No fitness impact of the knockout of the two main components of mimivirus genomic fiber and fibril layer

Abstract

AbstractThe Mimivirus 1.2Mb genome is organized into a nucleocapsid-like genomic fiber composed of a mixture of two GMC-oxidoreductase paralogs (1). Surprisingly, these proteins also constitute the glycosylated fibril layer that decorates the virion (2). The individual inactivation of each gene by homologous recombination-based knockout (KO) confirmed that they are not essential. The resulting genomic fiber corresponds to a 5- and a 6-start helix, similar to the wild type structures, and is composed of the remaining GMC-oxidoreductase. A model is proposed explaining the transition from a 6- to a 5-start helix and their coexistence. The two single mutants and the double mutant, for which both genes have been KO, showed equivalent fitness in laboratory conditions with no impact on the glycosylation of the layer of fibrils surrounding the capsids. Their major protein components can be easily exchanged, as highlighted by the double KO fibril layer composition. Accordingly, we show that the proteins composing the fibril layer can already be different between different members of the family (moumouviruses and megaviruses), echoing their differences in glycan composition (3, 4). These results obtained on two distinct essential processes (genome packaging and virion infectivity) illustrate the exceptional functional redundancy of theMimiviridae, which may be the major evolutionary force behind their giant genomes.One-Sentence Summary: Functional redundancy warrants mimivirus genomic fiber and fibril layer formation.Subject Area: Biological sciences/Microbiology/Virology/Viral evolution; Biological sciences/Microbiology/Virology/Virus structures