Molecular cloning, structure and expressional profiles of two novel single-exon genes (PoCCR6A and PoCCR6B) in the Japanese flounder (Paralichthys olivaceus)

• Two chemokine receptors (PoCCR6A and PoCCR6B) of the Japanese flounder were cloned and characterized.
• PoCCR6A and PoCCR6B were the first reported single-exonic CCR6 genes among various species.
• PoCCR6A and PoCCR6B were highly expressed in mucosal tissues of the Japanese flounder.
• PoCCR6A and PoCCR6B exhibited upregulated expression patterns during bacterial infection in four immune tissues.

CCR6 is an important binding receptor of CCL20 and beta-defensins, and has multiple functions in the innate and acquired immune responses. In this study, we cloned the PoCCR6A and PoCCR6B genes of the Japanese flounder and studied the gene structure and expression patterns of these two genes in bacterial infection. The full-length PoCCR6A cDNA is 1415 bp and the open reading frame (ORF) is 1113 bp, encoding a 370-amino-acid peptide. The full-length PoCCR6B cDNA is 2193 bp and the ORF is 1029 bp, encoding a 363-amino-acid peptide. The structures of PoCCR6A and PoCCR6B indicate that they are single-exon genes. The predicted proteins encoded by PoCCR6A and PoCCR6B have the typical G-protein-coupled receptor (GPCR) family signature of seven transmembrane domains and several conserved structural features. A tissue distribution analysis showed that PoCCR6A is predominately expressed in the intestine, gill, and blood, and PoCCR6B in the gill, spleen, and liver. The expression patterns of the two chemokine receptors were analyzed during bacterial infection. In spleen and kidney, the expression of PoCCR6A was significantly upregulated at 24 h after infection, whereas the expression of PoCCR6B was steady at these time points. While in intestine, both of them were upregulated at 6 h–12 h after infection, and in gill the expression levels of them were upregulated at 24 h. The patterns of expression suggested that PoCCR6A and PoCCR6B play an important role in the immune response of the Japanese flounder, especially in the mucosal tissues.