Transcriptional activation of c-jun during the G₀/G₁ transition in mouse fibroblasts

Abstract

Before quiescent cells can respond to mitogens and progress through the G₁ phase of cell growth, new messenger RNA synthesis is required¹. The G₁ phase seems to be a critical point of control in the cell cycle, where normal cells deprived of growth factors halt cycling while transformed cells do not, suggesting that regulatory genes, uncontrolled in the neoplastic phenotype, are expressed during the G₀ to G₁ transition. Some of these may code for nuclear proteins that participate in the transactivation of genes required for the progression through G₁. The observed changes in expression of the proto-oncogenes c-fos and c-myc, following stimulation of fibroblasts with growth factors^2–7, support this notion as recent evidence suggests that c-FOS and c-MYC proteins can function as transactivating factors^8–12. Moreover, the rapid induction of several genes in fibroblasts coding for putative transacting factors during the G₀ to G₁ transition has been recently reported^13–16. Here we present the nucleotide sequence of a mouse cDNA clone coding for a 334 residue protein which shows 80% similarity with v-JUN¹⁷ and more than 98% similarity with the human c-JUN sequence^18,19. We have demonstrated that in quiescent fibroblasts c-jun transcription is rapidly induced during the G₀ to G₁ transition.