Abstract
The sulfolipid sulfoquinovosyldiacylglycerol is found in almost all photosynthetic and few non-photosynthetic organisms. Studies on the biosynthesis and function of this unusual lipid were performed mostly in bacteria. In the present work the higher plant Arabidopsis thaliana was used as model organism. One part of this study focused on the biosynthesis of sulfolipid, in particular the formation of the sulfoquinovosyl-headgroup. Therefor a genetic approach was chosen. The SQD1 gene of Arabidopsis was expressed in E. coli and purified. The SQD1 protein was first characterized using a structural model and the results were proven biochemical. It was possible to identify NAD+ as cofactor and UDP-Glucose as substrate of SQD1. Crystallization and the resulting structure of SQD1confirmed the structural model and the biochemical characterization. Considering all these results a reaction mechanism for the SQD1 protein was proposed.
In the second part of this work the regulation and functions of sulfolipid in Arabidopsis were investigated. Under phosphate-limiting conditions drastic changes in the lipid composition as well as strong induction of SQD1 mRNA and SQD1 protein were observed. Furthermore the pho1 mutant of Arabidopsis was used for physiological experiments. In spite of dramatic changes in the lipid composition this mutant shows no changes in the light acclimation in comparison to the wild type.
It is suggested that adjustments of lipid composition of thylakoid membranes allow the cell to maintain the function of essential processes that are restricted to these membranes, such as photosynthesis. In particular a certain amount of anionic thylakoid lipid is maintained by substituting sulfolipid for phosphatidylglycerol under reduced phosphate availability. | 
