CP43 ', the isiA gene product, functions as an excitation energy dissipator in the cyanobacterium Synechococcus sp PCC 7942

Under conditions of iron deficiency certain cyanobacteria induce a chloroph yll (Chl)-binding protein, CP43 ', which is encoded by the isiA gene. We ha ve previously suggested that CP43' functions as a nonradiative dissipator o f light energy. To further substantiate its functional role an isiA overexp ression construct was introduced into the genome of a cyanobacterium Synech ococcus sp. PCC 7942 (giving isiA(oe) cells). The presence of functional CP 43' in isiA(oe) cells was confirmed by Western blot as well as by the prese nce of a characteristic blueshift of the red Chi a absorption peak and a no table increase in the 77 K fluorescence peak at 685 nm. Compared to wildtyp e cells isiA(oe) cells, with induced CP43', had both smaller functional ant enna size and decreased yields of room temperature Chi fluorescence at vari ous light irradiances. These observations strongly suggest that isiA(oe) ce lls, with induced CP43', have an increased capacity for dissipating light e nergy as heat. In agreement with this hypothesis isiA(oe) cells were also m ore resistant to photoinhibition of photosynthesis than wild-type cells. Ba sed on these results we have further strengthened the hypothesis that CP43 ' functions as a nonradiative dissipator of light energy, thus protecting p hotosystem II from excessive excitation under iron-deficient conditions.