Regulation of photosynthesis -Cytochrome b6f in redox regulation -Two novel proteases acting on an N-terminal peptide of LHCII

Detta är en avhandling från Department of Plant Biochemistry, Lund University

Sammanfattning: In higher plants photosynthesis takes place in the chloroplast. This work focuses on two different processes that effects photosynthesis. Firstly, we studied the events occurring prior to the phosphorylation of LHCII. Secondly, we also studied two novel proteases, with the potential to degrade LHCII.

In oxygenic photosynthesis two different photosystems partake in the gathering of light, PSI and PSII. They use slightly different wavelength for photochemistry. Since the photosystems are coupled in series, a light favouring one photosystem over the other leads to unbalanced electron transport. This unbalanced electron transport causes the interconnecting electron transport chain to be either reduced or oxidized. In this work we have seen, using Blue-native gels and P32-labelled protein assays, that the b6f-complex becomes phosphorylated during reducing conditions. We also suggest different amino acids as candidates for this phosphorylation.

Cytochrome b6f complex is vital for regulatory events taking place in the chloroplast. In the b6f-complex there are four different hemes, three of them (cyt f, heme bH and bL) are known to be involved in conveying electrons from the plastoquinone pool to PSI, whereas the fourth heme (heme ci), function remains unknown. An important first step to understand electron flow through this complex is to be able to study the redox centra individually. Using purified b6f-complex together with magnetic circular dicroism (MCD), we monitored the reduction of the individual hemes. We also assigned spectral features to the newly discovered heme ci. With these results and further measurements will gives the tool to correlated phosphorylation of b6f-complex with redox state of the hemes in the complex.

Two novel chloroplast proteases are described. One protease was found in the stroma and the other protease is associated with the thylakoid membrane. Both proteases are capable of cleaving a peptide resembling the N-terminal part of LHCII. The stromal protease identified as a glutamyl endo peptidase was given the name cGEP. The thylakoid-associated protease seems to be under redox-control, activated by different reducing compounds, with a pH-optima around 7, and decreased activity around pH 8.