Short- and long-term redox regulation of photosynthetic light energy distribution and photosystem stoichiometry by acetate metabolism in the green alga, Chlamydobotrys stellata

The effect of acetate metabolism on the light energy distribution between t he two photosystems, on the PS II/PS I stoichiometry and on the expression of psbA and psbB and psaA genes was investigated in the green alga, Chlamyd obotrys stellata during autotrophic (CO2), mixotrophic (CO2 plus acetate) a nd photoheterotrophic (only acetate) cultivation. It was observed that acet ate assimilation in the glyoxylate cycle resulted in a large drop in the AT P content and a concomitant increase in the NADPH content of the cells. The combined effect of high NADPH concentration and linear electron transport brought about an over-reduction of the inter-photosystem electron transport components. The reduced state of the inter-photosystem components initiate d a state 1/state 2 transition of LHC II and a decrease in the PS II/PS I r atio. The PS II/PS I ratio was reduced because the synthesis of PS II react ion centers was repressed and that of the PS I reaction centers was slightl y enhanced by acetate cultivation. The amount of PsbA and PsbB proteins of PS II and the abundance of psbA mRNA decreased. The abundance of PS I PsaA protein and psaA mRNA were only slightly increased. All of the acetate-indu ced effects were reversible when the cells were transferred back to an acet ate-free medium. Our observations demonstrate that the expression of the PS II psbA and psbB and PS I psaA genes is regulated by the redox state of th e inter-photosystem components at the transcriptional level. Experiments ca rried out in the presence of DBMIB which facilitates the reduction of plast oquinone pool indicate that the expression of genes encoding the components of PS II and PS I are controlled by the redox state of a component (cytoch rome b/f complex) located behind the plastoquinone pool.