Architecture of a transcribing-translating expressome

Area of Science:

• Molecular Biology
• Microbiology
• Structural Biology

Background:

• Transcription and translation are coupled in bacteria, but the underlying mechanism remains poorly understood.
• Understanding this coupling is crucial for comprehending gene expression regulation in prokaryotes.

Purpose of the Study:

• To elucidate the structural basis of transcription-translation coupling in *Escherichia coli*.
• To identify the molecular components and interactions involved in the formation of a transcribing-translating complex.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the expressome complex.
• In vitro biochemical assays and in vivo growth defect analyses were performed to investigate the role of specific protein domains.

Main Results:

• A defined "expressome" complex formed by RNA polymerase (RNAP) and the ribosome was structurally characterized.
• Continuous protection of ~30 nucleotides of mRNA was observed from the RNAP active center to the ribosome decoding center.
• The RNAP alpha subunit's carboxyl-terminal domain was identified as critical for RNAP-ribosome interaction and coupling.

Conclusions:

• The expressome structure provides a mechanistic explanation for how translation is coupled to transcription.
• This coupling, facilitated by the expressome, actively prevents transcriptional abnormalities like pausing and termination.
• The findings reveal a fundamental aspect of bacterial gene expression regulation and cellular fitness.