Largemouth bass (Micropterus salmoides), a member of the family Centrarchidae, is a freshwater fish species originating from North America, introduced into Guangdong Province, China, in 1983(Guo et al., 2020. Nowadays, owing to its breeding advantages such as high adaptability, rapid growth and delicious flesh, largemouth bass becomes one of the most popular and important aquaculture species with extensive cultivation and breeding (Huang et al., 2021), and its annual production has already reached 477,808 tons (Ministry of Agriculture & Rural Affairs Fisheries Administration Bureau, 2020). Unfortunately, a new viral disease impacted the largemouth bass aquaculture industry in Guangdong Province, China, in 2011, resulting in 200,000 fish deaths and substantial economic losses (Ma et al., 2013). Subsequently, rhabdovirus was confirmed as the pathogen and several rhabdovirus isolates were reported (

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Bcl2 family proteins play a critical role in cell death or survival. BAX, the death‐promoting protein of bcl2 family, mediated mitochondrial pathway inducing cells’ apoptosis in mammal. MiRNAs have been implicated as negative regulators down‐regulating genes’ expression after post‐transcriptional level. At present, little is known about the regulatory mechanism of miRNA on the Bcl2 family proteins during CyHV‐2 infection in silver crucian carp (Carassius auratus gibelio). In this study, the ccBAX (silver crucian carp BAX) gene was cloned and expressed, and polyclonal antibodies were raised in mouse against the purified ccBAX‐GST fusion protein. The structure analysis indicated that ccBAX protein included four conserve domains (BH1, BH2, BH3 and transmembrane domains) and the expression of ccBAX protein occurred throughout the cells. Furthermore, two miRNAs (miR‐124 and miRNA‐29b) were identified to negatively regulate ccBAX gene expression in GiCF cell. miR‐124 was found to suppress the expression of WT‐ccBAX (wild type), but not the MT‐ccBAX (mutant). Overall, the results demonstrated that the expression of the ccBAX gene was significantly down‐regulated by miR‐124 in silver crucian carp (Carassius auratus gibelio) during CyHV‐2 infection.

Herpesviruses have been reported to be able to encode and express functional viral microRNAs that target both viral and cellular transcripts. In our previous studies, we found a new miRNA miR-KT-635 encoded by Cyprinid herpesvirus 2, which is predicted to target viral genes and cellular genes involved in innate immune signalling pathway and apoptosis. However, the function and target gene of miR-KT-635 are not proved. In this study, the regulating target gene of miR-KT-635 was proved as the viral gene ORF23 directly, the target point sequence on gene was verified and miR-KT-635 was identified to regulate the expression of ORF23 protein. According to the bioinformatics analysis, the tRNA domain and ribosome domain in the protein sequence of ORF23 were found to share high homology with R2i and P53R2i, which are related to the ribonucleotide reductase small subunit in the host (transform NTP to dNTP). Within expectations, silencing of viral ORF23 or transfecting miR-KT-635 mimics in Carassius auratus gibelio caudal fin cell line (GiCF) could suppress viral propagation significantly.