In photosystem I of higher plants and cyanobacteria, phylloquinone (PhQ) acts as the secondary electron acceptor occupying the A1 site between A0 and the Fe-S clusters of the electron transport chain. Phylloquinone-less mutants, menA and menB, of the cyanobacterium Synechocystis sp. PCC 6803 have plastoquinone (PQ) occupying the A1 site and is capable of functioning as the electron acceptor in place of PhQ. Here we report experiments in which the growth medium of the mutant cells was supplemented with highly reducing 9,10-anthraquinones (AQ). As a result, the menA and menB mutants appeared to utilize the anthraquinone in the A1 site. The potentials of the supplemented AQ’s are significantly more reducing than either PhQ and PQ by as much as 400mV. Because of this, the quinones may approach or exceed the potential of the primary acceptor, A0, and lead to unusual electron transition behaviors. The function of quinone molecules in the A1 site of the mutants was monitored by a home built pump probe laser system. The probe beam monitors the recombination of the electron from the terminal FeS clusters of PSI. The P700 back reaction kinetic lifetime changes as a function of quinone. The plastoquinone mutants shows a 3ms lifetime and samples containing AQs (e.g. 1-NH2-AQ) having longer lifetimes (100ms). In decreasing the temperature, lifetimes change as a function of electron transfer pathways options. Glycerol is used as a cryogen, but also seems to have a direct effect on the lifetimes. Room temperature and low temperature experiments with 1-NH2-AQ in various glycerol concentrations showed an altered lifetime with extended lifetimes at room temperature, but short lifetimes at lower temperatures. We suspect that the 1-NH2-AQ is being utilized in the electron transfer process at room temperature, but at low temperatures it is reverting to residual PQ that may still be present. We are currently characterizing and exploring these observations to probe the effect of glycerol and searching for other methods by which to observe the electron transfer process at lower temperatures more effectively.

Characterization of 1-amino-9,10-anthraquinone in the A1 site of PS I complexes of the plastoquinone-containing mutants of Synechocystis sp. PCC 6803

253rd National Meeting (2017)

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