Abstract
Escherichia phage N4 was isolated in 1966 in Italy and has remained a genomic orphan for a long time. It encodes an extremely large virion-associated RNA polymerase unique for bacterial viruses that became characteristic for this group. In recent years, due to new and relatively inexpensive sequencing techniques the number of publicly available phage genome sequences expanded rapidly. This revealed new members of the N4-like phage group, from 33 members in 2015 to 115 N4-like viruses in 2020. Using new technologies and methods for classification, the Bacterial and Archaeal Viruses Subcommittee of the International Committee on Taxonomy of Viruses (ICTV) has moved the classification and taxonomy of bacterial viruses from mere morphological approaches to genomic and proteomic methods. The analysis of 115 N4-like genomes resulted in a huge reassessment of this group and the proposal of a new family “Schitoviridae”, including eight subfamilies and numerous new genera.
Highlights
Escherichia phage N4 is a virulent phage that was originally isolated by Gian Carlo Schito from sewers in Genoa (Italy) in 1966 [1]
The distinct clustering of the newly proposed family was confirmed with a gene-sharing network analysis using vConTACT2 (Figure 3), where the N4-like viruses cluster clearly separates from all other dsDNA bacterial viruses
N4-like viruses infecting Achromobacter xylosoxidans were grouped into four genera in the proposed subfamily “Rothmandenesvirinae” in honour of Lucia Rothman-Denes, who worked on N4 and its RNA polymerases
Summary
Escherichia phage N4 is a virulent phage that was originally isolated by Gian Carlo Schito from sewers in Genoa (Italy) in 1966 [1]. The last report of the Bacterial and Archaeal Viruses Subcommittee [11] presented the new taxonomic classifications and reassessments that were achieved in 2018 and 2019 and listed a new order (Tubulavirales), ten new families, 22 new sub-families, 424 new genera and 964 new species, which still represent only a fraction of the genomes currently available. It has to be taken into account that ICTV does not classify viral strains or variants, i.e., those phage isolates with genomes that show ≥95% DNA sequence identity with an exemplar isolate of a species [12]. This study provides further insight into the diversity and taxonomy of N4-like viruses using different approaches like genome-based phylogeny for deeper classification
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