Megalocytivirus， a DNA virus belonging to the Iridoviridae family， is a severe pathogen to a wide range of marine and freshwater fish. In this study， using turbot (Scophthalmus maximus) as a host model， we examined the immunoprotective property of one megalocytivirus gene， ORF75， in the form of DNA vaccine (named pORF75). Immunofluorescence microscopy and RT-PCR analysis showed that P444， the protein encoded by ORF75， was naturally produced in the tissues of turbot during megalocytivirus infection， and that the vaccine gene in pORF75 was expressed in fish cells transfected with pORF75 and in the tissues of turbot immunized with pORF75. Following vaccination of turbot with pORF75， a high level of survival (73%) was observed against a lethal megalocytivirus challenge. Consistently， viral replication in the vaccinated fish was significantly inhibited. Immune response analysis showed that pORF75-vaccinated fish (i) exhibited upregulated expression of the genes involved in innate and adaptive immunity， (ii) possessed specific memory immune cells that showed significant response to secondary antigen stimulation， and (iii) produced specific serum antibodies which， when co-introduced into turbot with megalocytivirus， blocked viral replication. Furthermore， whole-genome transcriptome analysis revealed that ORF75 knockdown altered the transcription of 43 viral genes. Taken together， these results indicate that ORF75 encoded a highly protective immunogen that is also a global transcription regulator of megalocytivirus.