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An alternative methionine aminopeptidase, MAP-2, is required for nitrogen starvation and high-light acclimation in the cyanobacterium Synechocystis sp. PCC 6803

¹ Institut für Mikrobiologie und Molekularbiologie, Universität Giessen, Germany;
² Institut für Biologie, Humboldt-Universität zu Berlin, 10115 Berlin;
³ Universität Tübingen

ABSTRACT

Methionine aminopeptidases (MetAPs or MAPs, encoded by map genes) are ubiquitous and pivotal enzymes for protein maturation in all living organisms. Whereas most bacteria harbour only one map gene, many cyanobacterial genomes contain two map paralogues, the genome of Synechocystis sp. PCC 6803 even three. The physiological function of multiple map paralogues remains elusive so far. This communication reports for the first time differential MetAP function in a cyanobacterium. In Synechocystis sp. PCC 6803, the universally conserved map-3 gene (sll0555) is predominantly expressed in exponentially growing cells and appears to be housekeeping. By contrast, expression of map-2 (slr0918) and map-1 (slr0786) genes increases during stress conditions. The map-1 paralogue is only transiently expressed, whereas the widely distributed map-2 gene appears to be the major MetAP during stress conditions. A map-2 deficient Synechocystis mutant shows a subtle impairment of photosystem II properties already under non-stressed conditions. In particular, the binding site for the quinone QB is affected, indicating specific N-terminal methionine processing requirements of photosystem II components. MAP-2 specific processing becomes essential under certain stress conditions, since the map-2 deficient mutant is severely impaired in surviving conditions of prolonged nitrogen starvation and high light exposure.

⁴ E-mail: karl.forchhammer{at}uni-tuebingen.de