Identification and expression of SRF targeted by miR-133a during early development of Paralichthys olivaceus.

Serum response factor (SRF) is a MADS-box transcription factor that regulates the expression of genes involved in development, metabolism, cell proliferation, and differentiation. In the present study, we cloned the full-length SRF cDNA which includes the coding region of 1503 bp, a 573-bp 5'untranslated region (UTR) and a 400-bp 3'-UTR. The deduced 501 amino acid sequence of the SRF protein contained a MADS domain and NLS at the N terminus, similar to other organisms, and it also is highly phylogenetically conserved. SRF mRNA is ubiquitously expressed in various tissues, with the highest level in the kidneys, and it is also highly expressed during the embryonic and metamorphic stages. During metamorphosis, the SRF mRNA levels are down-regulated by exogenous thyroid hormone (TH) at 17 dph and by thiourea (TU) at 29, 36, and 41 dph, whereas SRF mRNA levels were significantly up-regulated by the added exogenous TH to the TU-treated larvae at 41 dph, which indicates that thyroid hormone is essential for expression of SRF mRNA, so, higher levels of TH did not result in changes of SRF mRNA levels, while TH deficiency or inhibited by the non-specific TU toxicity cause down-regulation of SRF mRNA, which indicated that TH can indirectly affect the SRF mRNA levels. Meanwhile, using a luciferase reporter assay, we verified that SRF is a common target gene of miR-133a which is a muscle-specific microRNA (miRNA), which indicated that SRF may be involved in the signaling pathway of miRNA that regulates muscle development.