UC San Diego

Photosynthetic organisms are the primary source for food and fuel in this planet. Microalgae have the potential to become a platform for the production of natural products, therapeutics and ultimately biofuels. Achieving consistent and efficient photosynthetic growth under fluctuating environmental conditions constitute a key aspect that needs optimization. Photosystem II is regarded as a major bottleneck within this respect. Due to the intrinsic nature of PSII, long evolutionary history, and large number of interacting proteins and cofactors, PSII is said to be in a "frozen metabolic" state making further advantageous evolution by natural processes or by directed mutagenesis in the laboratory extremely challenging. However, previous studies have shown that PSII complexes from a wide variety of species have evolved unique properties, and corresponding unique amino acid sequences, that might be responsible for these distinctive photosynthetic characteristics. The goal of this project is to develop the strains and genetic tools that will allow us to express complete sets of Photosystem II core genes in the homogeneous and tractable algae host, Chlamydomonas reinhardtii. This in turn will allow for studying the natural structural diversity of PSII from a variety of organisms. These tools, and the scientific knowledge derived from them, will be necessary for selecting, understanding, and engineering specific photosynthetic phenotypes to improve the productivity of photoautotrophic organisms. In order to achieve this goal several scientific questions had to be answered. First we proved that heterologous PSII regulatory sequences weren't flexibly recognized in the chloroplast of C. reinhardtii. Second we showed that it is possible to recapitulate photosynthetic phenotypes from a different species (cyanobacterium) by expressing a single, yet key, PSII component (D1) in our eukaryotic algae host. We then generated a six-gene core-complex PSII knock-out strain ([Delta]psbADCBEF) and successfully complemented it with set of genes from three microalgae : C. reinhardtii, Volvox carteri and Scenedesmus obliquus (Chlorophyceae). Finally we tested PSII genes from the green microalgae Chlorella vulgaris (Trebouxyophyceae) and the plant Arabidopsis thaliana (Magnoliophyta), and showed that there are limitations to the extent of compatibility between PSII sub-units when dealing with organisms that do not belong to the same class (taxon)