Uncoupling of chloroplast reproductive events from cell cycle division processes by 5-fluorodeoxyuridine in the alga Scenedesmus quadricauda

FdUrd (5-fluorodeoxyuridine), a specific inhibitor of thymidylate synthase, was used to study the relationship between reproductive processes in chloroplast and nucleocytoplasmic compartments of the chlorococcal alga Scenedesmus quadricauda. The courses of DNA replication and nuclear division in both the compartments were followed in populations synchronised by the alternation of light and dark periods. DAPI-staining of DNA-containing structures was used for their visualisation and quantification. In contrast with cellular reproductive events, those in chloroplasts were not substantially affected by the presence of FdUrd (25 mu g/ml). It was shown that FdUrd specifically blocked nucDNA replication but not ptDNA replication. Thus, cells which had attained commitment to ptDNA replication, fission of pt-nuclei and chloroplast kinesis triggered and terminated these processes while the corresponding cellular processes were blocked. The courses of reproductive processes in chloroplasts were also substantially unaffected in cells grown in the presence of FdUrd for the whole cell cycle. This provided evidence that attainment of commitment to and termination of the entire sequence of reproductive events, including chloroplast fission, were controlled by different mechanisms than the reproductive processes in the nucleocytoplasmic compartment.