Abstract
The phAPEC6 genome encodes 551 predicted gene products, with the vast majority (83%) of unknown function. Of these, 62 have been identified as virion-associated proteins by mass spectrometry (ESI-MS/MS), including the major capsid protein (Gp225; present in 1620 copies), which shows a HK97 capsid protein-based fold. Cryo-electron microscopy experiments showed that the 350-kbp DNA molecule of Escherichia coli virus phAPEC6 is packaged in at least 15 concentric layers in the phage capsid. A capsid inner body rod is also present, measuring about 91 nm by 18 nm and oriented along the portal axis. In the phAPEC6 contractile tail, 25 hexameric stacked rings can be distinguished, built of the identified tail sheath protein (Gp277). Cryo-EM reconstruction reveals the base of the unique hairy fibers observed during an initial transmission electron microscopy (TEM) analysis. These very unusual filaments are ordered at three annular positions along the contractile sheath, as well as around the capsid, and may be involved in host interaction.
Highlights
Jumbo phages are tailed phages with a genome size of more than 200 kb [1]
More and more jumbo phages infecting different bacterial genera like Bacillus, Escherichia, Klebsiella, Pseudomonas, Yersinia, Erwinia and Ralstonia [2,3] have been isolated from a range of different environments, including water, soil, marine sediments, plant tissues, silkworms, composts and animal feces
Avian pathogenic Escherichia coli virus phAPEC6 was isolated during a search for candidates for phage therapy applications, but this phage drew our attention because of its large phage head, “whisker-like” tail fiber proteins and hairy appendages on the capsid and tail (Figure 1)
Summary
Jumbo phages are tailed phages with a genome size of more than 200 kb [1]. the largest described jumbo phage, Bacillus megaterium phage G (498 kb), was discovered over 40 years ago, most giant phages primarily remained unnoticed, due to their large virion size in combination with classical phage isolation techniques. Large viruses can display specific structures like long, wavy and curly tail fibers, which have rarely been observed [9], and their large genomes encode DNA polymerases, RNA polymerases, endolysins, chitinases, glycoside hydrolases, lyases and many other genes with unknown functions [3]. As such, they are a distinct and diverse group of phages, but only roughly 100 different jumbo phages have been isolated since their discovery [10,11], and only a few have been structurally classified [8,12].
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