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| Megavirus By Chantal Abergel, CC BY-SA 3.0 |
Download Episode (11.2 MB, 12.3 minutes)
Show notes:
Microbe of the episode: Terasakiella pusilla
News item
Journal Paper:
Schulz F, Yutin N, Ivanova NN, Ortega DR, Lee TK, Vierheilig J, Daims H, Horn M, Wagner M, Jensen GJ, Kyrpides NC, Koonin EV, Woyke T. 2017. Giant viruses with an expanded complement of translation system components. Science 356:82–85.
Other interesting stories:
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Episode outline:
Background: Giant viruses are cool
More and more being discovered; Mimi first
Ep 135 pandora, 161 pitho
Many infect amoebas, but some other things like Cafeteria roenbergensis, marine flagellate
Some bigger in genome size, some in capsid size, some both
Lots of unknown genes/functions, but many do lots of things that hosts usually do for virus
Ex: translation systems
Speculation about where viruses come from
1: Started as own branch of tree of life, lost genes until no longer alive
Not impossible; Rickettsia bacteria can’t live outside host
Mitochondria, etc
2: Started as viruses somehow, gained genes from hosts to grow larger
Maybe originally a self-replicating genetic accident, like transposon + more
What’s new: Now, scientists publishing in Science have discovered a new giant virus that adds some more evidence toward one of these hypotheses!
Methods: Sequenced genomes found in wastewater treatment plant in Klosterneuburg, Austria
Found many giant virus-like genes, enough to assemble 1.57-Mb genome
Named it Klosneuvirus; bigger genome than Mimi, Mega, and Pitho, smaller than Pandora
Distinct genome features place it with others
GC distribution, tetranucleotide patterns, homogeneous distribution of genes
Seems like pure, solid genome
Found 0.3 um particles like giant icosahedral viruses using EM
Also discovered some other giant viruses related to Klosneuvirus based on gene sequences
Though not as big
Some overlap with Mimivirus family, but a lot different
Seem to be related evolutionarily
Designated as subfamily Klosneuvirinae in family Mimiviridae
Seem to infect Cercozoa, group of single-cell eukaryotes that are hard to define
Compared genes among members of family to reconstruct historical path
Times and places of gaining clumps of genes, other times losing clumps
Lots more gaining than losing though, so small ancestor grows into multiple big lineages
As expected from adapting to different hosts
Most interesting: lots of translation-related proteins and tRNAs
Much more complete than Mimiviruses
Enough for 14-19 out of 20 amino acids
If giant viruses were reduced forms of previously free-living organisms, would expect these proteins to be unique and unrelated to anything but each other
Were they? No
Actually come from various different eukaryotic microbes; hosts
Two proteins were very different
Probably cos under selection to evolve away though
3 shared ancestor with Mimi, 4 acquired separately
Summary: new giant virus discovered, actually new branch of existing family. Infect eukaryotes other than amoebae. Have more translation-related proteins than others in family, but seemingly acquired from host rather than coming from 4th domain of life.
Applications and implications: Not as exciting as some mysterious lost alien group of organisms
But still pretty interesting; viruses that take large chunks of host machinery for own
What do I think: What motivation to take on more responsibility/work?
Can only guess
But may be good for taking over
Shut down host’s machinery and use own most efficiently, no competition or regulation
Still exciting work to be done with giant viruses
Trick might eventually be recognizing and distinguishing from non-virus organisms
Bigger they get, the more cell-like
Push up against the boundary of “alive” at some point
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