NusB

NusB is a bacterial transcription factor that plays a central role in transcription antitermination. It is encoded by the nusB gene and is a member of the Nus factor complex, which also includes NusA and NusE (also known as S10). NusB functions together with NusE and the BoxA RNA element to prevent premature termination of transcription by RNA polymerase at certain operons, most notably the ribosomal RNA (rRNA) operons, thereby promoting continuous transcription required for ribosome biogenesis.

Mechanism and interactions: NusB binds to the BoxA RNA sequence within nascent rRNA transcripts and interacts with NusE/S10 to form a complex that facilitates antitermination. This complex helps recruit and stabilize the transcription machinery in a way that allows RNA polymerase to read through terminator signals, ensuring high-level production of rRNA. NusA also participates in the larger antitermination complex, modulating the activity of RNA polymerase during this process.

Distribution and characteristics: NusB is widely conserved among bacteria and is often encoded in operons with nusA and nusE. The protein is typically around 130–150 amino acids in length and is found in the cytoplasm. While not universally essential, NusB is important for efficient rRNA synthesis and normal growth in many bacterial species; mutations or deletions can lead to reduced ribosome production and slower growth.

Structure and evolution: Structural studies in several bacteria show that NusB is a compact, conserved fold that provides a surface for RNA binding and protein–protein interactions with NusE and NusA. The NusB–NusE–BoxA interaction is a key feature of bacterial transcription regulation and reflects an ancient mechanism coordinating transcription elongation with ribosome biogenesis.