A key gene of the small RNA pathway in the flounder, Paralichthys olivaceus: identification and functional characterization of dicer.

Dicer is critical for producing mature microRNAs (miRNAs) from precursor molecules and small interfering RNAs and plays an important role in controlling development and metabolism. In the present study, we cloned the flounder dicer gene, which is 6585 nucleotides (nt), including a 5'-untranslated region (UTR) of 231 nt, a 3'-UTR of 663 nt and an open reading frame of 5691 nt encoding a polypeptide of 1897 amino acids, and analyzed the conservation and expression pattern of dicer. The tissue distribution analysis indicated that dicer is abundantly expressed in the brain, heart, liver, spleen, stomach, kidney, gill, muscle, intestine and gonad of adult fish. Temporal expression analysis indicated that dicer mRNA is highly expressed during the embryonic and early larval stages, and exhibits low expression during the metamorphic stages. Treatment with thyroid hormone (TH) or thiourea indirectly or directly up-regulated dicer mRNA levels at 17 and 23 dph, whereas treatment with TH down-regulated dicer mRNA levels at 36 dph. The dicer-specific siRNA significantly down-regulated dicer mRNA and pol-let-7d levels, while pol-let-7d precursor levels were not differentially changed compared with the control (NC). These results demonstrated that dicer plays a key role in development and metabolism through the production of mature miRNAs, providing basic information for further studies concerning the role of dicer in Paralichthys olivaceus development.