PEN1 catalyses RNA primer removal during plastid DNA replication in maize

Area of Science:

• Molecular Biology
• Plant Science
• Genetics

Background:

• Plastid DNA (ptDNA) replication requires RNA primer removal before DNA fragment ligation.
• The enzymes and mechanisms responsible for ptDNA primer excision remain largely unknown.
• Understanding this process is critical for comprehending ptDNA replication fidelity.

Purpose of the Study:

• To identify and characterize the enzyme responsible for RNA primer removal in maize plastids.
• To elucidate the mechanism and structural basis of this enzymatic activity.
• To investigate the in vivo consequences of impaired primer removal on plastid function.

Main Methods:

• Cloning and characterization of a maize gene encoding a plastid-localized exonuclease (PEN1).
• In vitro reconstitution of plastid RNA primer removal assays.
• Determination of the crystal structure of PEN1 in complex with double-stranded DNA.
• Analysis of developmental phenotypes in maize mutants lacking functional PEN1.

Main Results:

• A novel Mn2+-dependent 5'-3' exonuclease, PEN1, was identified in maize plastids.
• PEN1 efficiently cleaves RNA primers, enabling complete ribonucleotide excision.
• PEN1 mutation leads to developmental defects and accumulation of ptDNA breaks, impairing plastid function.
• The crystal structure reveals the mechanism of PEN1's exonuclease activity.

Conclusions:

• PEN1 is the key enzyme responsible for RNA primer removal during maize plastid DNA replication.
• This study elucidates the molecular mechanism and structural basis of PEN1 activity.
• The findings fill a significant knowledge gap in ptDNA replication and maintenance.