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Photosynthesis gene expression in Rhodobacter sphaeroides is regulated by redox changes which are linked to electron transport

Department of Microbiology, The University of Queensland, Brisbane 4072, Australia

¹Author for correspondence: Alastair G. McEwan. Tel: +61 7 3365 4878. Fax: +61 7 3365 4620. e-mail: mcewan@biosci.uq.edu.au

ABSTRACT

The effect of the respiratory electron acceptor nitrous oxide on the synthesis of the photosynthetic apparatus of Rhodobacter sphaeroides was examined. In phototrophically grown cells, the addition of nitrous oxide caused a reduction in the level of light-harvesting complex I and light-harvesting complex II under conditions of high light intensity (200 W m^-2) and low light intensity (16 W m^-2). 5-Aminolaevulinate synthase activity was decreased during growth in the presence of nitrous oxide and this limited production of spectral complexes since addition of exogenous 5-aminolaevulinic acid partially suppressed the effect of nitrous oxide. The effect of nitrous oxide on the expression of the operons encoding the pigment-binding proteins of light-harvesting complex I (puf), light-harvesting complex II (puc) and the two isoenzymes of 5-aminolaevulinate synthase (hemA and hemT) were measured using transcriptional fusions to lacZ. Nitrous oxide caused a decrease in puf and hemA transcription. However, there was an apparent increase in the expression of puc and hemT transcriptional fusions. The level of light-harvesting complexes in cells grown in the dark with different electron acceptors was also examined. Cells grown anaerobically with DMSO had a higher level of light-harvesting complexes than those grown anaerobically with nitrous oxide as electron acceptor. Cells grown aerobically had the lowest level of light-harvesting complexes. It is proposed that FnrL-dependent and FnrL-independent expression of photosynthesis genes is modulated by redox changes elicited by nitrous oxide respiration.

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