A pentose, as a cytosine nucleobase modification in Shewanella phage Thanatos genomic DNA, mediates enhanced resistance toward host restriction systems.

Co-evolution of bacterial defense systems and phage counter-defense mechanisms has resulted in an intricate biological interplay between bacteriophages and their prey. In order to evade nuclease-based mechanisms that target DNA, various bacteriophages modify their nucleobases, which impedes or even inhibits the recognition and restriction by endonucleases. We found that Shewanella phage Thanatos DNA is insensitive to multiple restriction enzymes and also to Cas I-Fv and Cas9 cleavage. Furthermore, with nanopore sequencing, the phage DNA showed severely impaired basecalling. In addition to an adenine methylation, the data indicated an additional, much more substantial nucleobase modification. Using liquid chromatography-mass spectrometry (LC-MS), we identified an unknown configuration of a deoxypentose attached to cytosine as an undiscovered modification of phage DNA, which is present in Thanatos genomic DNA and likely mediates resistance to restriction endonucleases, as well as reducing Cas nuclease activity significantly. To elucidate the underlying enzyme functions, we identified structural homologs of Thanatos proteins among known glycosyltransferase folds and experimentally proved a UDP-xylose pyrophosphorylase function of the phage protein TH1_063 by in vitro. Inactivation of TH1_060 leads to an almost complete inhibition of phage propagation, indicating an important role of the cytosine modification in phage survival and/or proliferation. IMPORTANCE: Several phages extensively decorate their DNA building blocks, providing an effective protection against various host and phage-produced restriction systems. These modifications allow the phages to distinguish between their own DNA and that of the host, significantly increasing the establishment of the phage chromosome upon entry into the host and subsequent phage proliferation. Several different modifications have been previously identified and characterized. Here, we describe a hitherto unknown cytosine modification, consisting of a deoxypentose-putatively xylose-that provides protection against various bacterial restriction systems, including DNA-targeting CRISPR/Cas systems. Our findings expand the range of DNA modifications that phages use for protection.