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Mitochondria and primary plastids evolved more than 1 billion years ago from free-living bacteria via endosymbiosis. Organelle evolution was accompanied by a massive size reduction of the endosymbiont genome. Thousands of endosymbiont genes were transferred into the host nuclear genome and mechanisms for trafficking nuclear-encoded proteins into the endosymbiont-derived organelle evolved. Providing a multitude of new bioenergetic and biosynthetic abilities to the host, organellogenesis has been one of the most transformative forces during evolution of the eukaryotic cell. However, although the acquisition of mitochondria and plastids has been evolutionary very successful and has had a profound impact on ecosystems on this planet, organellogenesis has remained a very rare event.
Our group is fascinated by the evolutionary processes that enable the merger of two physiologically and genetically very distinct cells. In the resulting chimeric organism, complex interactions between prokaryote-derived organelles and the surrounding eukaryotic cell fine tune organelle metabolism, growth, and division in response to the state of the host cell and environmental factors. Our projects aim at gaining insight into the molecular mechanisms that underlie the formation of a chimeric organism. Due to the ancient origin of mitochondria and plastids, however, it is challenging to reconstruct evolution of the complex interaction networks between host and endosymbiont, based on studies of these organelles. Therefore, we study more recently evolved endosymbiotic systems: the amoeba Paulinella chromatophora, which harbors nascent photosynthetic organelles of cyanobacterial origin that are termed ‘chromatophores’, and the trypanosomatid Angomonas deanei, which harbors the endosymbiotic b-proteobacterium Candidatus Kinetoplastibacterium crithidii.
P. chromatophora is a cercozoan amoeba that occurs in the sediments of freshwater ponds and lakes. Each cell contains two photosynthetic chromatophores that morphologically resemble cyanobacteria. During cell division, one chromatophore is passed to the daughter cell leading to vertical inheritance of the chromatophore. Phylogenetic studies revealed that chromatophores evolved independently from and more recently than plastids from a cyanobacterium of the Prochlorococcus/Synechococcus clade. Compared to its free-living relatives, the chromatophore genome has been strongly reduced in size, but is still ten times larger than a typical plastid genome. Reductive genome evolution in the chromatophore resulted in the loss of many biosynthetic capabilities (e.g. amino acids and various cofactors), establishing a metabolic dependence of the chromatophore on its amoebal host. Interestingly, more than 30 genes that were lost from the chromatophore genome were identified in the nuclear genome of P. chromatophora. Most of these genes encode proteins involved in photosynthesis and light-acclimation. PsaE and PsaK are nuclear-encoded low molecular weight subunits of photosystem I. Even though a protein import machinery similar to the TIC/TOC complex in plastids is likely missing in the chromatophore, biochemical studies have shown that PsaE and PsaK are synthesized in the amoebal cytoplasm and are subsequently trafficked into chromatophores. There, they assemble with chromatophore-encoded subunits into photosystem I. Although mechanistic details for the protein import pathway are still elusive, localization studies suggest that these proteins pass through the secretory pathway prior to entry into the chromatophore. Therewith, the host has gained a tool to directly modulate the photosynthetic apparatus. However, we expect that many more molecular factors are critical for successfully integrating host–endosymbiont metabolic and regulatory networks.
Also in A. deanei, cell cycle of host and endosymbiont are tightly synchronized and the symbiont is vertically inherited. Although the endosymbiont genome is strongly reduced in size, it retained genes for the biosynthesis of heme and other cofactors and amino acids that are required by the host. Although in A. deanei large scale transfer of endosymbiont genes into the host nuclear genome has not been detected, many genes of bacterial origin are found in the host genome. These genes have likely been obtained by horizontal gene transfer from environmental bacteria.
Our research addresses the following key questions:
(1) To what extent resemble bacterial endosymbionts early eukaryotic cell organelles?
(2) How readily do bacterial symbionts become dependent on import of host-encoded proteins and what are the functions of these imported proteins?
(3) Through which pathways can cytoplasmically-synthesized proteins traffic into the endosymbiont (or endosymbiont-derived organelle)?
(4) Which molecular mechanisms are used by the host to control and manipulate a bacterial endosymbiont?
(5) Are some of these mechanisms conserved across eukaryotic phyla?
(6) Where does the genetic repertoire originate from that allows a eukaryotic host to interact with a bacterial endosymbiont?
These questions are addressed by phylogenetic, molecular biological, and protein biochemical approaches.
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