Structural basis of long-range transcription-translation coupling.

Area of Science:

• Molecular Biology
• Structural Biology
• Microbiology

Background:

• Transcription-translation coupling is essential for gene expression in bacteria like E. coli.
• Previously reported coupled transcription-translation complexes (TTC-B) involve direct interactions between RNA polymerase (RNAP) and ribosomes, bridged by factors like NusG and NusA.

Purpose of the Study:

• To characterize novel molecular assemblies mediating transcription-translation coupling.
• To define a new class of coupled transcription-translation complexes (TTC-LC) that accommodate longer mRNA spacers.

Main Methods:

• Determination of molecular structures of coupled transcription-translation complexes.
• Comparative analysis of TTC-B and the newly identified TTC-LC structures.
• Investigation of the functional implications of TTC-LC in transcription termination.

Main Results:

• A new class of complex, TTC-LC (long-range coupling), was identified, characterized by a ~60° rotation and ~70 Å translation of RNAP relative to the ribosome.
• TTC-LC features a ~70 Å gap between RNAP and ribosome, accommodating longer mRNA spacers by looping.
• TTC-LC functions as a dynamic intermediate in TTC-B assembly/disassembly and exhibits distinct transcription termination defects compared to TTC-B.

Conclusions:

• TTC-LC represents a distinct mode of transcription-translation coupling, facilitating gene expression when ribosomes are not in immediate proximity to RNAP.
• The structural and functional differences between TTC-B and TTC-LC provide insights into the dynamic regulation of coupled transcription-translation.
• Both TTC-B and TTC-LC show defects in Rho-dependent transcription termination, suggesting broader regulatory roles for these complexes.