Ammonium is one of the major nutrients for plants, and a ubiquitous intermediate in plant metabolism, but it is also known to be toxic to many organisms, in particular to plants and oxygenic photosynthetic microorganisms. Ammonium directly accelerates photo damage of PSII in Synechocystis sp. strain PCC6803, rather than affecting the repair of photo damaged PSII. Damage of PSII is triggered by a combined light and ammonium-induced destruction of the oxygen-evolving complex.
Wild-type Synechocystis cells can tolerate relatively high concentrations of ammonium because of efficient OSII repair, whereas mutants in PSII repair are highly sensitive towards ammonium . Ammonium tolerance requires all three psbA genes since mutants of any of the three single pasta genes are more sensitive to ammonium than wild-type cells. Even the poorly expressed psbA1 gene confers elevated tolerate towards ammonium .
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