Phages with genomes >200 kb and >500 kb are referred to as jumbophages and megaphages, respectively, and are rarely isolated using conventional methods. Al-Shayeb, Sachdeva et al. performed metagenomic sequencing of DNA isolated from diverse ecosystems (for example, humans, animals, oceans, sediments, soils, deep subsurface habitats and the built environment) and found hundreds of phage genomes >200 kb in length, including a 735 kb genome, which the authors claim is the largest phage genome reported to date. The phylogeny of the phages was constructed, and ten ‘huge phage’ clades were defined, which are distributed across a broad bacterial host range. Thirty five of the huge phage genomes were manually curated, and large inventories of genes were found, including novel CRISPR–Cas systems and genes involved in translation, which the authors suggest may intercept host translation to redirect protein synthesis towards viral replication.
Al-Shayeb, B., Sachdeva, R. et al. Clades of huge phages from across Earth’s ecosystems. Nature https://doi.org/10.1038/s41586-020-2007-4 (2020)
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York, A. Too big to be ignored. Nat Rev Microbiol 18, 192 (2020). https://doi.org/10.1038/s41579-020-0341-z