Last updated April 4, 2018 at 11:49 am
One of the largest viral genomes ever sequenced shows its role in the life cycle of plankton.
Scientists have sequenced the genome of a giant virus – 100 times larger than many other viruses – and discovered a bacterium-sized brute, heavily armed, in possession of stolen goods and not afraid to steal more.
The first species of giant virus was described in 1995, and since then scores of new varieties have been found, typically infecting marine bacteria. Almost all of the giant viruses studied in detail so far have been ones that use a bacterial genus known as Acanthamoeba as hosts. The genus is widespread, but not common.
To try to better understand the role of giant viruses in the life cycle of the dense bacterial colonies that contribute to plankton in fresh and salt water, researchers from the University of British Columbia in Canada opted to look at a different host, a common bacterium known as Bodo saltans.
The microbe plays host to its very own giant, known (perhaps unimaginatively) as Bodo saltans virus (BsV).
The researchers, led by Christoph Deeg, discovered a massive beast. BsV boasts 1.39 million bases of DNA – one of the largest viral genomes ever sequenced, and certainly the largest in a virus that infects zooplankton.
The genome reflects the dual tasks that BsV has to accomplish to be successful. On one hand, it has to be able to enter its host, defeat its defences and set about replicating itself. On the other, it also has to repel boarders – competing viruses for which B. saltans represents tempting prey.
To achieve the second outcome, BsV comes armed with a wide and vicious array of toxins, as well as enzymes that specialise in slicing up DNA – allowing it to effectively dismember its competitors.
Gigantism, the researchers suggest in a paper in the journal eLife, is the product of “genomic plasticity”, resulting in “rapid evolution”. BsV’s genome illustrates this is two interesting ways – by what it contains and what it does not.
As well as its offensive equipment, the DNA also contains genes that help in the infection process and combat host immune defences. These, Suttle and colleagues suggest, were originally stolen from the host species and incorporated by the virus.
BsV, however, is unusual, even among giant viruses, in that it lacks material known as transfer-RNA (tRNA) – an essential element of the replication process.
It does, however, carry genes that catalyse the repair of tRNA – strongly suggesting that it co-opts its host’s tRNA to achieve its ends.
The pitch at which the ongoing contest between host and infection – between parasitic predator and prey – is demonstrated by the fact that no less than 10% of the BsV genome is dedicated to defeating B. saltans’ defences.
The manner in which this is done, says Deeg, indicates that the battle is far from settled.
“These genes are actively being duplicated in an accordion-like mechanism in the periphery of the viral genome,” he says.
“This suggests that the virus is engaged in an evolutionary arms race with its host, and could offer on explanation of how the genomes of giant viruses could reach their impressive complexity.”