7-Deazaguanines in DNA: functional and structural elucidation of a DNA modification system

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Modified nucleosides, 2'-deoxy-7-cyano-7-deazaguanosine (dPreQ0) and 2'-deoxy-7-amido-7-deazaguanosine (dADG), are found in DNA.
• These nucleosides originate from the bacterial queuosine tRNA modification pathway and the dpd gene cluster, which encodes the Dpd restriction-modification system.

Purpose of the Study:

• To reconstitute the Dpd modification machinery in vitro.
• To elucidate the roles of individual proteins within the Dpd system.
• To determine the structural basis of the DNA modification process.

Main Methods:

• In vitro reconstitution of the Dpd modification machinery from Salmonella enterica serovar Montevideo.
• X-ray crystallography of DpdA.
• Small-angle X-ray scattering (SAXS) analysis of DpdA and DpdB.
• Biochemical assays to determine enzyme activities and identify active site residues.

Main Results:

• DpdA exhibits transglycosylase activity, exchanging guanine for preQ0 in DNA.
• DpdB possesses an unexpected ATPase activity essential for preQ0 insertion into DNA.
• Catalytically essential active site residues in DpdA were identified.
• A specific modification site for DpdA activity was determined.
• DpdC independently converts preQ0-modified DNA to ADG-modified DNA.

Conclusions:

• The Dpd system utilizes a novel mechanism for DNA modification involving ATPase activity for nucleoside insertion.
• The study reveals the distinct roles of DpdA, DpdB, and DpdC in the sequential modification of DNA.
• Structural and biochemical data provide insights into the Dpd restriction-modification system's function.