No abstract

The photosystem I reaction center core containing P700 and iron‐sulfur center FX 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 FX and 100% of P700 (determined by optical spectroscopy) but is totally devoid of iron‐sulfur centers FA and FB (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 FA and FB is further reflected in the decline of acid‐labile sulfide from 11.8 ± 0.4 S2− /P700 in the control particle to 4.6 ± 0.3 S2− /P700 in the reaction center core. This preparation represents the first isolation of an intact reaction center core incorporating the components P700 and FX but totally lacking FA and FB.

No abstract

A new photosystem I core has been isolated that is devoid of the bound iron-sulfur clusters but preserves electron flow from P700 to the intermediate electron acceptor A1. The particle is prepared by incubation of a Synechococcus sp. PCC 6301 photosystem I core protein (which contains electron acceptors A0, A1, and FX) with 3 M urea and 5 mM K3Fe(CN)6 to oxidatively denature the FX iron-sulfur cluster to the level of zero-valence sulfur. In this apo-FX preparation, over 90% of the flash-induced absorption change at 820 nm decays with a 10-microseconds half-time characteristic of the decay of the P700 triplet state formed from the backreaction of P700+ with an acceptor earlier than FX. Chemical reduction at high pH values with aminoiminomethanesulfinic acid results in kinetics identical with those seen in the P700 chlorophyll a protein prepared with sodium dodecyl sulfate (SDS-CP1, which contains only electron acceptor A0); the flash-induced absorption change decays primarily with a 25-ns half-time characteristic of the backreaction between P700+ and A0-, and the magnitude of the total absorption change is larger than can be accounted for by the P700 content alone. Addition of oxygen results in a reversion to the 10-microseconds kinetic decay component attributed to the decay of the P700 triplet state. At 77 K, the optical transient in the apo-FX preparation decays with a 200-microseconds half-time characteristic of the backreaction between P700+ and A1-.(ABSTRACT TRUNCATED AT 250 WORDS)