EXPRESSION OF THE XENOPUS-LAEVIS PROLACTIN AND THYROTROPIN GENES DURING METAMORPHOSIS

The cDNAs encoding Xenopus laevis prolactin (PRL) and the alpha and be ta subunits of thyroid-stimulating hormone (TSHalpha and TSHbeta, resp ectively) have been cloned from a pituitary library. Results of develo pmental RNA blot analysis contradict the long-held biological role for PRL as a juvenilizing hormone in amphibia. The pituitary gland of a p remetamorphic tadpole expresses PRL mRNA at very low levels. The abund ance of PRL mRNA increases late in metamorphosis as a response to thyr oid hormone (TH), suggesting that PRL is more likely to have a functio n in the frog than in the tadpole. TSHalpha and -beta mRNA levels incr ease through prometamorphosis; this rise does not appear to be regulat ed directly by TH. At climax, both TH and TSH mRNA levels drop. The se quential morphological changes that characterize prometamorphosis depe nd upon the gradual increase of endogenous TH, which peaks at climax. This increase in TH in turn depends upon the lack of a traditional thy roid-pituitary negative-feedback loop throughout prometamorphosis.