When researchers in 1992 took a closer look at Legionellosis, they came across big lumps visible in light microscopy inside amoeba, which they thought were gram positive bacteria. When they found out however, that those gram positive lumps were actually viruses, they earned themselves the name Mimivirus, as they were MImicking MIcrobes quite well. Since the initial realization that viruses can be just as big as some bacteria, even bigger strains have been discovered: Megavirus chilensis was the virus with the biggest capsid known until in 2013 pandoraviruses appeared on the virological landscape. Their capsids reach up to a micrometer in size and their genome is about 2.5 MB long! While that is the biggest viral genome known today, pithoviruses have the biggest capsids with around 1.5 micrometers.
Those newly discovered giant viruses are genomically diverse. A large part of their genetic information remains to be better understood, but some findings so far are already changing the way we look at the origins of cellular organisms. Mimiviruses encode proteins that resemble aminoacyl-tRNA-synthetases and a rudimentary immune system against virophages. Because they’re viruses however, they need a host to reproduce. The confusion starts when considering that those giant viruses might be older than cellular organisms. Hypotheses about Mimivirus’ crucial role in the development of life on earth, or about viral origins of the three domains of life. Others suggest that giant viruses represent remains of entire other “domains”.
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