A unique exonuclease ExoG cleaves between RNA and DNA in mitochondrial DNA replication

Area of Science:

• Mitochondrial biology
• Molecular genetics
• Enzymology

Background:

• Mitochondrial DNA (mtDNA) replication requires efficient RNA primer removal.
• Mitochondrial RNase H1 partially removes RNA primers, leaving a dinucleotide remnant.
• A specific 5"-exonuclease is needed to clear these residual RNA nucleotides.

Purpose of the Study:

• To identify a mitochondrial 5"-exonuclease capable of removing RNA dinucleotides from hybrid DNA.
• To elucidate the structural mechanism of this exonuclease's activity at RNA-DNA junctions.

Main Methods:

• Biochemical assays to test exonuclease activity on RNA/DNA substrates.
• X-ray crystallography to determine the structure of human Exonuclease G (ExoG) with various nucleic acid duplexes.

Main Results:

• Human mitochondrial Exonuclease G (ExoG) efficiently cleaves RNA dinucleotides at RNA-DNA junctions.
• Crystal structures reveal ExoG's specific recognition and cleavage mechanism for A-form RNA/DNA hybrids.
• ExoG functions as a unique 5"-exonuclease in mtDNA replication primer processing.

Conclusions:

• ExoG is crucial for flap-independent RNA primer removal during mtDNA replication.
• This study establishes the molecular basis for ExoG's role in maintaining mitochondrial genome integrity.