The variety of colors of cyanobacteria, red algae and cryptophytes is mainly due to the composition of their light-harvesting complexes, which can vary depending on the species. These light-harvesting complexes consist mainly of the proteins phycocyanin (blue) and phycoerythrin (red) and form the so-called phycobilisome.
Phycobilisomes enable these organisms to collect light of additional wavelengths that can no longer be absorbed by the chlorophylls (green gap). The coloration of these phycobiliproteins is due to covalently bound chromophores, which belong to the class of phycobilins. Phycobilins are linear tetrapyrroles that are formed by cleavage of the ring-shaped heme molecule. This cleavage is catalyzed by heme oxygenases, resulting in the precursor molecule of all known phycobilins, biliverdin IXα.
Biliverdin then serves as a substrate for the class of ferredoxin-dependent bilin reductases (FDBR). Phycocyanobilin (PCB) and phycoerythrobilin (PEB) are the most widely used phycobilins. The formation of PCB is catalyzed by PcyA; PebA and PebB are responsible for the synthesis of PEB. Biliverdin is the substrate of PebA, which reduces it to the intermediate dihydrobiliverdin (DHBV). PebA then transfers DHBV to PebB, which then synthesizes PEB. Both PebS and PcyX are new FDBRs that have a viral origin (cyanophages) and can synthesize PEB as a single enzyme.
In our research we are interested in the biosynthesis of the different phycobilins. We investigate the underlying reaction mechanisms and the structural properties required for the specific enzyme activities. The research of viral enzymes is a new field of our department and provides insight into the evolution of tetrapyrrole biosynthesis and its role in the ecosystem. The FDBRs PebS and PcyX belong to the auxiliary metabolic genes (AMG) found in phages and are expressed in their hosts. In this project, the influence of these genes on photosynthesis and the light-harvesting complex will be investigated in order to gain a detailed understanding of the influence of AMGs.
Phages
Viruses are omnipresent in our world and are found practically everywhere where life can be found. Although viruses themselves are not considered living organisms, they interact with them. Due to their non-independent replication or reproduction, they are dependent on the host's metabolism for their multiplication. If a host is infected with a virus, the host metabolism is reprogrammed by the virus in such a way that the host produces a large number of identical copies of the virus and releases the viral "offspring" into the environment through lysis. Viruses in general are responsible for genetic diversity due to genetic exchange in horizontal gene transfer and are considered to be the driving force of global geochemical cycles in the ocean. There they also influence the form of available nutrients, the end of algal blooms and can regulate the host population through lysis.
In addition to prochlorococci, synechococci are one of the most common and widespread organisms in the sea. Both belong to the cyanobacteria and are the most important marine primary producers globally. In view of their size, they are classified as picoplankton and are considered to be the smallest known photosynthesizing organisms. Due to the special light conditions in the sea, they have special light-harvesting complexes (phycobilisomes) that absorb incident light and utilize it for energy production through photosynthesis.
Viruses that infect cyanobacteria are called cyanophages. Interestingly, genes have been found in cyanophages that are not directly involved in virus replication and appear to originate from the host. These genes and the encoded proteins are actively involved in altering the metabolism of their host during infection in order to maintain essential host metabolic pathways. Because of this property, they are called AMGs(auxiliary metabolic genes). The function of these genes and their encoded proteins includes the maintenance of protein biosynthesis, nucleotide synthesis and carbon metabolism, as well as photosynthesis for energy production. In general, AMGs are thought to fulfill the same function as their cyanobacterial orthologs and are derived from their hosts or related bacteria. The proteins encoded by cyanophage AMGs are similar to a variety of enzymes responsible for maintaining energy production in the host when expressed in cyanobacteria during phage infection. The regulation of these genes and their influence on the host have not yet been sufficiently investigated.
The study of viral enzymes is a current research focus of our group and provides insight into the evolution of tetrapyrrole biosynthesis and its role in the ecosystem. In this project we investigate the role of selected AMGs on host metabolism and photosynthesis in particular.
Tetrapyrroles and phages
The color variety of cyanobacteria, red algae and cryptophytes depends on the composition of their light harvesting complexes that differs from organism to organism. These light harvesting complexes consist primarily of the proteins phycocyanin (blue) and phycoerythrin (red) and form the so-called phycobilisome.
Phycobilisomes enable the organisms to collect light of additional wavelengths, which can’t cannot be absorbed by chlorophyll (green gap). The color of these phycobiliproteins is based on the covalently bound chromophores, the so-called phycobilins. Phycobilins are linear tetrapyrroles resulting originating from the cleavage of the circular molecule heme. Heme oxygenases catalyze this cleavage, resulting in the product biliverdin IXα. Biliverdin IXα serves then as precursor for all known phycobilins.
Subsequently, biliverdin functions as the substrate for the protein class of ferredoxin-dependent bilin reductases (FDBR). The resulting bilins phycocyanobilin (PCB) and phycoerythrobilin (PEB) are the most abundant ones. The FDBR PcyA catalyzes the synthesis of PCB, whereas the PEB-synthesis gets catalyzed byrequires the action of both, PebA and PebB. There, biliverdin gets is reduced by PebA to dihydrobiliverdin (DHBV). Afterwards, PebA passes on the DHBV to PebB which synthesizes PEB. The newly discovered FDBRs PebS and PcyA PcyX are of a viral origin (cyanophages) and are able to catalyze the reduction as a single enzyme.
In our research we are interested in the biosynthesis of the different phycobilins. We investigate the reaction mechanisms and the structural properties that are essential for the enzyme activities. The research of the viral enzymes is a new field of our group and allows an insight in the evolution of the tetrapyrrole synthesis and its role in the ecosystem. The FDBRs PebS and PcyX of cyanophages belong to the viral auxiliary metabolic genes (AMGs), which are expressed in their hosts. In this project we investigate the influence of these AMGs on the photosynthesis and the light-harvesting complex assembly to gain a detailed understanding of the influence of AMGs.