Structural biologists have found that two viruses with very different DNA share remarkably similar molecular structures. While most biologists rely on genetic analyses to untangle relationships between organisms, the authors of this study believe that such structural similarity reveals an ancient relationship between the two viruses. One is a bacteriophage, which infects bacteria, the other an adenovirus, which causes colds in people.
While scientists have long been sketching the family trees of animals and plants, evolutionary relationships among viruses are only now becoming evident. In earlier work, structural biologist Roger Burnett and his colleagues at The Wistar Institute in Philadelphia had examined PRD1, a common virus that infects Escherichia coli and other bacteria. They discovered that the principal protein that makes up the virus's shell was architecturally similar to the corresponding protein in adenovirus, on which Burnett focused much of his work.
To better understand this parallel, Burnett's team probed the structure of PRD1. The researchers used electron microscopy to get the overall structure of the phage particle, then filled in the details with x-ray crystallography. The structures of PRD1 and the adenovirus are "strikingly similar," they report in the October issue of the journal Structure. Traits in common include shape and function of shell proteins and methods of attaching to the host, and similar reproduction schemes. Despite widely disparate DNA sequences, Burnett thinks the two viruses could have shared a common ancestor, because key parts of individual proteins are resemble each other in both species. This has allowed the viruses to produce similar structures even as their DNA diverged.
"The evidence makes it hard to imagine they didn't come from some common ancestor," agrees Roger Hendrix, a biologist at the University of Pittsburgh. The results, he says, support a growing body of evidence pointing to a billion-year-old family resemblance between bacterial viruses and human viruses--implying that viruses may be more closely related to one another than previously thought.