On the complexity of giant viruses

Icosahedral dsDNA viruses utilize a single portal for genome delivery and packaging. The structural similarity of these portals in diverse viruses, as well as their invariable positioning at a unique icosahedral vertex, led to the consensus that a highly conserved vertex-portal architecture is essential for viral DNA translocations. By using high resolution imaging techniques we found the first exception to this paradigm. We demonstrate that genome delivery and packaging in the largest virus heretofore identified, the Acanthamoeba polyphaga Mimivirus, occur through two distinct portals that allow for highly effective translocation of the huge viral genome. Moreover, we show that the replication and assembly of the Mimivirus proceed exclusively in the host cytoplasm within highly ordered viral factories. These observations provide new insights on the enormous complexity that characterizes the infection cycles of large viruses.