Studies on the NusB protein of Escherichia coli expression and determination of secondary-structure elements by multinuclear NMR spectroscopy

The product of the nusB gene of Escherichia coli modulates the efficiency of transcription termination at nut (N utilization) sites of various bacterial and bacteriophage λ genes. Similar control mechanisms operate in eukaryotic viruses (e.g. human immunodeficiency virus). A recombinant strain of E. coli producing relatively large amounts of NusB protein (about 10% of cell protein) was constructed. The protein could be purified with high yield by anion-exchange chromatography followed by gel-permeation chromatography. The protein is a monomer of 15.6 kDa as shown by analytical ultracentrifugation. Structural studies were performed using protein samples labelled with ¹⁵N, ¹³C and ²H in various combinations. Heteronuclear three-dimensional triple-resonance NMR experiments combined with a semi-automatic assignment procedure yielded the sequential assignment of the ¹H, ¹³C and ¹⁵N backbone resonances. Based on experimentally derived scalar couplings, chemical shift values, amide-exchange data, and a semiquantitative interpretation of NOE data, the secondary structure of NusB has classified as α helical, comprising seven α helices.