Structural basis for converting a general transcription factor into an operon-specific virulence regulator

Area of Science:

• Molecular Biology
• Structural Biology
• Protein Evolution

Background:

• RfaH is a paralog of the general transcription factor NusG.
• RfaH is recruited to elongating RNA polymerase at specific regulatory sites.

Purpose of the Study:

• To determine the X-ray structure of Escherichia coli RfaH.
• To understand the structural differences between RfaH and NusG.
• To elucidate the mechanism of RfaH binding to RNA polymerase.

Main Methods:

• X-ray crystallography of Escherichia coli RfaH.

Main Results:

• The RfaH structure reveals two domains: an N-terminal domain similar to NusG and a distinct alpha-helical coiled-coil C domain.
• This represents an extreme example of protein fold evolution, transitioning from beta-barrel to all-alpha-helix.
• A hydrophobic cavity in the N domain, exposed in RfaH upon DNA binding, is proposed as the RNA polymerase-binding site.

Conclusions:

• RfaH's unique domain structure facilitates its regulatory role in transcription.
• The unmasking of the RNA polymerase-binding site via DNA interaction is a key regulatory mechanism.
• RfaH likely binds to the beta' subunit coiled coil of RNA polymerase.