Characterization and Complete Genome Sequences of Three N4-Like Roseobacter Phages Isolated from the South China Sea

Three bacteriophages (RD-1410W1-01, RD-1410Ws-07, and DS-1410Ws-06) were isolated from the surface water of Sanya Bay, northern South China Sea, on two marine bacteria type strains of the Roseobacter lineage. These phages have an isometric head and a short tail, morphologically belonging to the Podoviridae family. Two of these phages can infect four of seven marine roseobacter strains tested and the other one can infect three of them, showing relatively broader host ranges compared to known N4-like roseophages. One-step growth curves showed that these phages have similar short latent periods (1-2 h) but highly variable burst sizes (27-341 pfu cell(-1)). Their complete genomes show high level of similarities to known N4-like roseophages in terms of genome size, G + C content, gene content, and arrangement. The morphological and genomic features of these phages indicate that they belong to the N4likevirus genus. Moreover, comparative genomic analysis based on 43 N4-like phages (10 roseobacter phages and 33 phages infecting other lineages of bacteria) revealed a core genome of 18 genes shared by all the 43 phages and 38 genes shared by all the ten roseophages. The 38 core genes of N4-like roseophages nearly make up 70 % of each genome in length. Phylogenetic analysis based on the concatenated core gene products showed that our phage isolates represent two new phyletic branches, suggesting the broad genetic diversity of marine N4-like roseophages remains.