ACCUMULATION OF PROTOONCOGENE C-ERB-A RELATED TRANSCRIPTS DURING XENOPUS DEVELOPMENT - ASSOCIATION WITH EARLY ACQUISITION OF RESPONSE TO THYROID-HORMONE AND ESTROGEN

The expression of genes encoding receptors for estrogen and thyroid hormones, as well as total c-erb-A related transcripts was determined in unfertilized eggs, all stages of embryonic and larval development and in adult tissues of Xenopus, by quantitative Northern and slot-blot hybridization. DNA and antisense RNA probes complementary to Xenopus c-myc, cytoskeletal actin and albumin mRNAs served as controls or developmental markers. Hybridization to full-length chicken c-erb-A cDNA at moderate stringency revealed a complex biphasic ontogenic pattern for several c-erb-A related mRNAs in all tissues and at all developmental stages, an increase of 4-fold in the accumulation of these transcripts occurring before metamorphosis (stages 30 to 40-42) and followed by a gradual build-up after mid-metamorphosis (stage 56). Using full-length or ligand-binding domain fragments of thyroid hormone (TR) and estrogen (ER) cDNAs under stringent hybridization conditions, transcripts of TR and ER were detected from stages 44 and 54 onwards, respectively. The α and β forms of TR mRNAs exhibited different patterns of accumulation during development, the former transcript being present in substantially higher amounts at all developmental stages. The distinct patterns of accumulation of TR and ER mRNAs could be correlated with the differential pattern of early developmental acquisition of sensitivity of Xenopus larval tissues to thyroid hormone and estrogen.