Part of the processive rRNA transcription and antitermination complex (rrnTAC). The complex forms an RNA-chaperone ring around the RNA exit tunnel of RNA polymerase (RNAP). It supports rapid transcription and antitermination of rRNA operons, cotranscriptional rRNA folding, and annealing of distal rRNA regions to allow correct ribosome biogenesis (PubMed:32871103). 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 (PubMed:7045592, PubMed:7678781, PubMed:11884128, PubMed:14973028, PubMed:15716433, PubMed:16109710). The affinity of NusB for the boxA RNA sequence is significantly increased in the presence of the ribosomal protein S10 (PubMed:16109710, PubMed:11884128). NusB may serve as a loading factor that ensures efficient entry of S10 into the transcription complexes (PubMed:19111659). It also modulates the rrn boxA-mediated transcription elongation rates (PubMed:10383769). Monomer in solution (PubMed:10881193, PubMed:16109710). Forms a heterodimer with the ribosomal protein S10 (PubMed:1731086, PubMed:11884128, PubMed:16109710, PubMed:19111659). The rRNA transcription and antitermination complex (rrnTAC) consists of RNAP, NusA, NusB, NusE (rpsJ), NusG, SubB, ribosomal protein S4, DNA and precursor rRNA; S4 is more flexible than other subunits (PubMed:32871103). The N-terminal region functions as an arginine-rich RNA-binding motif (ARM). Inactivation of the gene causes cold-sensitive cell growth. Belongs to the NusB family.