Bacteria encode diverse anti-phage systems, such as CRISPR-Cas and restriction modification (RM), which limit infection by targeting phage DNA. We identified a DNA modification in phages, i.e., 5-arabinosyl-hydroxy-cytosine (5ara-hC), which adds arabinose to cytosines via a hydroxy linkage and protects phage from DNA targeting. The hydroxy linkage was common among arabinoslyated phages, with some arabinosylated phages encoding arabinose-5ara-hC transferases (Aat) that add a second or third arabinose to DNA. DNA arabinosylation enables evasion from DNA-targeting type I CRISPR-Cas and type II RM systems. However, arabinosylated phages remain sensitive to RNA-targeting CRISPR-Cas (type III and VI) and promiscuous type IV restriction endonucleases. 5ara-hC enables evasion of glycosylase defenses that target phages with glucosylated hydroxymethyl cytosines, and 5ara-ara-hC protects against some defenses capable of targeting 5ara-hC-modified phages. Collectively, this work identifies DNA modifications that enable phages to evade multiple defenses yet remain vulnerable to some systems that target RNA or modified nucleobases.

Serratia sp. ATCC 39006 is a Gram-negative bacterium that has been used to study the function of phage defences, such as CRISPR-Cas, and phage counter-defence mechanisms. To expand our phage collection to study the phage-host interaction with Serratia sp. ATCC 39006, we isolated the T4-like myovirus LC53 in Ōtepoti Dunedin, Aotearoa New Zealand. Morphological, phenotypic and genomic characterization revealed that LC53 is virulent and similar to other Serratia, Erwinia and Kosakonia phages belonging to the genus Winklervirus. Using a transposon mutant library, we identified the host ompW gene as essential for phage infection, suggesting that it encodes the phage receptor. The genome of LC53 encodes all the characteristic T4-like core proteins involved in phage DNA replication and generation of viral particles. Furthermore, our bioinformatic analysis suggests that the transcriptional organization of LC53 is similar to that of Escherichia coli phage T4. Importantly, LC53 encodes 18 tRNAs, which likely compensate for differences in GC content between phage and host genomes. Overall, this study describes a newly isolated phage infecting Serratia sp. ATCC 39006 that expands the diversity of phages available to study phage-host interactions.

In recent years, bacteriophage research has been boosted by a rising interest in using phage therapy to treat antibiotic-resistant bacterial infections. In addition, there is a desire to use phages and their unique proteins for specific biocontrol applications and diagnostics. However, the ability to manipulate phage genomes to understand and control gene functions, or alter phage properties such as host range, has remained challenging due to a lack of universal selectable markers. Here, we discuss the state-of-the-art techniques to engineer and select desired phage genomes using advances in cell-free methodologies and clustered regularly interspaced short palindromic repeats-CRISPR associated protein (CRISPR-Cas) counter-selection approaches.