Structure of the 30S translation initiation complex coupled to paused RNA polymerase and its potential for riboregulation

In many bacterial species, transcription and translation can be coupled physically, with potential impact on the rates and efficiency of gene expression. Here, we present structural evidence from cryo-EM demonstrating that a bacterial RNA polymerase that is paused proximally to the promoter can associate with the pioneering 30S translation initiation complex (30S IC) through mutual binding of the transcription factor NusG. These findings suggest that the physical link between transcription and translation can be established prior to commitment to protein synthesis. Although the mRNA is embedded in this ‘early expressome’ complex, it can nonetheless interact with small regulatory RNA (sRNA) and be targeted for cleavage in the protein-coding region by the RNA degradosome assembly in vitro . The potential tagging of transcripts with sRNA during pioneering and subsequent stages of translation initiation, when the 30S IC is at the 5′ end of a polyribosome, may support surveillance processes that ensure efficient and rapid termination of gene expression in response to regulatory signals.