ADRV 12L: A Ranaviral Putative Rad2 Family Protein Involved in DNA Recombination and Repair

The Andrias davidianus ranavirus (ADRV) is a member of the family Iridoviridae and belongs to the nucleocytoplasmic large DNA viruses. Based on genomic analysis, an ADRV-encoding protein, ADRV 12L, and its homologs from other iridoviruses were predicted as Rad2 family proteins based on the conserved amino acids, domains, and secondary structures. Expression analysis showed that the transcription of ADRV 12L started at 4 h post infection, and its expression was not inhibited by a DNA-replication inhibitor. Meanwhile, immunofluorescence localization showed that ADRV 12L mainly localized in viral factories and colocalized with the viral nascent DNA, which hinted at a possible role in DNA replication. Furthermore, a mutant ADRV lacking 12L (ADRV-Δ12L) was constructed. In both luciferase assays based on homologous recombination (HR) and double-strand break repair (DSBR) that followed, ADRV-Δ12L induced less luciferase activity than the wild-type ADRV, indicating that HR and DSBR were impaired in ADRV-Δ12L infected cells. In addition, infection with ADRV-Δ12L resulted in smaller plaque sizes and lower viral titers than that with wild-type ADRV, indicating an important role for 12L in efficient virus infection. Therefore, the results suggest that Rad2 homologs encoded by iridovirus have important roles in HR- and DSBR-process of the viral DNA and, thus, affect virus replication and the production of progeny virions.