Molecular Characterization and Expression Analysis of Toll-like Receptor 9 Gene in Turbot, Scophthalmus Maximus

Toll-like receptors (TLRs) have emerged as key sensors of invading microbes by recognizing pathogen-associated molecular patterns and activating innate immune responses. TLR9 functions as a pattern-recognition receptor that recognizes unmethylated CpG motifs in bacterial and viral DNA. In this study, the full cDNA and genomic sequences of tlr9 in Scophthalmus maximus (smtlr9) were identified and characterized. The full-length cDNA of smtlr9 was of 3754 bp encoding a polypeptide of 1066 amino acid residues. The genome sequence of smtlr9 was composed of 4083 nucleotides, including three exons and two introns. The putative Smtlr9 protein was characterized by a signal peptide sequence, a leucine-rich repeat domain (LRD) containing 15 leucine-rich repeats (LRRs), a transmembrane region, and a Toll/interleukin-1 receptor (TIR) domain. The putative protein shows the highest sequence identity (36.0~73.6%) to human and fish TLR9s. Phylogenetic analysis grouped Smtlr9 with other teleost Tlr9s. The smtlr9 mRNA was constitutively expressed in all tissues examined, with the highest level in the head kidney and kidney. The smtlr9 transcription level was up-regulated by ODN2395 in the muscle, head kidney, spleen, and gills.