Recruitment of a foreign quinone into the A₁ site of photosystem I: Characterization of a menB rubA double deletion mutant in Synechococcus sp. PCC 7002 devoid of F_X, F_A, and F_B and containing plastoquinone or exchanged 9,10-anthraquinone

A photosystem I (PS I) complex containing plastoquinone-9 (PQ-9) but devoid of F_X, F_B, and F_A was isolated and characterized from a mutant strain of Synechococcus sp. PCC 7002 in which the menB and rubA genes were insertionally inactivated. In isolated PS I trimers, the decay of P700⁺ measured in the near-IR and the decay of A ₁^- measured in the near-UV were found to be biphasic, with (averaged) room temperature lifetimes of 12 and 350 μs. The decay-associated spectra of both kinetic phases are characteristic of the oxidized minus reduced difference spectrum of a semiquinone, consistent with charge recombination between P700⁺ and PQ-9^-. The amplitude of the flash-induced absorbance changes in both the near-IR and the near-UV show that approximately one-half of the A₁ binding sites are either empty or nonfunctional. A spin-polarized chlorophyll triplet is observed by time-resolved EPR, and it is attributed to the ³P700 product of P700 ⁺A₀^- charge recombination via the T₀ spin level in those PS I complexes that do not contain a functional quinone. In those A₁ sites that are occupied, the P700⁺Q^- polarization pattern indicates that PQ-9 is oriented in a similar manner to that in the menB mutant. When excess 9,10-anthraquinone is added in vitro, it displaces PQ-9 and occupies the A₁ binding site more readily than in the menB mutant. This can be explained by a greater accessibility to the A ₁ site in the menB rubA mutant due to the absence of F_X and the stromal ridge polypeptides. The relatively low binding affinity of 9,10-anthraquinone allows it to be readily removed from the A₁ site by washing. However, all A₁ sites are shown to bind napthoquinones with high affinity and thus are proven to be functionally competent in quinone binding. The ability to readily displace PQ-9 from, the A₁ site makes the menB rubA mutant ideal for introducing novel quinones, particularly anthraquinones, into PS I.