Abstract
Iron-limiting growth conditions were established for Synechococcus sp. PCC 7002. Time-resolved optical spectroscopy of Fe-starved cells revealed a light-induced 520 nm spectral signal that may represent the electrochromic bandshift of carotenoids. The isiA and isiB were inactivated, respectively. Only the isiA mutant showed differences in growth rate and pigment content relative to the wild type grown in Fe-depleted medium. Further, the signal did not appear in Fe-starved isiA mutant. We conclude the signal serves as a marker for Fe-starvation and is associated with the IsiA protein. Under iron-limitation, IsiA associates with PS I, forms the PS I-IsiA supercomplex and increases the overall light-harvesting efficiency. We propose the spectral signal observed actually arises from the β–carotene near the quinones in PS I when the electron acceptors are transiently reduced by high light intensity. Under the intensity regime employed, the PS I-IsiA supercomplex in Fe-starved cells processes more energy per unit time than the PS I in Fe-replete cells, and thus backs up electrons to quinones more prominently. Time-resolved optical and EPR spectroscopies further demonstrated the carotenoid electrochromic bandshift signal and the semiquinone signal.View Large Image | View Hi-Res Image | Download PowerPoint Slide
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