We introduced an exogenous plasmid, pCmVCAT, into the unicellular green alga, Chlamydomonas reinhardtii by electroporation. Using in situ hybridization, we located the plasmid DNA inside the chloroplast (Cp) in a selected transformant. In this transformant, the plasmid DNA clusters were frequently transmitted unequally into daughter cells in dividing cells. Plasmid rescue and Southern blot analysis showed that the free plasmid existed as unaltered form. We constructed a probe that was suitable for simultaneous quantitative detection of Cp, nuclear and plasmid DNA. Using this probe and quantitative Southern blot analysis, we detected that the Cp located plasmid responded differently to several growth conditions that affected Cp DNA content. In nitrogen deprivation or in the presence of sublethal concentration of cadmium, the Cp DNA copy number was reduced while the content of Cp located plasmid DNA actually increased. Treatment with 5'-fluorodeoxyuridine (FdUrd) decreased the amount of both Cp DNA and Cp located plasmid DNA. We proposed a hypothesis to explain this experimental data.
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