Specific switching on of silent egg protein genes in vitro by an S-100 fraction in isolated nuclei from male Xenopus.

A soluble post-nuclear extract (S-100) specifically switches on the permanently silent vitellogenin genes in male Xenopus liver nuclei in vitro. Using cloned cDNAs to albumin, globin and vitellogenin mRNAs as hybridization probes, and heparin as an inhibitor of initiation of transcription, run-on assays with nuclei faithfully reproduced the transcriptional status of these genes in vivo. In the absence of heparin, a liver S-100 extract from female or male Xenopus treated with estradiol, but not from hormonally untreated males, induced the de novo transcription of vitellogenin genes in nuclei from hormonally naive male hepatocytes. The activation of the dormant genes was dependent on pre-incubation of nuclei with the competent extract for a minimum of 45 min at 24 degrees C before transcription was initiated. The same extract did not alter significantly the transcription of albumin and globin genes. The S-100 fraction from hormonally stimulated male hepatocytes exhibited a high degree of tissue specificity; it did not activate the transcription of vitellogenin genes in Xenopus erythrocyte and oviduct nuclei, and only slightly in nuclei from Xenopus XTC-2 cells, while a soluble extract from oviduct only slightly induced their transcription in male hepatocyte nuclei. This is the first demonstration of a hormone- and tissue-specific de novo activation of dormant genes by a soluble cell extract in isolated nuclei and offers a new approach to analyzing steroid hormone action and tissue-specific transcription factors in the regulation of gene expression.