Developmental expression of the Xenopus laevis fos protooncogene.

We have isolated the Xenopus laevis homologue of the protooncogene fos. Nucleotide sequence analysis of the complementary DNA revealed a coding domain for a 370-amino acid protein which shares extensive sequence homology with the avian and mammalian fos protein. During Xenopus development, the fos mRNA is present in the oocyte and egg stages and declines to relatively low levels by the mid-blastula transition. Transient expression of fos can be seen during the late neurula and again at the tadpole stage. The fos mRNA is uniformly distributed in the vegetal and animal poles of the oocytes. In a Xenopus kidney cell line, the fos gene can be transcriptionally activated by serum growth factors and 12-O-tetradecanoylphorbol-13-acetate, and the fos mRNA can be superinduced by cycloheximide. The X. laevis fos protein migrates as a series of bands around Mr 55,000 and is localized in the nucleus. Both the Xenopus fos RNA and fos proteins exhibit a short half-life analogous to that observed with their mammalian counterpart. The Xenopus fos protein readily associates with murine jun (AP-1) oncoprotein, and the complex binds efficiently to AP-1 recognition elements. Furthermore, the Xenopus fos protein collaborates with mammalian jun protein to activate transcription from 12-O-tetradecanoylphorbol-13-acetate-responsive element. The differential expression of fos during Xenopus development and its ability to regulate transcription in association with jun suggest a role during embryogenesis.