Protein S10 is a component of the 30S ribosomal subunit and participates together with NusB protein in processive transcription antitermination. The molecular mechanisms by which S10 can act as a translation or a transcription factor are not understood. We used complementation assays and recombineering to delineate regions of S10 dispensable for antitermination, and determined the crystal structure of a transcriptionally active NusB-S10 complex. In this complex, S10 adopts the same fold as in the 30S subunit and is blocked from simultaneous association with the ribosome. Mass spectrometric mapping of UV-induced crosslinks revealed that the NusB-S10 complex presents an intermolecular, composite, and contiguous binding surface for RNAs containing BoxA antitermination signals. Furthermore, S10 overproduction complemented a nusB null phenotype. These data demonstrate that S10 and NusB together form a BoxA-binding module, that NusB facilitates entry of S10 into the transcription machinery, and that S10 represents a central hub in processive antitermination.