FUNCTIONAL-ANALYSIS OF THE IRON-STRESS INDUCED CP-43'-POLYPEPTIDE OF PS-II IN THE CYANOBACTERIUM SYNECHOCOCCUS SP PCC-7942

Under conditions of iron-stress, the Photosystem II associated chlorop hyll a protein complex designated CP 43', which is encoded by the isiA gene, becomes the major pigment-protein complex in Synechococcus sp. PCC 7942. The isiB gene, which is located immediately downstream of is iA, encodes the protein flavodoxin, which can functionally replace fer redoxin under conditions of iron stress. We have constructed two cyano bacterial insertion mutants which are lacking (i) the CP 43' apoprotei n (designated isiA(-)) and (ii) flavodoxin (designated isiB(-)). The f unction of CP 43' was studied by comparing the cell characteristics, P S II functional absorption cross-sections and Chl a fluorescence param eters from the wild-type, isiA(-) and isiB(-) strains grown under iron -stressed conditions. In all strains grown under iron deprivation, the cell number doubling time was maintained despite marked changes in pi gment composition and other cell characteristics. This indicates that iron-starved cells remained viable and that their altered phenotype su ggests an adequate acclimation to low iron even in absence of CP 43' a nd/or flavodoxin. Under both iron conditions, no differences were dete cted between the three strains in the functional absorption crosssecti on of PS II determined from single turnover flash saturation curves of Chl a fluorescence. This demonstrates that CP 43' is not part of the functional light-harvesting antenna for PS II. In the wild-type and th e isiB(-) strain grown under iron-deficient conditions, CP 43' was pre sent in the thylakoid membrane as an uncoupled Chl-protein complex. Th is was indicated by (1) an increase of the yield of prompt Chl a fluor escence (F-o) and (2) the persistence after PS II trap closure of a fa st fluorescence decay component showing a maximum at 685 nm.