Characterization of rapidly labelled RNA associated with DNA in Chlorella

[Evidence of DNA-synthesis in hypocotyls and cytoledons of inhibited and non-inhibited seedlings of Sinapis alba]

Etiolated elongating hypocotyls of Sinapis alba synthesize DNA 40-72 h after sowing. The increase in DNA content is about 62.5%. Four-hour pulse experiments showed an increasing rate of [(32)P]orthophosphate incorporation; the maximum rate was obtained 64-68 h after sowing. In the cotyledons DNA increase up to the 56th h after sowing; later the DNA-content remains constant. During this time the incorporation of (32)P into the DNA of the cotyledons decreases continuously.After simultaneous application of Fluordesoxyuridin (FdUrd) (4×10(-5)M) and thymidine (4×10(-4)M) or thymidylate (4×10(-4)M) 40-64 h after sowing, the growth rate was the same as that in H2O treated controls, whereas application of FdUrd (4×10(-5)M) alone inhibited growth.In the presence of FdUrd (4×10(-5)M) thymidine (4×10(-4)M) and thymidylate (4×10(-4)M) enhanced the specific activity of the DNA up to respectively 68% and 63% of that in the H2O treated controls. Hydroxyurea (4 mg/ml) inhibited DNA-synthesis and growth in the same manner as FdUrd.The prolonged DNA-synthesis and the reversal of the FdUrd effect by thymidine or thymidylate are discussed.