|Comparision of relative sizes of viruses to that of E. Coli, a common bacterium. Credit: New Scientist|
In all of that DNA are lots of genes which are unlike any so far identified. The discoverers suggest that this category of viruses may be heirs to a fourth domain of life, outside of eukaryotes, bacteria, and archaea. Importantly, these are bona fide viruses, lacking the genes encoding the ribosomal machinery and other key features of bacteria.
These viruses are so different from what is commonly encountered that there must be something unusual about their lifestyle. While I was reading about this discovery, I came across some interesting commentary from some of the scientists who had made the earlier Mimivirus discovery, that it's no coincidence that these viruses were all found in amoebas. Amoebas are free-dwelling eukaryotes which ingest just about anything large enough. For example, if they are exposed to latex particles of about 0.5 microns, they will take them up, even though they are indigestible.
A lot of what amoebas internalize in the real world--such as bacteria-- serve as food. But many microbes don't get eaten after ingestion. They seem to persist, in a symbiotic relationship with the amoeba. The scientists were able to demonstrate that a single amoeba could be host to two different bacteria and a virus-- all at the same time. In such an complex and possibly competitive environment, having many genes might be necessary to defend against unexpected visitors. In support of this idea, bacterial strains which can live inside amoebas also have larger genomes than their close relatives which do not. And amoebas themselves can have enormous genomes: one species has a genome of 670,000 megabases-- more than 200x the human genome of 2,900 megabases!
So something about the amoebas and the microbes which infect them allows or even requires them to have very big genomes. The physical size of these giant viruses may be a function of needing to fit all that DNA into a package, or might be a separate phenomenon. For example, the Mimivirus is already right at the threshold of the latex beads that the amoeba will ingest. Could their size help the viruses to get ingested into their preferred home? That might explain why thus far these really big viruses have only been found in environments favored by amoebas.