A novel virus potentially evolved from the N4-like viruses represents a unique viral family: Poorviridae.

Pseudoalteromonas are widely distributed in marine extreme habitats and exhibit diverse extracellular protease activity, which is essential for marine biogeochemical cycles. However, our understanding of viruses that infect Pseudoalteromonas remains limited. This study isolated a virus infecting Pseudoalteromonas nigrifaciens from Xiaogang in Qingdao, China. vB_PunP_Y3 comprises a linear, double-strand DNA genome with a length of 48,854 bp, encoding 52 putative open reading frames. Transmission electron microscopy demonstrates the short-tailed morphology of vB_PunP_Y3. Phylogenetic and genome-content-based analysis indicate that vB_PunP_Y3 represents a novel virus family named as Poorviridae, along with three high-quality uncultivated viral genomes. Biogeographical analyses show that Poorviridae is distributed across five viral ecological zones, and is predominantly detected in the Antarctic, Arctic, and bathypelagic zones. Comparative genomics analyses identified three of the seven hallmark proteins of N4-like viruses (DNA polymerase, major capsid protein, and virion-encapsulated RNA polymerase) from vB_PunP_Y3, combing with the protein tertiary structures of the major capsid protein, suggesting that vB_PunP_Y3 might evolve from the N4-like viruses.

Importance: vB_PunP_Y3 is a unique strain containing three of the seven hallmark proteins of N4-like viruses, but is grouped into a novel family-level viral cluster with three uncultured viruses from metagenomics, named Poorviridae. This study enhanced the understanding about the genetic diversity, evolution, and distribution of Pseudoalteromonas viruses and provided insights into the novel evolution mechanism of marine viruses.