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For millions of years, viruses have participated in a far-flung, import-export business, exchanging fragments of themselves with both viral and non-viral agents and acquiring new features. What these tiny entities lack in outward complexity, they make up for with their astonishing abilities to swap out modular genomic components and ceaselessly reinvent themselves. In new research appearing in the journal mBio, Arvind Varsani and his colleagues investigate a recently discovered class of viruses that have taken the characteristic versatility of the viral world to new heights. Referred to as cruciviruses, these minute forms reveal a fusion of components from both RNA and DNA viruses, proving that these previously distinct genomic domains can, under proper conditions, intermingle, producing a hybrid or chimeric viral variant. Varsani, a virologist at the Arizona State Univeristy Biodesign Center for Fundamental and Applied Microbiomics, is deeply intrigued with these new viruses, which are starting to crop up in greater abundance and diversity in a wide range of environments. "It is great to see the research groups that first identified cruciviruses around the same time teaming up for the sharing and mining of metagenomic data with an aim to identify a larger diversity of cruciviruses," says Varsani, an associate professor with ASU School of Life Sciences.
New virus in town
Crucivirus sequences were identified by Varsani's colleague and co-author Kenneth M. Stedman and his group at Portland State University. The team detected the viruses flourishing in an extreme environment—Boiling Springs Lake (BSL) in Lassen Volcanic National Park, Northern California. Around the same time, Varsani and Mya Breitbart's research group identified a crucivirus in a dragonfly sample from Florida. Since their discovery in 2012, cruciviruses have been found in diverse environments around the world, from lakes in upstate New York and Florida, to the Antarctic and deep-sea sediments. Some 80 distinct cruciviruses had been identified, prior to the current study, which expands the number to 461. The first cruciviruses were identified using a technique known as viral metagenomics, in which viral genetic material obtained directly from the environment is sequenced rather than being cultivated or cultured from a host species or natural reservoir. The results of these early investigations revealed peculiar genetic sequences, radically distinct from anything that had been seen before. This sequences clearly displayed the signature of a DNA virus, yet also contained a gene that appeared to be derived from an RNA virus. Using a shotgun approach to trawl through a potentially vast sequence space, viral metagenomics enables researchers to identify all of the genomic patterns present in an environmental sample, then separate out distinct viral sequences, like a fisherman retrieving a variety of sea creatures from his net. The technique has revolutionized the discipline of virology. In addition to identifying a galaxy of previously unknown viruses, metagenomics has offered up exciting clues about genetic diversity and is helping to unlock some of the secrets of viral evolution, all without the need to initially isolate viral species or cultivate viruses in the lab.
Form and function
Cruciviruses belong to a broader class of viruses known as CRESS, (for circular Rep-encoding single-stranded) DNA viruses which have recently been classified into the phylum Cressdnaviricota. The defining characteristic of such viruses is their mode of replication, which relies on a specific component, known as the Rep protein. The Rep protein is important for guiding the replication method of these viruses, known as rolling circle DNA replication. Presence of the Rep protein and rolling circle replication pinpoints a virus as belonging to cressdnaviruses and helps researchers untangle the devilishly complex relationships and lineages found in the viral world. In addition to the Rep found in cressdnaviruses, cruciviruses contain another centrally important feature—a capsid protein that is similar to that previously found only in RNA viruses. Capsids are vitally important, forming the outer shell or envelope that encloses the virus's identity—its genetic sequence. The capsid shelters the vital nucleic acids sequestered within from digestion by host cell enzymes, enables virus particles to attach themselves to host cells and allows viruses to evade host cell defenses. Finally, capsids contain specialized features that give the virus its ability to puncture the host cell membrane and inject viral nucleic acid into the cell's cytoplasm. Analysis indicates that the capsid protein of cruciviruses is closely related to the capsid protein of another virus from the family Tombusviridae —a single-stranded RNA virus known to infect plants. This hybrid viral character, containing both DNA- and RNA virus derived coding components, is what makes cruciviruses so unique...
Original Study published in mBio:
