Apparent redistribution of electron flow between redox cofactors branches in photosystem I from the menB variants of cyanobacterium Synechocystis sp. PCC 6803.

Abstract

Interruption of the menB gene in cyanobacterium Synechocystis sp. PCC 6803 results in replacement of the native phylloquinones at the A₁ sites of photosystem I (PS I) by plastoquinone-9 (PQ) and a slowdown in the rate of forward electron transfer (ET) from plastosemiquinone to 4Fe4S clusters by a factor of ~ 1000 compared with the wild type (WT) (Semenov et al. 2000). We used PS I samples from two menB variants containing either loosely bound PQ (menB-PQ_L) or its tightly bound analogue (menB-PQ_T). Flash-induced optical changes were used to characterize both menB-PQ_L and menB-PQ_T samples. Photoelectric membrane potential (Δψ) generation was monitored in proteoliposomes containing PS I complexes from menB variants using direct electrometry. Sub-microsecond Δψ kinetic phase attributed to ET from P₇₀₀ to A₁ was followed by sub-millisecond components of Δψ generation with bi-exponential kinetics: τ₁ ≈ 5 µs (relative amplitude ~ 60%) and τ₂ ≈ 30 µs (~ 40%) in menB-PQ_T, and τ₁ ≈ 10 µs (~ 70%) and τ₂ ≈ 100 µs (~ 30%) in menB-PQ_L. These components were assigned to ET in the B and A branches (A_1B and A_1A) to the 4Fe4S clusters, respectively. The observed characteristic lifetimes are in quantitative agreement with theoretical slowdown of these reactions according to the Marcus theory and Moser-Dutton approximation, in accordance with the model in which ET is exergonic in both branches of WT PS I. In contrast to WT complexes, PS I from the menB variant exhibited a functional predominance of ET along the B-branch, presumably because A_1A site remains vacant in a fraction of PS I from both menB variants.