Investigating photosynthesis

In today’s highlighted article, the authors use traditional and far more modern biochemistry to uncover why photosynthesis is inhibited by Streptomyces spp., and characterise a previously unknown step in cyclic electron flow. This is also a good opportunity to point out these great photosynthesis outreach and education resources from Science and Plants for Schools. Admittedly, they don’t have anything on AA-sensitive CEF, because it’s unlikely anyone without a plant biochemistry PhD needs to know about that! But they have brilliant basic photosynthesis teaching resources, including these amazing algae-jelly-balls.

Photosynthesis background: Broadly speaking, photosynthesis is the process by which light energy from the sun is absorbed by Photosystems I and II (PSI and PSII), where it is channelled into electron transport chains and stored in ATP and NADPH. One electron carrier is ferredoxin (Fd).

There are two types of electron flow, cyclic and linear (CEF and LEF), which generate ATP. Though they are different processes, both CEF and LEF require PSI and PSII, two other thylakoid proteins, PGR5 and PGRL1, and electron carriers Fd, plastoquinone (PQ) and plastocyanin.

In CEF there are two processes by which electrons are transferred from Fd to PQ. One is a characterised NADH dehydrogenase-like compex dependent pathway, and all that is known about the other is that it is sensitive to antimycin A (AA), a product of Streptomyces spp. which inhibits CEF.

For background on AA-inhibition and the divergent electron transfer pathways in CEF, see Joët et al. (2001; Plant Phys. 24:1919). For more information on PGR5 and PGRL1, see DalCorso et al. (2008; Cell 132:273).

Gaps in knowledge of CEF:

New from Hertle et al.: Titration and Western blotting experiments showed that PGR5 and PGRL1 dimerize to each other. Six cysteine residues were conserved in all PGRL1 proteins. Hertle et al. made mutant PGRL1 proteins in which one or more cysteine residues were substituted for serine, and tested the varients for their capacity to bind PGR5, iron, and to promote AA-sensitive CEF. They worked out that all six cysteines were essential for AA-sensitive CEF, while specific cysteines were involved in binding iron and PGR5.

In vitro assays demonstrated that PGRL1 is capable of transferring electrons between Fd and PQ analogue DMBQ in the presence of PGR5, and this reaction was inhibited in the presence of AA. Hertle et al. show clearly that PGRL1 is physically a fit for a ferredoxin-plastoquinone reductase and make a strong case for it being the mediator between Fd and PQ in CEF, and the AA-sensitive step in the cyclic electron flow.

Highlighted article: Alexander P. Hertle, Thomas Blunder, Tobias Wunder, Paolo Pesaresi, Mathias Pribil, Ute Armbruster, Dario Leister (2013) PGRL1 Is the Elusive Ferredoxin-Plastoquinone Reductase in Photosynthetic Cyclic Electron Flow. Molecular Cell – 03 January 2013, 10.1016/j.molcel.2012.11.030

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