The Synechocystis Manganese Exporter Mnx Is Essential for Manganese Homeostasis in Cyanobacteria

The essential micronutrient manganese (Mn) functions as redox-active cofactor in active sites of enzymes and, thus, is involved in various physiological reactions. Moreover, in oxygenic photosynthetic organisms, Mn is of special importance, since it is central to the oxygen-evolving complex in photosystem II. Although Mn is an essential micronutrient, increased amounts are detrimental to the organism; thus, only a small window exists for beneficial concentrations. Accordingly, Mn homeostasis must be carefully maintained. In contrast to the well-studied uptake mechanisms in cyanobacteria, it is largely unknown how Mn is distributed to the different compartments inside the cell. We identified a protein with so far unknown function as a hypothetical Mn transporter in the cyanobacterial model strain Synechocystis sp. PCC 6803 and named this protein Mnx for Mn exporter. The knockout mutant Delta mnx showed increased sensitivity toward externally supplied Mn and Mn toxicity symptoms, which could be linked to intracellular Mn accumulation. Mn-54 chase experiments demonstrated that the mutant was not able to release Mn from the internal pool. Microscopic analysis of a Mnx:: yellow fluorescent protein fusion showed that the protein resides in the thylakoid membrane. Heterologous expression of mnx suppressed the Mn-sensitive phenotype of the Saccharomyces cerevisiae mutant Delta pmr1. Our results indicate that Mnx functions as a thylakoid Mn transporter and is a key player in maintaining Mn homeostasis in Synechocystis sp. PCC 6803. We propose that Mn export from the cytoplasm into the thylakoid lumen is crucial to prevent toxic cytoplasmic Mn accumulation and to ensure Mn provision to photosystem II.