Replication of chloroplast DNA in Chlamydomonas reinhardi during vegetative cell cycle: its mode and regulation.

The chloroplast is a semiautonomous, self-duplicatinig unit in cytoplasm equipped with its own hereditary apparatus.1-3 In Chlamydomonas,4-7 Acetabularia,8 Euglena,9' 10 and higher plant leaves,6' 11, 12 it has been observed that chloroplasts contain their own DNA distinct from chromosomal DNA. Moreover, activities of DNA-dependent RNA polymerase and of enzymes responsible for amino acid incorporation have been observed in cell-free preparations of chloroplasts.3 There has been no direct proof, however, that DNA in the chloroplast is the carrier of genes that control the function and duplication of the chloroplast in the cell. In this paper we present unequivocal evidence for (1) a semiconservative mode of chloroplast DNA replication, and (2) a difference in time of replication of chloroplast DNA and chromosomal DNA during the vegetative growth cycle of Chlarnydomonas reinhardi. These results show for the first time that replication of chloroplast DNA is normal (semiconservative) and regulated independently from that of chromosomal DNA. Regular replication of chloroplast DNA ensures its physical conservation and regular transmission to succeeding generations. The results also support the idea that the chloroplast has a genome and self-duplicating mechanism of its own.13