Abstract
Krüppel-like factors (KLFs) are transcription factors that bind DNA via three tandem zinc fingers, critically regulating cellular processes like proliferation, differentiation, and apoptosis. Among KLFs, KLF9 is particularly notable for its immunological roles in mammalian immune systems, where it regulates tumor growth, modulates interferon-related genes and inflammatory cytokines. However, the potential role of klf9 in fish immune responses remains poorly understood. In this study, we characterized turbot (Scophthalmus maximus L.) klf9 (Sm_klf9) and explored its role against bacterial infection. In our results, the full length of Sm_klf9 was 3,372 bp, including an open reading frame (ORF) of 684 bp. In phylogenetic analyses, Sm_klf9 grouped closely with the klf9 genes of Japanese flounder and Atlantic halibut, consistent with the broader phylogenetic relationships among flatfish species. Sm_klf9 exhibited widespread expression across all tested tissues in turbot with the highest expression level in blood. The expression of Sm_klf9 was significantly induced in a time-dependent manner in turbot intestine after bacterial infection (dramatic upregulation with 2.78-fold at 2 h, followed by significant downregulations with 2.29- and 2.74-fold at 12 h and 24 h, respectively), and the urgently significant induction at 2h suggested rapid response measures against bacterial infection at the early stages. The overexpression of Sm_klf9 up-regulated the key immune-related genes in the NF-κB signaling pathway, such as NF-κB2 and Myd88. Meanwhile, the overexpression of Sm_klf9 was found to accelerate the invasion of Edwardsiella tarda and promote the cell apoptosis in turbot intestine cells. Overall, the findings in this study indicated the important roles of Sm_klf9 in turbot immune system during pathogenic infection, which could provide a theoretical basis for investigating the key roles of Sm_klf9 in bacteriostatic ability and inflammatory regulations in turbot during bacterial infection.