There's a new paper out yesterday, about a little quirk of evolution between a cyanobacteria and a virus.

A little bit of background information. First, viruses are difficult to taxonomically distinguish. With the tiny size of viral genomes, barely any error-correction machinery is packed inside. Viruses depend on their replication rate to ensure survival rather than the fidelity of their copy, which leads to their rapid mutation rate, as well as the occasional packing of completely non-viral DNA (which we leverage to our benefit as generalized transduction, because it occurs often enough that we can depend on it occurring (although in my experience, not often enough.))

Sometimes the virus can pack a few extra genes from its host along with its viral DNA, especially if those genes came from other viruses that previously infected the same cell. So genes get exchanged like a bad case of mono, and tracking down individual lineages of evolution are nearly impossible. Forget those nice clear cladograms, where genes slowly evolve from precursor forms; instead, genes appear in viral genomes fully formed and fully functional, and you're not sure whether they evolved in this virus first, or just picked it up from another virus (or bacteria. or plant. or animal).

(Granted, evolution within strains of virus can be tracked through mutations, which is helpful for tracking the seasonal flu, but one good recombination between two strains and you get a pandemic, and your cladogram becomes one step closer to a family tree.)

So it's easy for viruses to accidentally pick up DNA from its host organism. Probably going to happen quite often, given how frequently cells are infected by viruses. But the question today is, what happens to the DNA that gets picked up?

Usually? Not much. The viral capsid often holds an exact amount of DNA, and the extra DNA just serves as filler to meet the threshold. Other times, the virus grabs antibiotic resistance genes, spreading antibiotic resistance to the next host the virus infects.

But how can the extra DNA become fixed in the viral population? only if the DNA provides some benefit to the virus itself, whether through benefiting the virus directly, or benefiting the host (and allowing it to produce more viruses).

Viruses steal host DNA, but how can it be useful to the virus? We'll get back to that question later.


Viruses are bad for cells (discounting most engineered viruses, including gene therapy and oncolytic viruses). Thus, the cells which are able to fight of viral infections are favoured by evolution.

The human body has both innate and adaptive immune systems, both of which play a role in fighting viruses. But single celled organisms don't have other cells to protect them from viruses; instead, they have internal nucleases to cut foreign invading DNA.

But bacteria have their own adaptive immune system: the CRISPR/Cas system. After cutting the viral DNA, the DNA segments are integrated into the CRISPR DNA array. This array serves as an archive of viral infections, from newest to oldest, and serve to guide nucleases to their matching viral DNA in order to more rapidly cut the viral DNA.

(The most lauded part of this system, of course, is the fact that it's a RNA-guided nuclease, which, compared to the next best thing (TALENs, which work through modular protein-based DNA binding) reduces times and labour by orders of magnitude.)


So what happens in this paper? Well, a pair of viruses have been discovered, which contains a CRIPSR DNA array, loaded with fragments of competing viruses.

Let me rephrase that. This is a virus which confers immunity to other viruses.

(on a tangential note, I find it odd that very few computer viruses have the same function. I've read of a few that downloaded security patches, but I can't find any references to them.)

I don't really need to state why this is beneficial to both the host and the virus. The host gains immunity to five other viruses, whereas the virus can replicate in an environment with competition from those viruses. Additionally, the Nostoc cyanobacteria form filaments, allowing for the transfer of transcribed CRISPR RNA between cells, without the transfer of the virus.


Importance? Not much, but it is a very interesting discovery, confirming once again that nature leverages every advantage it has to its disposal.

A virus, stealing the immune system of its host, and using it to its own advantage.

Now that's interesting science.

Tagged with biology, paper
Posted on2016-06-15 05:21
Last modified on2017-02-08 15:05

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