The discovery of Mimivirus in a French cooling tower amazed virologists and changed our view of the biology and evolution of giant viruses. Since then, many other giant viruses have been identified, and with three exceptions, they all appear to infect species of Acanthamoeba. Now a new member of the Mimivirus family has been discovered that infects the flagellated eukaryote Bodo saltans (pictured: image credit).
A unique feature of eukaryotic cells, which distinguishes them from bacteria, is the presence of a membrane-bound nucleus that contains the chromosomal DNA (illustrated; image credit). Surprisingly, a nucleus-like structure that forms during viral infection of bacteria is the site of viral DNA replication (link to paper).
During infection of Pseudomonas bacteria with the phage 2O1phi2-1, a separate compartment forms in which viral DNA replication takes place. A phage protein, gp105, makes up the outer layer of this compartment, which initially forms near one end of the cell, and then migrates to the center. The migration of the compartment takes place on a spindle made up of the tubulin-like protein PhuZ.
In addition to viral DNA, certain proteins gain entry into this compartment, including viral proteins involved in DNA and mRNA synthesis, and at least one host cell protein. Other proteins, such as those involved in translation and nucleotide synthesis, are excluded. This compartmentalization very much resembles that of the nucleus of eukaryotic cells.
Packaging of the viral DNA takes place on the surface of the viral nucleus. Empty phage capsids form at the bacterial cytoplasmic membrane, then migrate to the compartment where they attach firmly to the surface. By an unknown mechanism, DNA moves from the compartment into the capsid. Then capsids are released from the surface to further mature in the cytoplasm. The completed phages are released from the cell upon bacterial lysis.
These fascinating observations raise a number of unanswered questions. Does infection with other phages lead to assembly of a viral nucleus? How do molecules selectively move in and out of the structure?
Perhaps the most interesting question relates to the origin of viruses and cells. According to one hypothesis, self-replicating, virus-like nucleic acids might have first appeared on Earth, followed by cells without a nucleus. Was the nucleus a viral invention?
Viruses are tidily categorized into three groups according to the hosts they infect – bacteriophages, eukaryotic viruses, and archaeal viruses. Viruses do not infect hosts in another domain of life, and therefore lateral gene transfer is limited (giant DNA viruses might be exceptions). Now there is evidence for lateral gene transfer between eukaryotes and bacteriophages.
Proof of this unusual movement of DNA comes from studies of the obligate intracellular bacteria Wolbachia, which infects 40% of arthropods (pictured). Wolbachia are in turn infected with a bacteriophage called WO; nearly all sequenced Wolbachia genomes contain integrated WO DNA. Analysis of complete WO genome sequences revealed the presence of mutiple eukaryotic genes (link to paper) that comprise about half of the phage genome!
Ten different protein domains were identified in the eukaryotic genes of WO phage with four functions: toxins, host-microbe interactions, host cell suicide, and protein secretion through membranes.
One eukaryotic gene in phage WO is a black widow spider toxin called latrotoxin-CTD. Sequence analysis suggests that the spider toxin gene was transferred to phage WO within a Wolbachia genome (these bacteria are known to infect widow spiders).
It is not surprising that a virus of a bacterium that infects a eukaryotic cell might acquire eukaryotic genes, but the exact mechanism of gene transfer is unknown. Eukaryotic DNA might enter the WO genome while the particles are in the insect cell cytoplasm, or during packaging of viral DNA in the presence of animal DNA. Another possibility is transfer of eukaryotic DNA to the Wolbachia genome, and then to phage WO.
The fact that eukaryotic-like DNA sequences make up half of the phage WO genome suggests that they serve important functions for the virus. The functions ascribed to these eukaryotic genes suggest roles in cell lysis, modification of host proteins, and toxicity.
There are other examples of phage-infected obligate intracellular bacteria of Chlamydia, aphids, and tsetse flies. A study of these viral genomes should reveal whether lateral gene transfer between metazoans and bacteriophages is a common mechansim for augmenting functions of the viral genome.
On episode #34 of the science show This Week in Microbiology, Vincent, Michael, and Elio discuss changing populations of Emiliania huxleyi and their viruses in the North and Black Seas.
You can find TWiM #34 at microbeworld.org/twim.
As a college biology major during the 1970s I was taught that cells in which the genetic material is separated from the cytoplasm by a nuclear membrane – such as those of animals, fungi, plants, and protists – are called eukaryotes. In contrast, the DNA of bacteria is not bounded by such a structure, and hence these microbes are called prokaryotes, a name that means ‘before the nucleus’. This concept was accepted by biologists until the late-1970s, when Carl Woese used ribosomal RNA sequences to deduce the relationships among living organisms. He found that microorganisms previously thought to be bacteria, because they have no nucleus, were no more related to bacteria than to eukaryotes. He proposed that living organisms should be classified into three lineages, now called bacteria, archaea, and eukarya. Nevertheless, the prokaryotic classification is still used by many biologists. The following letter from Elio Schaecter, sent to TWiV, explains why:
“Regarding your discussion of the term prokaryote in TWiV #93, I want to pipe in as a combatant in the “P word” wars. I am firmly in the camp of the users of the term. Although the term has carried a phylogenetic burden, meaning that it originally implied a close evolutionary relationship between the Bacteria and the Archaea, no one I know uses it in that sense now. Among biologists, the three domains model is widely accepted, in fact, not even discussed. It’s true that there are leftover people who think that the prokaryote/eukaryote divide denotes a single evolutionary cleft, but that’s simply because any concept of science takes time to filter out.
“I maintain that the term, used in a broad sense, is extraordinarily useful. In a college textbook I co-authored called “Microbe”, we used the P word some 300 times. How come? Had we not used it, we would have had to say “Bacteria and Archaea” that many times (and being parsimonious of verbiage, we eschewed that). This usage illustrates the reality that these two groups of microbes, though they likely diverged very early on in evolution, share a large number of common properties. Their sizes tend to overlap, their overall body plan is generally very similar, they often occupy the same habitats, they share many homologous genes. Presented with an EM thin section, you could not tell a typical bacterium from a typical archaeon. So, dissimilar as they may be in one sense, they are very similar in a number of important attributes. Saying “prokaryotes” is much like saying “animals” or “plants,” large groups that are extremely heterogeneous and that diverged a long time ago (although certainly not as far back as the prokaryotes and eukaryotes). I agree, there is danger in the P word being misunderstood out in the big wide world, but there is none within the family of biologists.
“Anyhow, the battle has been met and, yeah!, the victors are clearly the users of the word prokaryote. The term is found all over the place, notwithstanding the astonishing campaign waged against it. Just look at titles of recent articles in major journals.
“There is a more serious issue. Making phylogeny the overarching master of relatedness is readily justified if one thinks in these terms only. But isn’t ecology just as important to understand biological behavior and relatedness? There is a tyranny to phylogeny, which demands that you view the world of living things in terms of where they came from, not what they are doing now.”
What does this nonmenclature issue have to do with virology? According to Patrick Forterre:
The discovery of unique viruses infecting archaea also corroborates the three domains concept from the virus perspective. Indeed, most viruses infecting archaea have nothing in common with those infecting bacteria, although they are still considered as “bacteriophages” by many virologists…David Prangishvili and myself have thus suggested to classify viruses into three categories, archaeoviruses, bacterioviruses and eukaryoviruses.
Woese, C. (1977). Phylogenetic Structure of the Prokaryotic Domain: The Primary Kingdoms Proceedings of the National Academy of Sciences, 74 (11), 5088-5090 DOI: 10.1073/pnas.74.11.5088
Prangishvili, D., Forterre, P., & Garrett, R. (2006). Viruses of the Archaea: a unifying view Nature Reviews Microbiology, 4 (11), 837-848 DOI: 10.1038/nrmicro1527