The response of anoxygenic photosynthetic bacteria to singlet oxygen

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Photosynthetic organisms generate singlet oxygen when light, oxygen and chlorophyll are simultaneously present. Little is known about bacterial defence mechanisms against singlet oxygen and their regulation. We will unravel the signal chains underlying the singlet oxygen response in anoxygenic photosynthetic bacteria. Our preliminary work on the response of the facultatively photosynthetic bacterium Rhodobacter sphaeroides to singlet oxygen revealed a role for the two sigma factor proteins sigma E and sigma 38, and for the Hfq protein, which is involved in regulation by small RNAs. The putative response regulator OmpR showed strongly increased synthesis rate in the presence of singlet oxygen. A sigma 38 deletion mutant is not only more sensitive to singlet oxygen but also shows strong aggregation of cells in liquid culture suggesting an altered composition of the cell envelope. Therefore the focus of our investigations will be on: 1. the signalling mechanisms in the sigma E and the sigma 38 dependent pathways, 2. the role of small regulatory RNAs, 3. the role of the OmpR protein and 4. the effect of singlet oxygen on membrane proteins. Some comparative studies will address the singlet oxygen response in Roseobacter denitrificans, which unlike R. sphaeroides expresses photosynthesis genes even at high oxygen tension. A long term goal is the identification of the mechanisms of sensing singlet oxygen.