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In 2017, scientists discovered what appeared to be an unkillable virus. Don't worry; it's not one that infects us.

It's a bacteriophage, meaning it infects bacteria. Which actually makes it all the more impressive, that it's so hard to kill. See, bacteria have been fighting off viral infections for billions of years.

And even though they lack the fancy cellular compartments that our cells have, they've come up with some really effective defenses. But this virus, dubbed the phiKZ jumbo phage, thwarted the best weapons from every bacteria scientists tested. And that turned out to be because it does something very un-virus-like: it basically builds a nucleus inside its host.

Technically, it's unclear if it gets to be called a nucleus, since that term generally refers to the double-membraned compartment where the genome is stored in plants, fungi, animals, etc. Just to be clear, bacteria aren't included in that. Their cells don't have nuclei... normally.

A phiKZ jumbo phage builds a nucleus inside its host bacterial cell, or at least something nucleus-like. And even though it's made of proteins instead of membranes, it both looks and acts like a nucleus. Just like in our cells, all the virus's DNA-reading and copying happens inside this structure, while RNA translation and protein-building occurs outside.

And the virus not only manages to put its own genetic material and proteins into this nucleus, it somehow gets the bacterial proteins it needs to replicate its DNA in there. So yes, we're talking about a virus that builds a nucleus-like thing inside an otherwise nucleus-free bacterium. It's as if the virus skipped a step in evolution, or took a mind-blowing leap.

It's like if you were anxiously awaiting your baby's first word, but instead of "mama", they busted out. Lafayette's rap from Hamilton. And this nucleus makes the virus virtually unkillable.

You see, the best weapons bacteria have against viral invaders attack the viruses' genetic material after it's inside their cells. Like, there are restriction modification (or RM) systems. In general, these are tag teams of two tiny enzymes.

One checks whether genetic material belongs in the cell, while the other chops up what doesn't belong. And some of the most effective virus killers are. CRISPR systems.

That CRISPR. The big, hulking genetic slicers that scientists have now co-opted for genetic engineering. They can be set to target pretty any sequence of DNA, so they can evolve to seek and destroy pretty much any virus.

But both of these rely on the bacteria being able to find the virus's genetic material, which is where the nucleus-like thing comes in. Even when researchers gave bacteria super amazing. CRISPR or RM systems from other bacteria, phiKZ jumbo phages kept on trucking.

That made scientists think the nucleus was protecting the virus's genome, but they wanted stronger evidence. So they started an Ocean's 11-esque endeavor to smuggle something inside the nucleus. Ultimately, they couldn't get a whole CRISPR system to fit, but they did manage to sneak an RM team in.

And finally, the researchers were able to slow down phiKZ replication. This strongly suggested the the nucleus-like compartment is what makes the virus so hard to kill. You might think such an effective strategy would be used by all viruses.

And in fact, scientists have discovered that other jumbo bacteriophages make these structures. But that doesn't make them totally unkillable. The virus still has to send RNAs out of this protective envelope to act as blueprints for the proteins that will be used by the next generation, and those are probably still vulnerable to attack.

So, this virus may not be invincible after all. Just really close to it. Thanks for watching this episode of SciShow.

If you enjoyed learning about this bizarre virus, you might enjoy our episode on the viruses that shaped humanity, because viruses aren't always bad. Sometimes, they give us the genetic tools to do some really interesting things. ♪♪♪.