Involved in transcription antitermination. Required for transcription of ribosomal RNA (rRNA) genes. Binds specifically to the boxA antiterminator sequence of the ribosomal RNA (rrn) operons. Monomer or homodimer; in equilibrium, with a preference for the monomer. Dimerization may be employed to package NusB in an inactive form until recruitment into antitermination complexes. Belongs to the NusB family.