Distinct Phage-Encoded Enzymes for Substitution of Deoxythymidine by Deoxyuridine in Phage Genomes

DNA base modification is a common strategy used by bacteriophages to evade host immune detection. A prominent example is dU-DNA, where thymidine is globally replaced with 2'-deoxyuridine. Despite its widespread occurrence, dU-DNA's biosynthetic pathways and functional roles remain incompletely understood. Here, different enzymes supporting dU-DNA biosynthesis in phage PBS1, Roseophage DSS3_VP1, and Yersiniophage PhiR1-37 are identified and characterized. The dU nucleotide precursor is supplied by phage dCTP deaminases (Dcds). Thymidine nucleotides are degraded by dTMP phosphatases (Dtms) in PBS1 and DSS3_VP1, and by a dTTP pyrophosphatase (Dtt) in PhiR1-37, preventing incorporation into phage DNA. The dU-DNA, isolated from Yersiniophage PhiR1-37 or synthesized by PCR, demonstrates resistance to cleavage by restriction enzymes recognizing thymidine-containing sequences, partial resistance to LbCas12a nuclease recognizing a TTTV PAM site, while remaining sensitive to SpCas9 nuclease recognizing a NGG PAM site. A phylogenetic analysis of PBS1 dCTP deaminase and closely related T4 phage dCMP deaminase suggest possible evolutionary origins from bacterial dCDP deaminases. Overall, these findings suggest independent acquisition of dU-DNA biosynthetic enzymes and pathways in the diverse phages, and support its protective function against different host-encoded nucleases.