Structural basis of transcription-translation coupling

Area of Science:

• Molecular Biology
• Microbiology
• Structural Biology

Background:

• Transcription and translation are coupled in bacteria, with RNA polymerase (RNAP) and ribosomes coordinating mRNA synthesis and protein synthesis.
• Transcription factors NusG and NusA are known to modulate this coupling process.
• The precise mechanisms of NusG bridging and NusA binding within transcription-translation complexes (TTCs) remained unclear.

Purpose of the Study:

• To elucidate the structural basis of transcription-translation coupling in *Escherichia coli*.
• To determine how transcription factors NusG and NusA interact with RNAP and ribosomes during coupled processes.
• To characterize different states of transcription-translation complexes (TTCs) based on mRNA spacer length.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine high-resolution structures of *Escherichia coli* transcription-translation complexes (TTCs).
• Complexes with varying lengths of messenger RNA (mRNA) spacers between RNAP and the ribosome were analyzed.
• Structural analysis focused on the binding interfaces and conformational states of RNAP, ribosomes, NusG, and NusA.

Main Results:

• Two distinct TTC states were identified based on mRNA spacer length: TTC-A (short spacers) and TTC-B (longer spacers).
• TTC-A structures showed a conformation incompatible with NusG bridging and NusA binding.
• TTC-B structures revealed a novel conformation enabling NusG bridging and NusA binding, detailing their interaction sites.

Conclusions:

• The study reveals distinct structural states of bacterial transcription-translation complexes (TTCs) dependent on mRNA spacer length.
• A new state, TTC-B, facilitates NusG and NusA binding, mediating NusG- and NusA-dependent transcription-translation coupling.
• These findings provide critical insights into the molecular mechanisms governing the coordination of gene expression in bacteria.