Summary: In the purple non-sulphur bacterium , genes encoding structural polypeptides of the lightharvesting (LH) and reaction center (RC) complexes incorporated into an intracytoplasmic photosynthetic membrane are induced upon lowering the oxygen tension in the media of aerobically growing cultures. When cultures are grown microaerophilically in the dark, an intracytoplasmic photosynthetic membrane develops gratuitously if a terminal oxidant, such as dimethylsulphoxide (DMSO) is present in the medium. The purpose of the present study was to determine whether in these conditions, photosynthetic genes are completely derepressed or whether they are still inducible in response to a lowering of oxygen tension. Oxygen induction of mRNA for the and operons was compared in dark aerobic cultures (20% O) shifted to low oxygen conditions (3% O) allowing growth microaerophilically with or without DMSO as an accessory terminal oxidant. The extent of the induction was similar in both growth conditions, 6 to 12-fold for mRNA and at least 400-fold for mRNA which encode the light-harvesting I (LHIα) and light-harvesting II (LHIIβ) polypeptides, respectively. The and operons were also induced by low oxygen tension in a mutant strain blocked in an early step of bacteriochlorophyll (BChl) synthesis, suggesting that the presence of BChl may not be a prerequisite for the normal oxygen regulation of the genes encoding the structural polypeptides of the photosynthetic apparatus.


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