Gibberellin‐enhanced transcription by isolated nuclei from cucumber hypocotyls

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Abstract
Nuclei isolated from apical 1 cm (gibberellin‐responsive region) of dark grown cucumber (Cucumis sativus L. cv National Pickling) hypocotyls and purified on Percoll step gradients were capable of in vitro transcription; however, only nuclei from the upper regions of the gradient gave GA4‐enhanced transcription. Addition of GA4 altered the transcriptional activity in favor of RNA polymerase II, from 50% in control to more than 90% in GA4‐treated nuclei. The GA4‐enhanced transcription was rapid and did not require extraction of nuclei in a medium with exogenous GA4 or preincubation of nuclei in GA4. Addition of [3H]GA4‐binding cytosolic protein to nuclei increased the overall transcription but did not affect the GA4‐enhanced transcription. Receptor‐binding assay indicated that these nuclei contain [3H]GA4‐binding sites which are heat‐labile and bound to nuclear matrix. Also, the nuclei seem to contain an inhibitor of GA4‐enhanced transcription which is sensitive to heat and removed by repeated washings. Nuclei from the non‐ responsive basal region of the hypocotyl or from the apical region of cucumber hypocotyls grown at 34°C did not show GA4‐enhanced transcription.