Unequal activation by estrogen of individual Xenopus vitellogenin genes during development.

Using a technique of filter hybridization under very stringent conditions to HindIII fragments of complementary DNA cloned in plasmids, we have measured the accumulation in hepatocytes of mRNA specified by each of the four vitellogenin genes (A1, A2, B1, B2) at different stages of development of Xenopus laevis. The ontogenic competence of embryonic liver to respond to the first exposure to estradiol-17 beta, in terms of activation of transcription of this multigene family, is acquired late in metamorphosis at around Nieuwkoop-Faber stage 58. Upon hormonal induction, the four mRNAs accumulate under non-steady-state conditions at different rates and to different extents at all developmental stages in vivo and in cultured adult hepatocytes. A1 and B1 mRNAs appear more rapidly and accumulate to levels that are five- to eightfold those specified by genes A2 and B2, with higher amounts of B1 than A1 mRNA. A threefold higher absolute rate of synthesis of A1 and B1 mRNAs in hepatocyte cultures, relative to the A2-B2 pair, suggests that hormonal regulation of differential accumulation of vitellogenin mRNA occurs at the transcriptional level. At the early developmental stages (up to stage 61) of acquired competence, there appears to be no fixed pattern of expression, but a pattern of unequal activation of individual genes of the Xenopus vitellogenin multigene family is established thereafter and then retained at all developmental stages of tadpoles, froglets, and in both male and female adults.