Expression of the isiA gene is essential for the survival of the cyanobacterium Synechococcus sp PCC7942 by protecting photosystem II from excess light under iron limitation

• Iron deficiency is known to suppress primary productivity in both marine and freshwater ecosystems. In response to iron deficiency, certain cyanobacteria induce a chlorophyll (Chl)-protein complex, CP43 ', which is encoded by the isiA gene. The deduced amino-acid sequence of CP43 ' predicts some structural similarity to the CP43 polypeptide of photosystem II, but the function of CP43 ' remains uncertain. In order to assess its physiological role, the isiA gene of a cyanobacterium, Synechococcus sp. PCC7942, was inactivated by insertion mutagenesis (giving isiA(-) cells). Compared with isiA- cells, under iron deprivation, wild-type cells showed both lower rates of photosystem Ii-mediated O-2 evolution at limiting light irradiances and decreased yields of room temperature Chi fluorescence at various irradiances. These observations strongly suggest that the decreased photosystem II activity in wild-type cells with CP43 ' is attributable to increased non-radiative dissipation of light energy. In agreement with this hypothesis, isiA- cells were more susceptible to photoinhibition of photosynthesis than wild-type cells, resulting in much slower growth rates under iron limitation. Based on these results, we suggest that CP43' functions as a non-radiative dissipator of light energy, thus protecting photosystem II from excessive excitation under iron-deficient conditions.

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