Expanding the Mimiviridae family using asparagine synthase as a sequence bait

Since the pioneering Global Ocean Sampling project, large-scale sequencing of environmental DNA has become a common approach to assess the biodiversity of diverse environments, with an emphasis on microbial populations: unicellular eukaryotes (“protists”), bacteria, archaea, and their innumerous associated viruses and phages. However, the global analysis of the viral diversity (“the virome”) from sequence data is fundamentally hampered by the lack of a universal gene that would allow their unambiguous identification and reliable separation from cellular microorganisms. The problem has been made even more difficult with the discovery of micron-sized giant viruses for which the usual fractionation protocol on a “sterilizing” filter is no longer an option. In the present proof-of-principle work we used actual metagenomic data to show that glutamine-hydrolysing asparagine synthase is a reliable sequence probe to discover new members of the Mimiviridae family, hint at the existence of a new family of large DNA viruses, and point out misidentified database entries.