the F1Fo ATPase. Finally, NADPH is used as a redox donor to assimilate CO2 via the Calvin cycle. Oxygenic photosynthesis, therefore, needs the transition metals manganese, iron, and copper, the earth alkali metals magnesium and calcium, and the halogen chlorine. The nitrogen-fixing nitrogenase complex contains iron and molybdenum. Anabaena sp. strain PCC 7120 may also be able to synthesize an alternative vanadium-iron-dependent nitrogenase (NifH2, all1455) and a vanadium-dependent chloroperoxidase (alr0672). Cobalt is part of the component cobalamin, and nickel is part of urease and of hydrogenase, an enzyme that reassimilates molecular hydrogen produced by the nitrogenases among other physiological functions. If we move along the periodic table of the elements in the first transition period from left to right, Anabaena has use for V, Mo of the second transition period (chromium of the first period is as chromate a sulfate antagonist and very toxic), Mn, Fe, Co, Ni, Cu, and finally Zn. Thus, Anabaena has an impressive need for transition and other metals and, like other organisms, must solve the problem of getting the correct metal to the right protein. In general, about 40% of all enzymes need metals as cofactors, ranking from Mg (16%) Zn (9%) Fe (8%) Mn (6%) Ca (2%) Co and Cu (1%) down to K, Na, Ni, V, Mo, W, and in one case Cd (60). The problem of metal allocation is especially difficult for transition metal cations because, at first glance, those of the first transition group (Mn, Fe, Co, Ni, Cu, and Zn) have similar ionic radii (Fig. 2). In the cell, these metal cations interfere with each other along the Irving-Williams series or “pecking order” (21), which has copper on top and manganese, essential for production of molecular oxygen, at the bottom as far as transition