DNA repair

Kohl S & Bock R 2009 Transposition of a bacterial insertion sequence in chloroplasts. Plant J 58:423-436.

• after cleavage at the IS150 borders, the double-strand breaks left behind in the plastid genome cannot be efficiently repaired by non-homologous end joining (i.e. religation)
• the genome molecules from which IS150 was removed, are either repaired by recombinational repair (using intact, i.e. IS150-containing, genome copies as template) or remain unrepaired, and thus represent dead-end products that are excluded from further replication, and which are condemned to degradation

• plastids apparently lack the capacity to repair double-strand breaks in their genome by non-homologous end joining (NHEJ)
• although the plastid DNA maps as a circular genome, a number of studies have demonstrated that, in vivo, the predominant genome conformations are linear molecules, and various derivatives thereof
• the lack of double-strand break repair by NHEJ in plastids implies that linear plastid genomes cannot be reconverted into circles by religation
• the only way to repair double-strand breaks would be recombinational repair (template-assisted repair) using an intact plastid genome molecule as template

• the absence of NHEJ activity from plastids may also explain another peculiarity of the plastid genome: its immunity to promiscuous DNA
• plastid and mitochondrial DNA sequences frequently invade the nuclear genome
• DNA sequences of plastid and nuclear origin are often found in higher plant mitochondrial genomes
• no nuclear or mitochondrial DNA sequence has ever been found in any plastid genome
• the absence of NHEJ activity would protect the plastid genome from invasion by promiscuous DNA
• non-homologous nuclear and mitochondrial sequences cannot integrate into the plastid genome by ligation into double-strand breaks