Translate environmental light conditions into a response.
Both projects 1 and 2 require that microbes intricately translate environmental light signals into a response. In both cases, the molecular mechanism(s) by which transcriptional control proteins interpret environmental signals into these outputs, whether it be the production of a chlorophyll or sunscreen molecule, is largely unknown. Therefore, in project 3, we will explore how photosynthetic organisms sense and respond to available environmental conditions. Here, we will place an emphasis on understanding how organisms use the redox state of the photosynthetic electron transport chain as an indicator of light intensity and how organisms perceive environmental UV-light signals and translate them into a response. One example reaction involved in chlorophyll biosynthesis that is mediated by light intensity is shown in Figure 1.
Figure 1: Photosynthetic bacteria are able to thrive in diverse environments due in part to their abilities to differentially express chlorophyll biosynthetic enzymes that perform equivalent reactions under different environmental conditions. One example is shown here: both light-dependent protochlorophyllide reductase (LPOR) and dark-operative protochlorophyllide oxidoreductase (DPOR) catalyze the conversion of protochlorophyllide to chlorophyllide in the penultimate step of chlorophyll biosynthesis1-4. Each enzyme catalyzes reduction of the double bond shown in pink. LPOR is essential under aerobic high light conditions, whereas DPOR works optimally under anaerobic conditions, or conditions where oxygenic photosynthesis is minimal (e.g. in the dark)1-4. Incredibly, cyanobacteria use the Photosynthetic Electron transport-Dependent Regulator to turn on the transcription of DPOR in response to low light intensity5,6. Although it is known that the Photosynthetic Electron transport-Dependent Regulator is important for acclimation to changing light environments5, the molecular details of how environmental light conditions are perceived and relayed remains to be elucidated.
1. Heyes, D. J. & Hunter, C. N. Making light work of enzyme catalysis: protochlorophyllide oxidoreductase. Trends Biochem Sci 30, 642-649, doi:10.1016/j.tibs.2005.09.001 (2005).
2. Fujita, Y., Tsujimoto, R. & Aoki, R. Evolutionary Aspects and Regulation of Tetrapyrrole Biosynthesis in Cyanobacteria under Aerobic and Anaerobic Environments. Life (Basel) 5, 1172-1203, doi:10.3390/life5021172 (2015).
3. Fujita, Y. Protochlorophyllide reduction: a key step in the greening of plants. Plant Cell Physiol 37, 411-421 (1996).
4. Reinbothe, C. et al. Chlorophyll biosynthesis: spotlight on protochlorophyllide reduction. Trends Plant Sci 15, 614-624, doi:10.1016/j.tplants.2010.07.002 (2010).
5. Nakamura, K. & Hihara, Y. Photon flux density-dependent gene expression in Synechocystis sp. PCC 6803 is regulated by a small, redox-responsive, LuxR-type regulator. J Biol Chem 281, 36758-36766, doi:10.1074/jbc.M606797200 (2006).
6. Horiuchi, M. et al. The PedR transcriptional regulator interacts with thioredoxin to connect photosynthesis with gene expression in cyanobacteria. Biochem J 431, 135-140, doi:10.1042/BJ20100789 (2010).