Base flipping in open complex formation at bacterial promoters.

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Transcription initiation in bacteria involves RNA polymerase binding promoter DNA.
• Formation of an open complex (RPo) requires strand separation of the promoter DNA.
• Sigma70 (σ70) and its homologs play a crucial role in this process.

Purpose of the Study:

• To review the role of σ70 in DNA strand separation during transcription initiation.
• To present evidence for strand separation initiation at the -11A base of the promoter.
• To explain the stabilization mechanisms of the open complex.

Main Methods:

• Utilized the fluorescent adenine analog 2-aminopurine to study DNA strand separation.
• Reviewed structural studies on the open complex formation.
• Analyzed the interactions between promoter DNA and σ70.

Main Results:

• Demonstrated that the -11A base on the non-template strand flips out of the DNA helix.
• Showed that the flipped -11A stacks with tyrosine 430 of σ70 within a hydrophobic pocket.
• Confirmed that additional non-template strand bases also flip out, stabilizing the open complex and preventing DNA re-annealing.

Conclusions:

• The flipping of specific DNA bases, particularly -11A, into σ70 hydrophobic pockets is critical for open complex formation and stability.
• This mechanism prevents the re-annealing of separated DNA strands, facilitating transcription initiation.
• Understanding these molecular interactions provides insight into the regulation of bacterial gene expression.