The crystal structure of NusB from Mycobacterium tuberculosis

Both prokaryotes and eukaryotes regulate transcription through mechanisms that suppress termination signals. An antitermination mechanism was first characterized in bacteriophage lambda. Bacteria have analogous machinery that regulates ribosomal RNA transcription and employs host factors, called the N-utilizing (where N stands for the phage lambda N protein) substances (Nus), NusA, NusB, NusE and NusG. Here we report the crystal structure of NusB from Mycobacterium tuberculosis, the bacterium that causes tuberculosis in humans. This molecule shares a similar tertiary structure with the related Escherichia coli protein but adopts a different quaternary organization. We show that, unlike the E. coli homolog, M. tuberculosis NusB is dimeric both in solution and in the crystal. These data help provide a framework for understanding the structural and biological function of NusB in the prokaryotic transcriptional antitermination complex.