Transcriptional activity and DNA supercoiling during early germination in maize

Abstract The possible link between the ability of DNA to be transcribed and its three-dimensional organization in the eukaryotic nucleus has been studied during germination of the plant embryo. In this model system there is a gradual increase in the transcriptional activity of chromatin and no nuclear DNA replication takes place for at least the first 20–24 h following water imbibition. Maize embryo nucleoids derived by nonionic detergent and high salt treatment from nuclei isolated at different time points of germination were analysed by sedimentation in a series of sucrose gradients containing increasing amounts of the intercalating agent ethidium bromide. The sedimentation of the dry embryo nucleoids shows no dependence on the concentration of ethidium bromide while the 16-h germinated embryo possesses the titration curve characteristic of the active plant genomes. On the other hand, DNA isolated from both the dry and the germinated material behaved upon EthBr intercalation as expected on theoretical grounds, i.e. its sedimentation rate decreased with increase of the EthBr concentration. The lack of a biphasic response to EthBr titration we interpret as an indication of a lack of supercoiled DNA loops in the dry embryo nuclei.