Expression of the isiA gene is essential for the survival of the cyanobacterium Synechococcus sp. PCC 7942 by protecting photosystem II from excess light under iron limitation

Iron deficiency is known to suppress primary productivity in both marine an d freshwater ecosystems. In response to iron deficiency, certain cyanobacte ria induce a chlorophyll (Chl)-protein complex, CP43 ', which is encoded by the isiA gene. The deduced amino-acid sequence of CP43 ' predicts some str uctural similarity to the CP43 polypeptide of photosystem II, but the funct ion of CP43 ' remains uncertain. In order to assess its physiological role, the isiA gene of a cyanobacterium, Synechococcus sp. PCC7942, was inactiva ted by insertion mutagenesis (giving isiA(-) cells). Compared with isiA- ce lls, under iron deprivation, wild-type cells showed both lower rates of pho tosystem Ii-mediated O-2 evolution at limiting light irradiances and decrea sed yields of room temperature Chi fluorescence at various irradiances. The se 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 o n these results, we suggest that CP43' functions as a non-radiative dissipa tor of light energy, thus protecting photosystem II from excessive excitati on under iron-deficient conditions.