An SI3-σ arch stabilizes cyanobacteria transcription initiation complex

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Bacterial transcription initiation relies on RNA polymerases (RNAPs) and sigma (σ) factors.
• σ factors are essential for promoter DNA recognition and unwinding.
• Cyanobacteria possess unique RNAP structures and transcription regulation mechanisms.

Purpose of the Study:

• To determine the near-atomic resolution structures of cyanobacterial transcription initiation complexes.
• To elucidate the structural basis of transcription initiation in cyanobacteria.
• To investigate the role of the "SI3-σ" arch in cyanobacterial transcription.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to obtain high-resolution structures.
• Structural analysis focused on the interaction between RNAP, σ factor, and promoter DNA.
• Functional assays assessed the impact of disrupting the "SI3-σ" arch on cell growth and stress response.

Main Results:

• Two cryo-EM structures revealed a novel "SI3-σ" arch interaction in cyanobacterial RNAP.
• This arch involves domain 2 of σA (σ2) and sequence insertion 3 (SI3) within the Trigger Loop (TL).
• The "SI3-σ" arch stabilizes the RNAP-promoter DNA open complex, facilitating initiation from diverse promoter classes.

Conclusions:

• Cyanobacteria utilize a unique "SI3-σ" arch mechanism for transcription initiation.
• Disruption of this arch negatively impacts cyanobacteria growth and stress response.
• The findings offer insights into cyanobacterial and chloroplast transcription regulation.