Weintraub H, Davis R, Lockshon D, Lassar A
Fred Hutchinson Cancer Research Center, Seattle, WA 98104.
Proc Natl Acad Sci U S A. 1990 Aug;87(15):5623-7. doi: 10.1073/pnas.87.15.5623.
MyoD is a master regulatory gene for myogenesis. Its product, the MyoD protein, appears to act by binding to muscle-specific enhancer sequences. We show that MyoD binds cooperatively to two sites in the muscle-specific creatine kinase enhancer; this is dramatically reflected in dissociation-rate measurements. A deletion of the acidic N terminus (residues 3-56) results in a protein that binds normally to single sites but fails to bind cooperatively to two adjacent sites, suggesting a role of the N terminus in cooperative interactions. In transfection assays, a reporter gene flanked by a single MyoD binding site fails to be activated by cotransfected MyoD expression vectors. In contrast, a reporter with two or more MyoD binding sites is activated by wild-type MyoD but not by N-terminally deleted MyoD. A reporter gene with a single binding site, although not activated by MyoD, can nonetheless compete for expression with a reporter gene containing three sites. Thus, in vivo, a single site can bind MyoD, but occupancy of two or more sites is required for subsequent transcriptional activation.
肌细胞生成素（MyoD）是成肌作用的主要调控基因。其产物MyoD蛋白似乎通过与肌肉特异性增强子序列结合来发挥作用。我们发现MyoD能协同结合到肌肉特异性肌酸激酶增强子的两个位点；这在解离速率测量中得到了显著体现。酸性N端（第3 - 56位氨基酸残基）的缺失导致一种蛋白，它能正常结合单个位点，但不能协同结合两个相邻位点，这表明N端在协同相互作用中发挥作用。在转染实验中，一个由单个MyoD结合位点侧翼的报告基因不会被共转染的MyoD表达载体激活。相反，一个带有两个或更多MyoD结合位点的报告基因会被野生型MyoD激活，但不会被N端缺失的MyoD激活。一个带有单个结合位点的报告基因，虽然不会被MyoD激活，但仍能与一个含有三个位点的报告基因竞争表达。因此，在体内，单个位点能结合MyoD，但后续转录激活需要两个或更多位点被占据。
