The chloroplast DNA (chDNA) in the unicellular green alga, Chlorella, consists of at least two DNA species. The major component (ch-DNAM) has the corrected "practical density" in CsCl of 1.7100 gm/ml, which is almost the same as the value for major nuclear DNA (nDNAM); the minor component has the value 1.6860 gm/ml. During the cell cycle effected by synchronous culture under a light-dark regimen, chDNA was synthesized near the middle of the light phase before nDNA synthesis. The content of chDNA quadrupled during the synthetic period. This fact corresponds to the observations that four chloroplasts can be found in large cells at about the end of the light phase and four daughter cells are produced from one mother cell at the end of the cell cycle. The algal chDNA was labelled actively with 14CO2 or [3H]thymidine during its synthesis.
To determine whether chDNA replicates semi-conservatively, we examined the density-labelling and density-shift of chDNA during its synthesis in the cell cycle. Since chDNAM has almost the same buoyant density as that of nDNAM in CsCl, we made CsCl equilibrium density gradients in preparative ultracentri-fuge on the total cell DNA containing chDNA which had been preferentially labelled with [3H]thymidine or 14CO2 during its synthesis. We here report the successful demonstration of the semi-conservative mode of replication in chDNAM, and discuss the remarkable characteristics of this algal chDNA.
Japan Society for Cell Biology