Single Review on past events

He not only calls himself Lepetit, in his research he is also interested in the smallest things of life: microorganisms. The biologist especially focuses on the process of photosynthesis and its regulation. During photosynthesis carbon dioxide and water are transformed with the aid of light in oxygen and carbohydrates. Without continuous supply of O₂ the atmosphere would be devoid of oxygen and life as we know it would not be possible.

In his presentation on “The history of photosynthesis evolution and its consequences on intracellular communication” Bernard Lepetit first explained the roots of photosynthesis: The first geological evidence for oxygen accumulation goes back to the ur-earth when oxygen was bound in H₂O and CO₂ and can be found as the presence of red banded iron formations and in deviations of the isotopic ratio of sulfur. Approximately 2.7 Giga years ago Cyanobacteria evolved having two photosystems, which each can be found alone in anoxygenic phototrophic bacteria. This occurred by the uptake of one photosynthetic bacterium by another photosynthetic bacterium which gave rise to a cyanobacterium containing two different, in serial operating photosystems. Then, approximately 2- 2.5 Giga years ago, more complex systems called eukaryotes evolved where a eukaryotic ancestor engulfed a proteobacterium and ‘domesticated ’ it as a mitochondrium. The original function of the mitochondria was probably bacterial photosynthesis, where light energy was used to acquire nitrogen. When oxygen concentration rose, they inverted the electron flow and used the oxygen to oxidize carbon in order to gain energy. 1.5 Giga years ago eukaryotic photosynthesis was invented by the engulfment of a cyanobacterium by a heterotrophic eukaryotic cell. This process is called primary endosymbiosis (the chloroplasts here are called ‘primary’ plastids), followed by the secondary endosymbiosis about 1.2 Giga years ago, where a heterotrophic cell engulfed a primary plastid containing photosynthetic eukaryote (the chloroplasts here are called ‘secondary’ plastids). An important consequence of the endosymbiosis was that the majority of genes from the endosymbiont were transferred from the endosymbiont to the host; also, the establishment of additional membranes surrounding the chloroplast required specific transporters for the transfer of metabolites and proteins.

In all photosynthetic eukaryotes, the chloroplast contains thousands of proteins and is surrounded by two to four membranes. Only the minority of these proteins is encoded as genes in the chloroplast itself due to the gene transfer during endosymbiosis. Consequently, almost all protein complexes in the chloroplasts are chimeric complexes of nuclear and plastid encoded proteins. This chimeric nature asks for a fine regulation of nuclear gene expression with a trigger inside the chloroplast (retrograde signaling). Bernard Lepetit wanted to find out more about retrograde signaling in algae with secondary plastids. To answer this question he worked with diatoms which he calls “beautiful unicellular algae” and “one of the most important ecological groups on the globe”. Therefore he investigated the possibility of a component in the chloroplast (PQ-pool) to be a sensor which can transduce a gene expression stimulus to the nucleus. He could demonstrate that the PQ-pool has an influence on the synthesis of specific photoprotective pigments inside the chloroplast. “The pigment pool size is regulated by the redox state of the PQ-pool inside the chloroplast; that means the PQ-pool is a trigger.” Expanding this finding, he obtained evidence for a stimulating function of the PQ-pool on nuclear gene expression of genes encoding for photoprotective proteins (LHCX). With that result he provided the first example of retrograde signaling in organisms with secondary plastids. His future work is focused on isolating samples for a large scale analysis of gene expression changes, investigating mutants impaired in the reduction of the PQ-pool towards their capacity of retrograde signaling (collaboration with Johann Lavaud, France), as well as establishing a system to measure the redox state of the PQ-pool.