Differential induction of vitellogenin gene transcription and total transcriptional activity by estrogen in Xenopus laevis liver.

The effects of estrogen on liver gene expression in Xenopus laevis were examined using a nuclear transcription "run-on" assay. Vitellogenin transcription was detected 2 h after a single dose of estradiol and reached a maximum on day 4. By 12 days after hormone treatment vitellogenin transcription declined to low levels. Within 3 h after estrogen, total transcriptional activity increased 9-fold relative to control values, reaching a maximum level of 40-fold by 12 h. Total transcription remained elevated throughout the following 12-day time course. These data indicate that changes in vitellogenin transcription and total nuclear transcription are uncoupled. Steady-state levels of vitellogenin mRNA demonstrated a close correlation with the level of vitellogenin gene transcription at all time points. In another series of experiments, animals treated repeatedly with estrogen demonstrated an elevated steady-state level of vitellogenin transcription, an elevated steady-state level of vitellogenin mRNA, and a constant elevated level of total nuclear transcriptional activity. Neither hormonal treatment regimen had an effect on the transcription of actin or induced the embryonic gene DG42. Finally, the increase in total transcriptional activity is associated with increased activities of RNA polymerase I and/or III in addition to stimulation of RNA polymerase II activity.