Chloroplast DNA Replication: Mechanism, Enzymes and Replication Origins

Chloroplasts contain circular DNA molecules which are found in low copy number in proplastids but are amplified to very high copy number in actively dividing leaf cells. A double displacement loop (D-loop) mechanism for chloroplast DNA (ctDNA) replication has been proposed, and pairs of replication origins which fit this model have been identified in some species. It appears that ctDNA replication is under the control of at least some nuclear gene products, as genes for DNA polymerase, topoisomerases, DNA primase and other accessory replication proteins have not been reported in the sequenced chloroplast genomes, and ctDNA replication remains active in the absence of active chloroplast transcription or translation. Only a few chloroplast replication proteins have been isolated, and to date most have not been characterized in detail. The mechanism by which ctDNA copy number is regulated during plant development is not known. In this review we summarize the current understanding of ctDNA replication.