Strzelecka T E, Clore G M, Gronenborn A M
Laboratory of Chemical Physics, National Institutes of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520, USA.
Structure. 1995 Oct 15;3(10):1087-95. doi: 10.1016/s0969-2126(01)00244-1.
The Mu Ner protein is a small (74 amino acids), basic, DNA-binding protein found in phage Mu. It belongs to a class of proteins, the cro and repressor proteins, that regulate the switch from the lysogenic to the lytic state of the phage life cycle. There is no significant sequence identity between Mu Ner and the cro proteins of other phages, despite their functional similarity. In addition, there is no significant sequence identity with any other DNA-binding proteins, with the exception of Ner from the related phage D108 and the Nlp protein of Escherichia coli. As the tertiary structures of Mu Ner and these two related proteins are unknown, it is clear that a three-dimensional (3D) structure of Mu Ner is essential in order to gain insight into its mode of DNA binding.
The 3D solution structure of Mu Ner has been solved by 3D and 4D heteronuclear magnetic resonance spectroscopy. The structure consists of five alpha helices, two of which comprise a helix-turn-helix (HTH) motif. Analysis of line broadening and disappearance of crosspeaks in a 1H-15N correlation spectrum of the Mu Ner-DNA complex suggests that residues in these two helices are most likely to be in contact with the DNA.
Like the functionally analogous cro proteins from phages lambda and 434, the Mu Ner protein possesses a HTH DNA recognition motif. The Ner protein from phage D108 and the Nlp protein from E. coli are likely to have very similar tertiary structures due to high amino-acid-sequence identity with Mu Ner.
Mu Ner蛋白是一种存在于噬菌体Mu中的小蛋白（74个氨基酸），呈碱性，具有DNA结合能力。它属于一类蛋白质，即cro蛋白和阻遏蛋白，这类蛋白调节噬菌体生命周期从溶原状态到裂解状态的转换。尽管Mu Ner与其他噬菌体的cro蛋白功能相似，但它们之间没有显著的序列同一性。此外，除了来自相关噬菌体D108的Ner蛋白和大肠杆菌的Nlp蛋白外，Mu Ner与其他任何DNA结合蛋白都没有显著的序列同一性。由于Mu Ner以及这两种相关蛋白的三级结构未知，显然Mu Ner的三维（3D）结构对于深入了解其DNA结合模式至关重要。
Mu Ner的3D溶液结构已通过3D和4D异核磁共振光谱法解析。该结构由五个α螺旋组成，其中两个构成螺旋-转角-螺旋（HTH）基序。对Mu Ner-DNA复合物的1H-15N相关光谱中的线宽展宽和交叉峰消失进行分析表明，这两个螺旋中的残基最有可能与DNA接触。
与来自噬菌体λ和434的功能类似的cro蛋白一样，Mu Ner蛋白具有HTH DNA识别基序。由于与Mu Ner具有高度的氨基酸序列同一性，噬菌体D108的Ner蛋白和大肠杆菌的Nlp蛋白可能具有非常相似的三级结构。
