Abstract
In bacteria, transcription and translation are coupled processes, in which movement of RNA polymerase (RNAP) synthesizing mRNA is coordinated with movement of the first ribosome translating mRNA. Coupling is modulated by the transcription factors NusG--which is thought to bridge RNAP and ribosome--and NusA. Here, we report cryo-EM structures of Escherichia coli transcription-translation complexes (TTCs) containing different-length mRNA spacers between RNAP and the ribosome active-center P-site. Structures of TTCs containing short spacers show a state incompatible with NusG bridging and NusA binding (TTC-A; previously termed “expressome”). Structures of TTCs containing longer spacers reveal two new states, one compatible with NusG bridging and both compatible with NusA binding (TTC-B and TTC-C). We propose that TTC-B mediates NusG-dependent transcription-translation coupling and TTC-C mediates NusG-independent transcription-translation coupling.
One Sentence Summary Cryo-EM defines states that mediate NusG-dependent and NusG-independent transcription-translation coupling in bacteria