What are some biological activities of viruses

Are viruses living beings?

Contradicting theories

Viruses are not living things. They do not breathe, cannot reproduce on their own and do not have any metabolism. This definition is questioned. The discovery of more and more giant viruses makes it necessary to reconsider their role and task in evolution and in the biosphere as well as their definition. Their origin is still in the dark. There are three theories in particular.

According to the prophage and proto-oncogene concept, the viruses developed from genes of prokaryotes or eukaryotes ("escape model"). The combination with nucleocapsid genes resulted in infectious bodies, the virions, which survive outside the host and can multiply within the host.

According to a similar theory, viruses could have originated from parasitic cells whose genome was reduced (e.g. amoeba theory, see below).

This is countered by the "virus-first model". It says that viruses arose before the first biological structures in a precellular RNA world (self-replicating molecules).

Giant viruses and their genetic analysis are now causing discussion. The question of whether the viruses originated before or after LUCA (Last Universal Cellular Ancestor) is re-asked. The French researcher Patrick Forterre from the Pasteur Institute in Paris presents a virocell concept that tries to classify viruses as living organisms in evolutionary history on the basis of giant viruses - especially since viruses have a much larger reservoir of genes than cellular organisms.

"This mimivirus looks like a new parasitic life form to me."

Jean-Michel Claverie

The first giant viruses

The first giant virus had already been described in 1975: the phage G, the Bacillus megaterium infected. Its DNA sequence was larger than that of small bacteria. For a long time it remained an inexplicable isolated case. No other giant viruses were found, either in filtrates or in plaques. They are simply too big for the 0.2 μm filters. And in Petri dishes they seem to multiply very slowly. They can only be detected using modern molecular analysis methods.

So the next discovery did not come until 2003. Researchers working with Jean-Michel Claverie and Didier Raoult from the University of Aix-Marseille II discovered in 1992 in Bradford, England, cooling circuits of air conditioning systems for legionella (Legionella pneumophila) examined and in the amoeba Acanthamoeba polyphaga identified a gram-positive cocci that she Bradfordcoccus called. It was not until eleven years later that they realized that the supposed bacterium is an icosahedral capsid. The virus envelope is covered with fibrils like a bacterium and has a diameter of 400 nm; thus it is the largest virus known to date. Its double-stranded (ds) DNA is 1.2 million kilobase pairs (kb) long and has 981 coding sequences, including genes that were previously only known from living beings. The researchers called their find "Mimicking Virus" or "Mimivirus" for short, because it has the mimicry of a bacterium.

In 2010, the Cafeteria roenbergensis Virus, CroV for short, was discovered off the coast of Texas in the Gulf of Mexico (Fig. 1). Its dsDNA is 618 kb long and carries 544 genes; of these, 274 genes are detected during infection of the flagellate Cafeteria roenbergensis expressed for protein synthesis. For comparison: A "normal" virus has only about ten genes, whereas a bacterium has at least 470 (Mycoplasma genitalium). The bacterium with the smallest genome (145 kb) is currently considered to be Hodgkinia cicadicola who have favourited the endosymbiotic table in the cicada Diciceroprocta semicincta lives.

While Acanthamoeba only lives in fresh water is the heterotrophic flagellate cafeteria (it belongs to the bicosoecida within the stramenopiles or heteroconta) very widespread in marine surface waters, in deep sea sediments and even in hydrothermal vents and can make up up to 20% of the nanoplankton that is at the beginning of the sea's food chain. It is 4 to 6 μm long (without flagella) and phagocytotically feeds on bacteria and viruses.

If such an important organism is attacked by viruses, that is reason enough to take a closer look at their role and influence. The extraordinarily complex CroV has many genes that are involved in DNA replication, transcription and translation and even in protein modification and carbohydrate metabolism of its host. This suggests that CroV has an autonomous strategy in multiplying in cafeteria pursued (see below "Killing the winner").

In the cafeteria

Cafeteria is said to have got its name from its discoverer Curtis Suttle, University of British Columbia, after he spent hours in a cafeteria discussing these flagellates with his students.

Lots of girens - lots of theories

More and more giant viruses are being discovered. They are now called Giren to differentiate them from "normal" viruses. Giren not only have an unusually large number of genes, they also produce a lot of tRNA, which is necessary for protein synthesis. It is hardly surprising that they also have atypical sequences such as introns and inteins. This enables them to modify the sequences of their DNA and the proteins they synthesize.

Since there is no gene that is common to all viruses, their monophyletic origin is unlikely. Some researchers suspect that all girens come from the same gene pool as the prokaryotes and eukaryotes. Others only see a lively horizontal gene transfer between these groups.

The amoeba theory is particularly original. Since amoebas usually take up a particularly large number of different microorganisms in their cytoplasm, they are to be regarded as a kind of "marketplace" for the microorganisms that can exchange their genes there. Here the giren could have been created by recombinations and reductions of DNA. Giren, which build up a layer of fibrils on their capsid, resemble gram-positive bacteria and are large enough to trigger the phagocytotic reflex in amoeba.

Most girens are grouped together to form the NCLDV (Nucleocytoplasmic Large DNA Viruses). Although these are very variable in their morphology, they have 47 genes in common. The VCLDV are currently divided into five to six subgroups (Tab. 1).

Tab. 1: Giren (VCLDV) and their properties; small selection [7].

DNA (length) coding for it
Acanthamoeba polyphaga mimivirus (APMV)
Pseudomonas chlororaphis phage
Bacillus megaterium phage G.
Shrimp WSSV (White spot syndrome virus)
Chlorovirus PBCV-1
(Paramecium bursaria chlorella Virus 1)
Cafeteria roenbergensis Virus (CroV) **
Heterocapsa circularisquama virus
* gram positive bacteria; ** other researchers count CroV to the Mimiviridae

Concept of the virocell

According to the classic definition, a virion, i.e. a complete virus with genome and envelope outside of a cell, is not a living being because it cannot reproduce. Patrick Forterre (see above) and Claudiu Bandea (Centers for Disease Control and Prevention, USA), on the other hand, do not see the virion as the decisive stage of the viral life cycle, but rather the intracellular form of the virus.

The replication of the mimivirus in Acanthamoeba can be observed in the light microscope: the virions arise on clearly defined spherical structures. Due to the viral infection of the amoeba, its metabolism is completely reorganized; a "virocell" arises with a "viral factory" (Forterre), an intracellular parasitic organism with metabolic activity.

The Mamavirus, a close relative of the Mimivirus, can in turn be infected by tiny Sputnik viruses, which reduces the rate of reproduction. The viral factory thus has all the properties of a living cell: reproduction and metabolic activity and even susceptibility to "disease".

Analogous to the thesis by Jacques Monod (1910 - 1976; Nobel Prize 1965) that it is every cell's dream to become two cells, Forterre says that it is a virocell's dream to create hundreds of new virocells by releasing the virions. He contrasts the virocell with the ribocells of the prokaryotes and eukaryotes; the viral factory of the virocell corresponds to the nucleus of the ribocell.

Coevolution

Viral factories and cell nuclei both reside in the cytoplasm, and both membranes are constructed from the endoplasmic reticulum. Since they coexist in the infected cell, there are many possibilities for genetic exchange and coevolution. New genetic analyzes suggest that 13% of the genes of archaea and bacteria were introduced by viruses and similar mobile elements. Up to 40% of the human genome is said to come from retroviruses. Telomeres and centromeres are in each case derived from retroviruses. 85% of the maize genome is said to be made up of transposons, which in turn come from viruses.

For all these reasons, viruses should be older than the ribocells of the descendants of LUCA.

Viruses played (and still do) a critical role in the evolution of life by forcing their hosts to develop defense strategies. It is assumed that pathogenic bacteria obtained their virulence genes from viruses, which was useful because they were able to defend themselves against their unicellular predators. In the course of evolution, they attacked and used the virulence genes to infect eukaryotic cells.

Coevolution also plays a role with Giren. Here we come full circle cafeteria.

Killing the winner

The population size of an organism in its environment is determined by its growth and mortality. Since bacteria and archaea in the pelagic region (water away from the bank above the bottom zone) can survive a shortage of food (bottom-up control) for a long time, their mortality rate is largely determined by viral lysis and the hunger of predators (top-down control). The marine viruses infect both the bacteria and their enemies, but certain viruses only ever infect certain species.

This interaction is very complex and has not yet been clarified. In general, however, it can be said that viruses primarily attack the rapidly growing populations. This is called the phenomenon of "killing the winner". This mechanism can explain why so many different types of bacteria and phytoplankton can coexist. The viruses limit those that multiply more rapidly. Rare species, on the other hand, are less infected.

The share of viruses in total mortality is estimated at around 20%. With the calcareous alga Emiliania huxleyi, the algae of the year 2009, this proportion can also be 100% if it has multiplied en masse and with its "algae bloom" areas of over 100,000 km2 covered.

"The billion-year struggle between cells and viruses is the most important engine of evolution."

Patrick Forterre

Role in the nutrient cycle

In recent years it has become increasingly clear that viruses play an important role in marine ecosystems. As virions, they are ubiquitous and can have a concentration of up to 10, especially in coastal waters8 Particles per milliliter of water. Their number correlates with that of the bacteria and is typically five to ten times as high. Your DNA dissolved in seawater is an important part of the phosphorus cycle within the food web and food chains. The lysis of their hosts influences the composition and the amount of dissolved organic matter (DOM = dissolved organic matter); while dead organisms sink to the bottom, the DOM remains in the upper, euphotic (light-rich) zone of the water column and is available here as a nutrient for the microbes. A quarter of the primary production is said to flow through this viral degeneration process.

Endless potential

The usable potential of viruses is now growing to great heights. Their genes represent a pool of many as yet unknown gene products. It is interesting that many viral proteins are particularly small. Genetic engineering would also like to use this pool. For example, new restriction endonucleases have been found and new promoter and enhancer sequences that function in both mono- and dicotyledons. Nanotechnology is also developing new products using viruses.

Viruses could become the new interface between biology, chemistry and materials science. Biosensors, catalytic or enzymatic nanofactories, nanoelectronics - there are currently no limits to the imagination. In addition, many Giren can still be discovered in the vastness and depths of the seas.

Perhaps phage therapy will also be honored again. It was founded in 1920 by the French-Canadian Felix d‘Herelle (1873 - 1949). In 1935 he conducted research at the institute of his friend George Eliava in Tbilisi (Georgia, then Soviet Union) to use phages to treat wound infections and diarrhea. Despite penicillin and other antibiotics, there are still phage preparations available over the counter in Russian pharmacies today.


literature

[1] Patrick Forterre. Manipulation of cellular syntheses and the nature of viruses: The virocell concept. Comptes Rendus Chimie 2010, in press; doi: 10.1016 / j.crci.2010.06.007. [2] Matthias Fischer et al. Giant virus with a remarkable complement of genes infects marine zooplankton. Proc Natl Acad Sci USA 2010; 107 (45): 19508-19513. [3] Patrick Forterre. Defining Life: The Virus Viewpoint. Orig Life Evol Biosph 2010; 40: 151-160. [4] Eugene V Koonin et al. The ancient virus world and evolution of cells. Biol Direct 2006; 1: 29. [5] Mimivirus: Discovery of a giant virus. Press release by the CNRS (Center national de la recherche scientifique) in Paris on March 28, 2003; www.cnrs.fr/cw/en/pres/compress/mimivirus.htm.[6] Jakob Pernthaler. Predation on prokaryotes in the water column and its ecological implications. Nat Rev Microbiol 2005; 3: 537-546; doi: 10.1038 / nrmicro1180. [7] James L van Etten et al. DNA Viruses: The really big ones (Giruses). Annu Rev Microbiol 2010; 64: 83-99; doi: 10.1146 / annurev.micro.112408.134338.

author

Dr. Uwe Schulte, Osterholzallee 82, 71636 Ludwigsburg, [email protected]



DAZ 2011, No. 1, p. 84