Solution structure of the antitermination protein NusB of Escherichia coli: A novel all-helical fold for an RNA-binding protein

The NusB protein of Escherichia coli is involved in the regulation of rRNA biosynthesis by transcriptional antitermination. In cooperation with several other proteins, it binds to a dodecamer motif designated rrn boxA on the nascent rRNA. The antitermination proteins of E. coli are recruited in the replication cycle of bacteriophage λ, where they play an important role in switching from the lysogenic to the lytic cycle. Multidimensional heteronuclear NMR experiments were performed with recombinant NusB protein labelled with ¹³C, ¹⁵N and ²H. The three-dimensional structure of the protein was solved from 1926 NMR-derived distances and 80 torsion angle restraints. The protein folds into an α/α-helical topology consisting of six helices; the arginine-rich N-terminus appears to be disordered. Complexation of the protein with an RNA dodecamer equivalent to the rrn boxA site results in chemical shift changes of numerous amide signals. The overall packing of the protein appears to be conserved, but the flexible N-terminus adopts a more rigid structure upon RNA binding, indicating that the N-terminus functions as an arginine-rich RNA-binding motif (ARM).