Many microbes live in extreme environments, encountering conditions that are very hot, very cold, highly acidic, or very salty. The viruses that infect such microbes must also be able to retain infectivity in extreme conditions. How do they do it?
Clues come from the observations that the genomes of viruses that infect Archaea in extreme geothermal environments encode proteins that have never been seen before. The idea is that such unusual proteins must endow these viruses with the ability to maintain infectivity under extreme conditions.
The hosts of Rudiviruses (rudi=small rod in Latin), the Archea Sulfobolus islandicus, live at high temperatures (80° C) and low pH (3.0). These non-enveloped viruses consist of double-stranded DNA wrapped in a helical manner with thousands of copies of a 134 amino acid protein (illustrated; image credit). The three-dimensional structure of Sulfobolus islandicus rod-shaped virus 2 (SIRV2) reveals a new type of organization of virus particles, and provides clues about how it retains infectivity in extreme environments.
Resolution of the SIRV2 structure reveals that it consists of dimers of a single protein which forms helices that are tightly wrapped around the DNA genome. The result is a coiled DNA protected by a coat of protein that stabilizes and protects the genome. Without DNA, over half of the capsid protein is unstructured. Only in the presence of DNA does the viral protein form an alpha helix that wraps around the nucleic acid.
The DNA genome of SIRV2 is in the A-form, in contrast to B-form DNA which is found in most other organisms. The two types of DNA differ in their geometry and dimensions. It was previously thought that A-DNA occurs only when the nucleic acid is dehydrated.
These two usual properties of SIRV2 are also found in gram positive bacteria which form desiccation and heat resistant spores when starved of nutrients. Sporulation is accompanied by a change in the bacterial genome from B-DNA to A-DNA, which is caused by the binding of small acid-soluble proteins. Like the SIRV2 capsid protein, small acid-soluble proteins of spore-forming bacteria are unstructured in solution, and become alpha helices when bound to DNA. These observations suggest that binding of the SIRV2 capsid protein changes the viral DNA to the A-form, conferring stability in extreme environments.