Structural basis for transcription regulation by alarmone ppGpp

Area of Science:

• Bacterial molecular biology
• Structural biology
• Biochemistry

Background:

• Guanosine-tetraphosphate (ppGpp) is a critical regulator of bacterial stringent control, a response to environmental stresses like amino acid starvation.
• Understanding ppGpp's interaction with RNA polymerase (RNAP) is crucial for deciphering bacterial adaptive mechanisms.

Purpose of the Study:

• To elucidate the structural basis of ppGpp binding to Thermus thermophilus RNAP holoenzyme.
• To investigate the role of ppGpp-DNA interactions in bacterial transcription regulation.

Main Methods:

• X-ray crystallography to determine the 2.7 Å resolution structure of RNAP-ppGpp complex.
• Biochemical assays to validate the functional significance of observed interactions.

Main Results:

• ppGpp binds to a conserved site near the RNAP active center in distinct orientations within the crystal structure.
• ppGpp binding exhibits symmetry in diphosphate interactions but flexibility in base orientation.
• Active site asymmetry correlates with different ppGpp binding modes.
• Experimental evidence supports base-specific contacts between ppGpp and DNA cytosines.

Conclusions:

• The distinct ppGpp binding modes and associated active site configurations suggest a dynamic regulatory mechanism.
• Base pairing between ppGpp and nontemplate DNA strand cytosines is proposed as a critical element in ppGpp-mediated transcription control.
• These findings highlight the importance of specific molecular interactions in bacterial stress adaptation.