An α Helix to β Barrel Domain Switch Transforms the Transcription Factor RfaH into a Translation Factor

NusG homologs regulate transcription and coupled processes in all living organisms. The Escherichia coli (E. coli) two-domain paralogs NusG and RfaH have conformationally identical N-terminal domains (NTDs) but dramatically different carboxy-terminal domains (CTDs), a β barrel in NusG and an α hairpin in RfaH. Both NTDs interact with elongating RNA polymerase (RNAP) to reduce pausing. In NusG, NTD and CTD are completely independent, and NusG-CTD interacts with termination factor Rho or ribosomal protein S10. In contrast, RfaH-CTD makes extensive contacts with RfaH-NTD to mask an RNAP-binding site therein. Upon RfaH interaction with its DNA target, the operon polarity suppressor (ops) DNA, RfaH-CTD is released, allowing RfaH-NTD to bind to RNAP. Here, we show that the released RfaH-CTD completely refolds from an all-α to an all-β conformation identical to that of NusG-CTD. As a consequence, RfaH-CTD binding to S10 is enabled and translation of RfaH-controlled operons is strongly potentiated. [Display omitted] [Display omitted] ► RfaH dramatically refolds upon recruitment to the transcription elongation complex ► The RfaH C-terminal domain (CTD) switches from an α-helical to a β sheet conformation ► Refolding is sufficient to enable RfaH as a regulator of translation ► Refolded RfaH-CTD binds ribosomal protein S10 and recruits the ribosome to mRNA RfaH dramatically refolds upon binding the transcription elongation complex, allowing it to recruit ribosomal protein S10 and providing a direct link between transcriptional elongation and translation initiation.