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The photosystem I reaction center core containing P700 and iron-sulfur center F X has been isolated from a Synechococcus photosystem I particle with 6.8 M urea at pH 10.0 followed by sucrose density ultracentrifugation. The reaction center core has retained > 90% of F X and 100% of P700 (determined by optical spectroscopy) but is totally devoid of iron-sulfur centers F A and F B (determined by optical and ESR spectroscopy). SDS-PAGE indicates the retention of the 57 kDa reaction center polypeptide(s) but the total absence of the 16.4 and 8.1 kDa polypeptides. The loss of F A and F B is further reflected in the decline of acid-labile sulfide from 11.8 +/- 0.4 S 2- /P700 in the control particle to 4.6 +/- 0.3 S 2- /P700 in the reaction center core. This preparation represents the first isolation of an intact reaction center core incorporating the components P700 and F X but totally lacking F A and F B .