Abstract
This study aims to elucidate the molecular mechanism of an alternative electron flow (AEF) functioning under suppressed (CO2-limited) photosynthesis in the cyanobacterium Synechocystis sp. PCC 6803. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], reaches a maximum shortly after the onset of actinic illumination. Thereafter, Y(II) transiently decreases concomitantly with a decrease in the photosynthetic oxygen evolution rate and then recovers to a rate that is close to the initial maximum. These results show that CO2 limitation suppresses photosynthesis and induces AEF. In contrast to the wild type, Synechocystis sp. PCC 6803 mutants deficient in the genes encoding FLAVODIIRON2 (FLV2) and FLV4 proteins show no recovery of Y(II) after prolonged illumination. However, Synechocystis sp. PCC 6803 mutants deficient in genes encoding proteins functioning in photorespiration show AEF activity similar to the wild type. In contrast to Synechocystis sp. PCC 6803, the cyanobacterium Synechococcus elongatus PCC 7942 has no FLV proteins with high homology to FLV2 and FLV4 in Synechocystis sp. PCC 6803. This lack of FLV2/4 may explain why AEF is not induced under CO2-limited photosynthesis in S. elongatus PCC 7942. As the glutathione S-transferase fusion protein overexpressed in Escherichia coli exhibits NADH-dependent oxygen reduction to water, we suggest that FLV2 and FLV4 mediate oxygen-dependent AEF in Synechocystis sp. PCC 6803 when electron acceptors such as CO2 are not available.
Highlights
This study aims to elucidate the molecular mechanism of an alternative electron flow (AEF) functioning under suppressed (CO2limited) photosynthesis in the cyanobacterium Synechocystis sp
We determined oxygen evolution rates at the three times marked in Figure 1 as a (CO2 in the medium not yet depleted by photosynthesis), b, and g
We aimed at elucidating the molecular mechanism of AEF under CO2-limited photosynthesis in Synechocystis sp
Summary
This study aims to elucidate the molecular mechanism of an alternative electron flow (AEF) functioning under suppressed (CO2limited) photosynthesis in the cyanobacterium Synechocystis sp. Thereafter, Y(II) transiently decreases concomitantly with a decrease in the photosynthetic oxygen evolution rate and recovers to a rate that is close to the initial maximum These results show that CO2 limitation suppresses photosynthesis and induces AEF. PCC 6803 mutants deficient in genes encoding proteins functioning in photorespiration show AEF activity similar to the wild type. The photooxidation and photoreduction cycles of the reaction center Chls in PSI and PSII become uncoupled from the production of NADPH, inducing alternative electron flow (AEF) pathways (Mullineaux, 2014). Flores et al (2005) suggested that the nitrate assimilation pathway functions in AEF when the cells live in medium containing nitrate
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