Characterization of the promoter region of bovine SIX4: Roles of E-box and MyoD in the regulation of basal transcription.

The sine oculis homeobox 4 (SIX4) gene belongs to the Six gene family and encodes an evolutionarily conserved transcription factor. Previous studies have demonstrated that SIX4 plays an essential role in proper muscle regeneration. However, the mechanisms regulating SIX4 transcription remain elusive. In the present study, we determined that bovine SIX4 was highly expressed in the longissimus thoracis and in undifferentiated Qinchuan cattle muscle cells (QCMCs) and that its protein localizes to both the cytoplasm and the nucleus. To elucidate the bovine the molecular mechanisms of SIX4 regulation, 1.3 kb of the 5'-regulatory region was obtained. MyoD and Ebox recognition sites were identified in the core promoter region at -522/-193 of the bovine SIX4 using a series of 5' deletion promoter plasmids in luciferase reporter assays. An electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay in combination with site-directed mutation and siRNA interference demonstrated that MyoD binding occurs at MyoD and Ebox recognition sites through direct and indirect mechanisms and play important roles in the transcriptional regulation of the bovine SIX4 promoter. Taken together, these interactions provide insight into the regulatory mechanisms of SIX4 transcription in mediating skeletal muscle growth in cattle.