All living organisms share the same DNA bases: adenine, guanine, cytosine and thymine. All except bacteriophages, viruses that infect a wide range of bacteria belonging to families as diverse as cyanobacteria, proteobacteria and actinobacteria. Among them, some siphoviruses have DNA in which adenine is totally replaced by another base, aminoadenine, through an unknown mechanism. Until now, only the enzyme responsible for the biosynthesis of aminoadenine had been identified in a siphovirus.

To understand how aminoadenine is integrated into the DNA of the virus, the researchers explored the databases available to them and searched for proteins homologous to the enzyme involved in aminoadenine biosynthesis. This led them to observe that other viruses in the siphovirus family contain a DNA polymerase enzyme, in addition to the homologous protein.

The characterization of this new DNA polymerase revealed its ability to use aminoadenine in place of adenine during DNA synthesis. This base modification makes the virus DNA resistant to the "restriction enzymes" of the infected bacteria, whose mission is to destroy any detected foreign DNA, thereby allowing it to survive in the host.

Although this study was conducted on four siphoviruses, the finding can likely be extrapolated to many other viruses in this family.

This work is the result of a collaboration between the Genoscope (CEA-Jacob), TESSSI (The European Syndicate of Synthetic Scientists and Industrialists), and the Institut Pasteur.