CHLOROPLAST DNA-REPLICATION - MECHANISM, ENZYMES AND REPLICATION ORIGINS

Chloroplasts contain circular DNA molecules which are found in low cop y number in proplastids but are amplified to very high copy number in actively dividing leaf cells. A double displacement loop (D-loop) mech anism for chloroplast DNA (ctDNA) replication has been proposed, and p airs of replication origins which fit this model have been identified in some species. It appears that ctDNA replication is under the contro l 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 trans cription or translation. Only a few chloroplast replication proteins h ave been isolated, and to date most have not been characterized in det ail. The mechanism by which ctDNA copy number is regulated during plan t development is not known. In this review we summarize the current un derstanding of ctDNA replication.