Quiescence-dependent activation of the p20K promoter in growth-arrested chicken embryo fibroblasts.
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We described the synthesis of a quiescence-specific p20K protein in quiescent chicken heart mesenchymal cells and contact-inhibited chicken embryo fibroblasts (Bédard, P.-A., Balk, S.D., Gunther, H.S., Morisi, A., and Erikson, R.L. (1987a) Mol. Cell. Biol. 7, 1450-1459). We now report that the expression of p20K is enhanced in cells rendered quiescent by other conditions of growth arrest such as serum starvation or treatment with hydroxyurea. Chicken embryo fibroblasts transformed by the Rous sarcoma virus also expressed p20K upon serum/medium depletion. In all conditions investigated, the synthesis of p20K was correlated with decreased DNA synthesis, indicating that growth arrest regulates the expression of p20K in fibroblasts. The abundance of p20K mRNAs was elevated in quiescent cells, and the p20K gene was more active in nuclear run-on transcription assays in conditions of growth arrest. The p20K gene was isolated, and the promoter region was analyzed in transient expression assays. Serum starvation increased the activity of the promoter, indicating that the expression of p20K is controlled at least in part at the transcriptional level. Deletion analysis revealed that a region of less than 217 base pairs (bp) is sufficient for quiescence-dependent activity of the promoter. Within this region, a segment of 48 bp was essential to basal and quiescence-induced activity of the promoter in dividing and nondividing cells, respectively. The 48-bp region enhanced the activity of a minimal heterologous promoter in quiescent cells but had no effect in conditions of proliferation, indicating that it functions as a quiescence-responsive unit (QRU). Therefore, the transcription of the p20K gene in quiescent fibroblasts is controlled by the one or several growth-regulated elements located in the 48-bp QRU of the promoter.
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