We reported earlier [Smart, L. B., Warren, P. V., Golbeck, J. H., & McIntosh, L. (1993) Proc. Natl. Acad. Sci. U.S.A. 90,1132-1136] that the site-directed conversion of cysteine-565 to serine (C565S) in PsaB of Synechocystis sp. PCC 6803 leads to an accumulation of photosystem I polypeptides and the low-temperature photoreduction of the terminal electron acceptors FA and FB. In this paper, we report the occurrence of a [3Fe-4S]1+,0 cluster in dodecyl maltoside-solubilized photosystem I complexes prepared from the C565S mutant. The [3Fe-4S] cluster is reducible with dithionite at pH 6.5, implying a midpoint potential considerably more oxidizing than either FA or FB. Similar to the behavior of Fx, the [3Fe-4S] cluster undergoes partial, reversible photoreduction when the complex is illuminated at 15 K, and complete photoreduction when the sample is illuminated during freezing. Contrary to the result expected in the presence of a relatively high-potential Fx, there is significant low-temperature and room temperature photoreduction of FA and FB in the C565S complex. Although the FA and FB resonances are more intense when the complex is frozen during illumination, they still account for <60% of FA and FB found by chemical reduction. When the FA and FB clusters are prereduced with dithionite at pH 10.0, a new set of resonances appear upon illumination at g = 2.015,1.941, and 1.811, and disappear on subsequent darkness. The species giving rise to this signal is most likely a mixed-ligand [4Fe-4S]2+,1+ cluster located in the Fx site. This acceptor, denoted Fx′, has different ESR properties from wild-type Fx, but it has a redox potential low enough to transfer an electron to FA and FB. We suggest that Fx′ is assembled as a functional [4Fe-4S] cluster in vivo but in a minority of centers Fx′ loses an iron, resulting in a population of [3Fe-4S] clusters which cannot pass the electron forward to FA or FB. The redox behavior of the [3Fe-4S] and mixed-ligand [4Fe-4S] clusters is consistent with the participation of Fx in electron transfer from A1 - to FA and FB.
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