Under conditions of iron deficiency certain cyanobacteria induce a chlorophyll (Chl)-binding protein, CP43′, which is encoded by the isiA gene. We have previously suggested that CP43′ functions as a nonradiative dissipator of light energy. To further substantiate its functional role an isiA overexpression construct was introduced into the genome of a cyanobacterium Synechococcus sp. PCC 7942 (giving isiA^oe cells). The presence of functional CP43′ in isiA^oe cells was confirmed by Western blot as well as by the presence of a characteristic blueshift of the red Chl a absorption peak and a notable increase in the 77 K fluorescence peak at 685 nm. Compared to wild-type cells isiA^oe cells, with induced CP43′, had both smaller functional antenna size and decreased yields of room temperature Chl fluorescence at various light irradiances. These observations strongly suggest that isiA^oe cells, with induced CP43′, have an increased capacity for dissipating light energy as heat. In agreement with this hypothesis isiA^oe cells were also more resistant to photoinhibition of photosynthesis than wild-type cells. Based on these results we have further strengthened the hypothesis that CP43′ functions as a nonradiative dissipator of light energy, thus protecting photosystem II from excessive excitation under iron-deficient conditions.