Perception and transmission of light signals in purple bacteria
Final Report Abstract
The goal of this project was to elucidate light-dependent signal chains in R. sphaeroides, which affect the expression of photosynthesis genes. We identified three flavin-binding proteins, which belong to different classes of photoreceptors and show a photocycle. 1) The BLUF domain of the anti-repressor protein AppA. BLUF domain proteins are now accepted as a new class of photoreceptors. 2) The cryptochrome CryB, 3) and a short LOV domain protein. Surprisingly all three photoreceptors absorb identical light qualities (blue light) and affect the expression of photosynthesis genes. This finding makes it almost impossible to unravel the contribution of the individual photoreceptors to blue-light dependent signaling in R. sphaeroides. Among these photoreceptors AppA has the unique ability to integrate redox and light signals. Our results revealed a heme bound to the C-terminal part of AppA, which influences binding to the transcriptional repressor PpsR. The heme is involved in redox- and light-dependent signaling. We also demonstrated a modular structure of the AppA domains: both domains fulfill their function even if not covalently linked and the BLUF domain from a eukaryotic organism can functionally replace the BLUF domain of AppA and signal to the AppA C- terminal part. The AppA BLUF domain has a central role in blue light-dependent signaling in R. sphaeroides, since its lack abolishes the effect of blue light on photosynthesis gene expression almost completely. CryB is the first cryptochrome-like bacterial protein, for which a clear biological function could be demonstrated. Although it has some contribution to photorepair activity in vivo, we could not demonstrate repair activity in vitro. CryB shows however high affinity to single stranded DNA and RNA in vitro. CryB is involved in the regulation of photosynthesis genes and is under control of the alternative sigma factor RpoHII. RpoHII has a major role in the response to photooxidative stress. The short LOV domain protein of R. sphaeroides is unusual, since it contains a longer C- terminal J-alpha helix when compared to other short LOV proteins without output domain. It is associated with the membrane. Microarray studies revealed that LOV has a repressing effect on photosynthesis gene expression and also affects genes of the RpoE-regulon. RpoE targets genes for stress responses in R. sphaeroides. The signal chain from LOV to photosynthesis gene expression is still under investigation. Besides the signal chains that are initiated by photoreceptors we unraveled an additional light-dependent signal chain that acts on photosynthesis gene expression. The photosynthetic electron transport resulting from light absorption by bacteriochlorophyll reduces the electron flow through components of the respiratory chain. This leads to a higer activity of the sensor kinase PrrB and consequently to higher levels of phosphorylated PrrA that activates photosynthesis genes. When only low levels of oxygen are present, this stimulating signal dominates over the repressing effect of light. This response mediated by bacteriochlorophyll absorption is however less sensitive to blue light than the response mediated by the AppA-PpsR system.
Publications
- (2002) A single flavoprotein, AppA, can integrate both redox and light signals in Rhodobacter sphaeroides. Mol. Microbiol. 45: 827-836
Braatsch, S., Gomelsky, M., Kuphal, S., Klug, G.
- (2002) BLUF: a novel FAD-binding domain involved in sensory transduction in microorganisms. Trends in Biochem. Sci. 27: 497-500
Gomelsky, M., Klug, G.
- (2004) A eukaryotic BLUF domain mediates light-dependent gene expression in the purple bacterium Rhodobacter sphaeroides 2.4.1. Proc. Natl. Acad. Sci. USA 101: 12306-12311
Han, Y., Braatsch, S., Osterloh, L., Klug, G.
- (2004) Blue light perception in bacteria. Photosynthesis Res. 79: 45- 47
Braatsch, S., Klug, G.
- (2004) ORF90, a gene required for photoreactivation in Rhodobacter capsulatus SB1003 encodes a cyclobutane pyrimidine dimer photolyase. Photosynthesis Res. 79: 167-177
Braatsch, S., Klug, G.
- (2004) Responses of the Rhodobacter sphaeroides transcriptome to blue light under semiaerobic conditions. J. Bacteriol. 186: 7726-7735
Braatsch, S., Moskvin, O., Klug, G., Gomelsky, M.
- (2005) A blue light dependent expression of photosynthesis genes under anaerobic conditions in Rhodobacter capsulatus and Rhodobacter sphaeroides is mediated by photosynthetic electron transport. Mol. Microbiol. 58: 903-914
Happ, H., Braatsch, S., Broschek, V., Osterloh, L., Klug, G.
- (2007) A heme cofactor is required for redox and light signaling by the AppA protein of Rhodobacter sphaeroides. Mol. Microbiol. 64: 1090-1104
Han, Y., Meyer, M.H.F., Keusgen, M., Klug, G.
- (2007) The AppA and PpsR proteins from Rhodobacter sphaeroides can establish a redox dependent signal chain but fail to transmit blue light signals in other bacteria. J. Bacteriol. 189: 2274-2282
Jäger, A., Braatsch, S., Haberzettl, K., Metz, S., Osterloh, L., Han, Y., Klug, G.
- (2007) The phrA gene of Rhodobacter sphaeroides encodes a photolyase and is regulated by singlet oxygen and peroxide in a σE dependent manner. Microbiology 153: 1842-1852
Hendrischk, A.-K., Braatsch, S., Glaeser, J., Klug, G.
- (2008) Regulation of genes by light. In: The Purple Phototrophic Bacteria. Advances in Photosynthesis and Respiration. eds. Hunter, C.N., Daldal, F., Thurnauer, M., and Beatty, J.T.; Vol 28, pp.155-179, Springer, Dodrecht, Netherlands
Klug, G., Masuda, S.
- (2009) A cryptochrome-like protein is involved in the regulation of photosynthesis genes in Rhodobacter sphaeroides. Mol. Microbiol. 74: 990-1003
Hendrischk, A-K., Frühwirth, S.M., Moldt, J, Pokorny, R., Metz, S., Kaiser, G., Jäger, A., Batschauer, A., Klug, G.
- (2009) Characterization of an unusual LOV domain protein in the alpha-proteobacterium Rhodobacter sphaeroides. Photochem. Photobiol. 85: 1254-1259
Hendrischk, A-K., Moldt, J., Frühwirth, S., Klug, G.
- (2009) In vivo studies on the light sensitivity of the photosynthetic gene regulators AppA/PpsR and PrrA/PrrB in Rhodobacter sphaeroides. J. Bacteriol., 191: 4473-4477
Metz, S., Jäger, A., Klug, G.
- (2010) In vivo effects on photosynthesis gene expression of base pair exchanges in the light responsive BLUF domain of AppA in Rhodobacter sphaeroides. Photochem. Photobiol. in press, Epub May 21
Metz, S., Hendriks, J., Jäger, A., Hellingwerf, K., Klug, G.
